Harold Hwang

All Publications

• Transport phase diagram and anomalous metallicity in superconducting infinite-layer nickelates. Nature communications Hsu, Y., Lee, K., Badoux, S., Duffy, C., Cuoghi, A., Wang, B. Y., Kool, A., Haik-Dunn, I., Hwang, H. Y., Hussey, N. E. 2024; 15 (1): 9863

  Abstract

  Despite obvious similarities in their electronic and crystallographic structures, it remains unclear whether the interactions that shape the normal and superconducting (SC) state properties of high-Tc cuprates and infinite-layer nickelates (ILNs) have the same origin. This question has been brought into sharper focus with recent studies on ILNs of improved crystallinity that reveal a SC dome of comparable extent and similar transport properties above Tc as the hole-doped cuprates. The evolution of these properties in the magnetic-field-induced normal state, however, has yet to be determined. Here, we examine the magnetotransport properties of new-generation Nd1-xSrxNiO2 films in the T0 limit across the phase diagram in fields up to 54 T. This extensive study reveals that the limiting low-T form of the normal-state resistivity in ILNs exhibits non-Fermi-liquid behaviour over an extended doping range inside the SC dome, rather than at a singular quantum critical point. While there are clear differences in the charge dynamics of ILNs and cuprates, most notably in the magnetoresistance, our findings reveal that both systems exhibit anomalous metallicity characteristic of a quantum critical phase.

  View details for DOI 10.1038/s41467-024-54135-y

  View details for PubMedID 39543141

• Author Correction: Highly confined epsilon-near-zero and surface phonon polaritons in SrTiO3membranes. Nature communications Xu, R., Crassee, I., Bechtel, H. A., Zhou, Y., Bercher, A., Korosec, L., Rischau, C. W., Teyssier, J., Crust, K. J., Lee, Y., Gilbert Corder, S. N., Li, J., Dionne, J. A., Hwang, H. Y., Kuzmenko, A. B., Liu, Y. 2024; 15 (1): 8545

  View details for DOI 10.1038/s41467-024-52983-2

  View details for PubMedID 39358377

• Flexoelectric Enhancement of Strain Gradient Elasticity Across a Ferroelectric-to-Paraelectric Phase Transition. Nano letters Harbola, V., Pesquera, D., Xu, R., Ashby, P. D., Martin, L. W., Hwang, H. Y. 2024

  Abstract

  We study the temperature dependent elastic properties of Ba0.8Sr0.2TiO3 freestanding membranes across the ferroelectric-to-paraelectric phase transition using an atomic force microscope. The bending rigidity of thin membranes can be stiffer compared to stretching due to strain gradient elasticity (SGE). We measure the Young's modulus of freestanding Ba0.8Sr0.2TiO3 drumheads in bending and stretching dominated deformation regimes on a variable temperature platform, finding a peak in the difference between the two Young's moduli obtained at the phase transition. This demonstrates a dependence of SGE on the dielectric properties of a material and alludes to a flexoelectric origin of an effective SGE.

  View details for DOI 10.1021/acs.nanolett.4c02946

  View details for PubMedID 39133234

• Tuning Exciton Emission via Ferroelectric Polarization at a Heterogeneous Interface between a Monolayer Transition Metal Dichalcogenide and a Perovskite Oxide Membrane. Nano letters Choi, J., Crust, K. J., Li, L., Lee, K., Luo, J., So, J. P., Watanabe, K., Taniguchi, T., Hwang, H. Y., Mak, K. F., Shan, J., Fuchs, G. D. 2024

  Abstract

  We demonstrate the integration of a thin BaTiO3 (BTO) membrane with monolayer MoSe2 in a dual-gate device that enables in situ manipulation of the BTO ferroelectric polarization with a voltage pulse. While two-dimensional (2D) transition metal dichalcogenides (TMDs) offer remarkable adaptability, their hybrid integration with other families of functional materials beyond the realm of 2D materials has been challenging. Released functional oxide membranes offer a solution for 2D/3D integration via stacking. 2D TMD excitons can serve as a local probe of the ferroelectric polarization in BTO at a heterogeneous interface. Using photoluminescence (PL) of MoSe2 excitons to optically read out the doping level, we find that the relative population of charge carriers in MoSe2 depends sensitively on the ferroelectric polarization. This finding points to a promising avenue for future-generation versatile sensing devices with high sensitivity, fast readout, and diverse applicability for advanced signal processing.

  View details for DOI 10.1021/acs.nanolett.4c01853

  View details for PubMedID 38996059

• Highly confined epsilon-near-zero and surface phonon polaritons in SrTiO3 membranes. Nature communications Xu, R., Crassee, I., Bechtel, H. A., Zhou, Y., Bercher, A., Korosec, L., Rischau, C. W., Teyssier, J., Crust, K. J., Lee, Y., Gilbert Corder, S. N., Li, J., Dionne, J. A., Hwang, H. Y., Kuzmenko, A. B., Liu, Y. 2024; 15 (1): 4743

  Abstract

  Recent theoretical studies have suggested that transition metal perovskite oxide membranes can enable surface phonon polaritons in the infrared range with low loss and much stronger subwavelength confinement than bulk crystals. Such modes, however, have not been experimentally observed so far. Here, using a combination of far-field Fourier-transform infrared (FTIR) spectroscopy and near-field synchrotron infrared nanospectroscopy (SINS) imaging, we study the phonon polaritons in a 100 nm thick freestanding crystalline membrane of SrTiO3 transferred on metallic and dielectric substrates. We observe a symmetric-antisymmetric mode splitting giving rise to epsilon-near-zero and Berreman modes as well as highly confined (by a factor of 10) propagating phonon polaritons, both of which result from the deep-subwavelength thickness of the membranes. Theoretical modeling based on the analytical finite-dipole model and numerical finite-difference methods fully corroborate the experimental results. Our work reveals the potential of oxide membranes as a promising platform for infrared photonics and polaritonics.

  View details for DOI 10.1038/s41467-024-47917-x

  View details for PubMedID 38834672

  View details for PubMedCentralID 9799068

• Superionic fluoride gate dielectrics with low diffusion barrier for two-dimensional electronics. Nature nanotechnology Meng, K., Li, Z., Chen, P., Ma, X., Huang, J., Li, J., Qin, F., Qiu, C., Zhang, Y., Zhang, D., Deng, Y., Yang, Y., Gu, G., Hwang, H. Y., Xue, Q. K., Cui, Y., Yuan, H. 2024

  Abstract

  Exploration of new dielectrics with a large capacitive coupling is an essential topic in modern electronics when conventional dielectrics suffer from the leakage issue near the breakdown limit. Here, to address this looming challenge, we demonstrate that rare-earth metal fluorides with extremely low ion migration barriers can generally exhibit an excellent capacitive coupling over 20 μF cm-2 (with an equivalent oxide thickness of ~0.15 nm and a large effective dielectric constant near 30) and great compatibility with scalable device manufacturing processes. Such a static dielectric capability of superionic fluorides is exemplified by MoS2 transistors exhibiting high on/off current ratios over 108, ultralow subthreshold swing of 65 mV dec-1 and ultralow leakage current density of ~10-6 A cm-2. Therefore, the fluoride-gated logic inverters can achieve notably higher static voltage gain values (surpassing ~167) compared with a conventional dielectric. Furthermore, the application of fluoride gating enables the demonstration of NAND, NOR, AND and OR logic circuits with low static energy consumption. In particular, the superconductor-insulator transition at the clean-limit Bi2Sr2CaCu2O8+δ can also be realized through fluoride gating. Our findings highlight fluoride dielectrics as a pioneering platform for advanced electronic applications and for tailoring emergent electronic states in condensed matter.

  View details for DOI 10.1038/s41565-024-01675-5

  View details for PubMedID 38750167

  View details for PubMedCentralID 4773419

• Scanning SQUID study of ferromagnetism and superconductivity in infinite-layer nickelates PHYSICAL REVIEW MATERIALS Shi, R. A., Wang, B., Iguchi, Y., Osada, M., Lee, K., Goodge, B. H., Kourkoutis, L. F., Hwang, H. Y., Moler, K. A. 2024; 8 (2)

  View details for DOI 10.1103/PhysRevMaterials.8.024802

  View details for Web of Science ID 001171649400003

• Interlayer engineering of Fe3GeTe2: From 3D superlattice to 2D monolayer. Proceedings of the National Academy of Sciences of the United States of America Wu, Y., Wang, B. Y., Yu, Y., Li, Y., Ribeiro, H. B., Wang, J., Xu, R., Liu, Y., Ye, Y., Zhou, J., Ke, F., Harbola, V., Heinz, T. F., Hwang, H. Y., Cui, Y. 2024; 121 (4): e2314454121

  Abstract

  The discoveries of ferromagnetism down to the atomically thin limit in van der Waals (vdW) crystals by mechanical exfoliation have enriched the family of magnetic thin films [C. Gong et al., Nature 546, 265-269 (2017) and B. Huang et al., Nature 546, 270-273 (2017)]. However, compared to the study of traditional magnetic thin films by physical deposition methods, the toolbox of the vdW crystals based on mechanical exfoliation and transfer suffers from low yield and ambient corrosion problem and now is facing new challenges to study magnetism. For example, the formation of magnetic superlattice is difficult in vdW crystals, which limits the study of the interlayer interaction in vdW crystals [M. Gibertini, M. Koperski, A. F. Morpurgo, K. S. Novoselov, Nat. Nanotechnol. 14, 408-419 (2019)]. Here, we report a strategy of interlayer engineering of the magnetic vdW crystal Fe3GeTe2 (FGT) by intercalating quaternary ammonium cations into the vdW spacing. Both three-dimensional (3D) vdW superlattice and two-dimensional (2D) vdW monolayer can be formed by using this method based on the amount of intercalant. On the one hand, the FGT superlattice shows a strong 3D critical behavior with a decreased coercivity and increased domain wall size, attributed to the co-engineering of the anisotropy, exchange interaction, and electron doping by intercalation. On the other hand, the 2D vdW few layers obtained by over-intercalation are capped with organic molecules from the bulk crystal, which not only enhances the ferromagnetic transition temperature (TC), but also substantially protects the thin samples from degradation, thus allowing the preparation of large-scale FGT ink in ambient environment.

  View details for DOI 10.1073/pnas.2314454121

  View details for PubMedID 38232283

• Twisted epitaxy of gold nanodisks grown between twisted substrate layers of molybdenum disulfide. Science (New York, N.Y.) Cui, Y., Wang, J., Li, Y., Wu, Y., Been, E., Zhang, Z., Zhou, J., Zhang, W., Hwang, H. Y., Sinclair, R., Cui, Y. 2024; 383 (6679): 212-219

  Abstract

  We expand the concept of epitaxy to a regime of "twisted epitaxy" with the epilayer crystal orientation between two substrates influenced by their relative orientation. We annealed nanometer-thick gold (Au) nanoparticles between two substrates of exfoliated hexagonal molybdenum disulfide (MoS2) with varying orientation of their basal planes with a mutual twist angle ranging from 0° to 60°. Transmission electron microscopy studies show that Au aligns midway between the top and bottom MoS2 when the twist angle of the bilayer is small (<~7°). For larger twist angles, Au has only a small misorientation with the bottom MoS2 that varies approximately sinusoidally with twist angle of the bilayer MoS2. Four-dimensional scanning transmission electron microscopy analysis further reveals a periodic strain variation (<|±0.5%|) in the Au nanodisks associated with the twisted epitaxy, consistent with the Moiré registry of the two MoS2 twisted layers.

  View details for DOI 10.1126/science.adk5947

  View details for PubMedID 38207038

• Evidence for d-wave superconductivity of infinite-layer nickelates from low-energy electrodynamics. Nature materials Cheng, B., Cheng, D., Lee, K., Luo, L., Chen, Z., Lee, Y., Wang, B. Y., Mootz, M., Perakis, I. E., Shen, Z. X., Hwang, H. Y., Wang, J. 2024

  Abstract

  The discovery of superconductivity in infinite-layer nickelates established another category of unconventional superconductors that shares structural and electronic similarities with cuprates. However, key issues of the superconducting pairing symmetry, gap amplitude and superconducting fluctuations are yet to be addressed. Here we utilize static and ultrafast terahertz spectroscopy to address these. We demonstrate that the equilibrium terahertz conductivity and non-equilibrium terahertz responses of an optimally Sr-doped nickelate film (superconducting transition temperature of Tc = 17 K) are in line with the electrodynamics of d-wave superconductivity in the dirty limit. The gap-to-Tc ratio (2Δ/kBTc) is found to be 3.4, indicating that the superconductivity falls in the weak coupling regime. In addition, we observed substantial superconducting fluctuations near Tc that do not extend into the deep normal state as the optimally hole-doped cuprates do. Our results support a d-wave system that closely resembles the electron-doped cuprates.

  View details for DOI 10.1038/s41563-023-01766-z

  View details for PubMedID 38182811

• Delamination-Assisted Ultrafast Wrinkle Formation in a Freestanding Film. Nano letters Su, Y., Zong, A., Kogar, A., Lu, D., Hong, S. S., Freelon, B., Rohwer, T., Wang, B. Y., Hwang, H. Y., Gedik, N. 2023

  Abstract

  Freestanding films provide a versatile platform for materials engineering thanks to additional structural motifs not found in films with a substrate. A ubiquitous example is wrinkles, yet little is known about how they can develop over as fast as a few picoseconds due to a lack of experimental probes to visualize their dynamics in real time on the nanoscopic scale. Here, we use time-resolved electron diffraction to directly observe light-activated wrinkling formation in freestanding La2/3Ca1/3MnO3 films. Via a "lock-in" analysis of oscillations in the diffraction peak position, intensity, and width, we quantitatively reconstructed how wrinkles develop on the time scale of lattice vibration. Contrary to the common assumption of fixed boundary conditions, we found that wrinkle development is associated with ultrafast delamination at the film boundaries. Our work provides a generic protocol to quantify wrinkling dynamics in freestanding films and highlights the importance of the film-substrate interaction in determining the properties of freestanding structures.

  View details for DOI 10.1021/acs.nanolett.3c02898

  View details for PubMedID 37988604

• Electronic band sculpted by oxygen vacancies and indispensable for dilute superconductivity PHYSICAL REVIEW RESEARCH Fauque, B., Collignon, C., Yoon, H., Ravi, X., Lin, X., Mazin, I. I., Hwang, H. Y., Behnia, K. 2023; 5 (3)

  View details for DOI 10.1103/PhysRevResearch.5.033080

  View details for Web of Science ID 001052955500004

• Visualizing Polar Distortions and Interface Effects with Multislice Ptychography. Microscopy and microanalysis : the official journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada Harikrishnan, K. P., Li, Y. E., Crust, K. J., Khandelwal, A., Shao, Y. T., Chen, Z., Zhang, C., Guguschev, C., Xu, R., Hwang, H. Y., Schlom, D. G., Muller, D. A. 2023; 29 (Supplement_1): 1626-1627

  View details for DOI 10.1093/micmic/ozad067.835

  View details for PubMedID 37613802

• Publisher Correction: Geometric frustration of Jahn-Teller order in the infinite-layer lattice. Nature Kim, W. J., Smeaton, M. A., Jia, C., Goodge, B. H., Cho, B. G., Lee, K., Osada, M., Jost, D., Ievlev, A. V., Moritz, B., Kourkoutis, L. F., Devereaux, T. P., Hwang, H. Y. 2023

  View details for DOI 10.1038/s41586-023-06432-7

  View details for PubMedID 37474799

• Linear-in-temperature resistivity for optimally superconducting (Nd,Sr)NiO2. Nature Lee, K., Wang, B. Y., Osada, M., Goodge, B. H., Wang, T. C., Lee, Y., Harvey, S., Kim, W. J., Yu, Y., Murthy, C., Raghu, S., Kourkoutis, L. F., Hwang, H. Y. 2023; 619 (7969): 288-292

  Abstract

  The occurrence of superconductivity in proximity to various strongly correlated phases of matter has drawn extensive focus on their normal state properties, to develop an understanding of the state from which superconductivity emerges1-4. The recent finding of superconductivity in layered nickelates raises similar interests5-8. However, transport measurements of doped infinite-layer nickelate thin films have been hampered by materials limitations of these metastable compounds: in particular, a high density of extended defects9-11. Here, by moving to a substrate (LaAlO3)0.3(Sr2TaAlO6)0.7 that better stabilizes the growth and reduction conditions, we can synthesize the doping series of Nd1-xSrxNiO2 essentially free from extended defects. In their absence, the normal state resistivity shows a low-temperature upturn in the underdoped regime, linear behaviour near optimal doping and quadratic temperature dependence for overdoping. This is phenomenologically similar to the copper oxides2,12 despite key distinctions-namely, the absence of an insulating parent compound5,6,9,10, multiband electronic structure13,14 and a Mott-Hubbard orbital alignment rather than the charge-transfer insulator of the copper oxides15,16. We further observe an enhancement of superconductivity, both in terms of transition temperature and range of doping. These results indicate a convergence in the electronic properties of both superconducting families as the scale of disorder in the nickelates is reduced.

  View details for DOI 10.1038/s41586-023-06129-x

  View details for PubMedID 37438595

  View details for PubMedCentralID 7812792

• Electrostatic modulation of the lateral carrier density profile in field effect devices with nonlinear dielectrics PHYSICAL REVIEW B Persky, E., Yoon, H., Xie, Y., Hwang, H. Y., Ruhman, J., Kalisky, B. 2023; 107 (19)

  View details for DOI 10.1103/PhysRevB.107.195427

  View details for Web of Science ID 001003445700003

• Effects of rare-earth magnetism on the superconducting upper critical field in infinite-layer nickelates. Science advances Wang, B. Y., Wang, T. C., Hsu, Y. T., Osada, M., Lee, K., Jia, C., Duffy, C., Li, D., Fowlie, J., Beasley, M. R., Devereaux, T. P., Fisher, I. R., Hussey, N. E., Hwang, H. Y. 2023; 9 (20): eadf6655

  Abstract

  The search for superconductivity in infinite-layer nickelates was motivated by analogy to the cuprates, and this perspective has framed much of the initial consideration of this material. However, a growing number of studies have highlighted the involvement of rare-earth orbitals; in that context, the consequences of varying the rare-earth element in the superconducting nickelates have been much debated. Here, we show notable differences in the magnitude and anisotropy of the superconducting upper critical field across the La-, Pr-, and Nd-nickelates. These distinctions originate from the 4f electron characteristics of the rare-earth ions in the lattice: They are absent for La3+, nonmagnetic for the Pr3+ singlet ground state, and magnetic for the Nd3+ Kramer's doublet. The unique polar and azimuthal angle-dependent magnetoresistance found in the Nd-nickelates can be understood to arise from the magnetic contribution of the Nd3+ 4f moments. Such robust and tunable superconductivity suggests potential in future high-field applications.

  View details for DOI 10.1126/sciadv.adf6655

  View details for PubMedID 37196089

• Resolving the polar interface of infinite-layer nickelate thin films. Nature materials Goodge, B. H., Geisler, B., Lee, K., Osada, M., Wang, B. Y., Li, D., Hwang, H. Y., Pentcheva, R., Kourkoutis, L. F. 2023

  Abstract

  Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and d electron filling, however, superconductivity in nickelates has thus far only been stabilized in thin-film geometry, raising questions about the polar interface between substrate and thin film. Here we conduct a detailed experimental and theoretical study of the prototypical interface between Nd1-xSrxNiO2 and SrTiO3. Atomic-resolution electron energy loss spectroscopy in the scanning transmission electron microscope reveals the formation of a single intermediate Nd(Ti,Ni)O3 layer. Density functional theory calculations with a Hubbard U term show how the observed structure alleviates the polar discontinuity. We explore the effects of oxygen occupancy, hole doping and cation structure to disentangle the contributions of each for reducing interface charge density. Resolving the non-trivial interface structure will be instructive for future synthesis of nickelate films on other substrates and in vertical heterostructures.

  View details for DOI 10.1038/s41563-023-01510-7

  View details for PubMedID 36973543

• Emergent chirality in a polar meron to skyrmion phase transition. Nature communications Shao, Y. T., Das, S., Hong, Z., Xu, R., Chandrika, S., Gómez-Ortiz, F., García-Fernández, P., Chen, L. Q., Hwang, H. Y., Junquera, J., Martin, L. W., Ramesh, R., Muller, D. A. 2023; 14 (1): 1355

  Abstract

  Polar skyrmions are predicted to emerge from the interplay of elastic, electrostatic and gradient energies, in contrast to the key role of the anti-symmetric Dzyalozhinskii-Moriya interaction in magnetic skyrmions. Here, we explore the reversible transition from a skyrmion state (topological charge of -1) to a two-dimensional, tetratic lattice of merons (with topological charge of -1/2) upon varying the temperature and elastic boundary conditions in [(PbTiO3)16/(SrTiO3)16]8 membranes. This topological phase transition is accompanied by a change in chirality, from zero-net chirality (in meronic phase) to net-handedness (in skyrmionic phase). We show how scanning electron diffraction provides a robust measure of the local polarization simultaneously with the strain state at sub-nm resolution, while also directly mapping the chirality of each skyrmion. Using this, we demonstrate strain as a crucial order parameter to drive isotropic-to-anisotropic structural transitions of chiral polar skyrmions to non-chiral merons, validated with X-ray reciprocal space mapping and phase-field simulations.

  View details for DOI 10.1038/s41467-023-36950-x

  View details for PubMedID 36907894

  View details for PubMedCentralID 5798329

• Observation of Coulomb blockade and Coulomb staircases in superconducting Pr0.8Sr0.2NiO2 films PHYSICAL REVIEW B Wang, R., Xiong, Y., Yan, H., Hu, X., Osada, M., Li, D., Hwang, H. Y., Song, C., Ma, X., Xue, Q. 2023; 107 (11)

  View details for DOI 10.1103/PhysRevB.107.115411

  View details for Web of Science ID 000962389700004

• Geometric frustration of Jahn-Teller order in the infinite-layer lattice. Nature Kim, W. J., Smeaton, M. A., Jia, C., Goodge, B. H., Cho, B., Lee, K., Osada, M., Jost, D., Ievlev, A. V., Moritz, B., Kourkoutis, L. F., Devereaux, T. P., Hwang, H. Y. 2023

  Abstract

  The Jahn-Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, has a key role in many symmetry-lowering crystal deformations1. Lattices of Jahn-Teller ions can induce a cooperative distortion, as exemplified by LaMnO3 (refs. 2,3). Although many examples occur in octahedrally4 or tetrahedrally5 coordinated transition metal oxides due to their high orbital degeneracy, this effect has yet to be manifested for square-planar anion coordination, as found in infinite-layer copper6,7, nickel8,9, iron10,11 and manganese oxides12. Here we synthesize single-crystal CaCoO2 thin films by topotactic reduction of the brownmillerite CaCoO2.5 phase. We observe a markedly distorted infinite-layer structure, with angstrom-scale displacements of the cations from their high-symmetry positions. This can be understood to originate from the Jahn-Teller degeneracy of the dxz and dyz orbitals in the d7 electronic configuration along with substantial ligand-transition metal mixing. A complex pattern of distortions arises in a [Formula: see text] tetragonal supercell, reflecting the competition between an ordered Jahn-Teller effect on the CoO2 sublattice and the geometric frustration of the associated displacements of the Ca sublattice, which are strongly coupled in the absence of apical oxygen. As a result of this competition, the CaCoO2 structure forms an extended two-in-two-out type of Co distortion following 'ice rules'13.

  View details for DOI 10.1038/s41586-022-05681-2

  View details for PubMedID 36813969

• Size-Induced Ferroelectricity in Antiferroelectric Oxide Membranes. Advanced materials (Deerfield Beach, Fla.) Xu, R., Crust, K. J., Harbola, V., Arras, R., Patel, K. Y., Prosandeev, S., Cao, H., Shao, Y. T., Behera, P., Caretta, L., Kim, W. J., Khandelwal, A., Acharya, M., Wang, M. M., Liu, Y., Barnard, E. S., Raja, A., Martin, L. W., Gu, X. W., Zhou, H., Ramesh, R., Muller, D. A., Bellaiche, L., Hwang, H. Y. 2023: e2210562

  Abstract

  Despite extensive studies on size effects in ferroelectrics, how structures and properties evolve in antiferroelectrics with reduced dimensions still remains elusive. Given the enormous potential of utilizing antiferroelectrics for high energy-density storage applications, understanding their size effects would provide key information for optimizing device performances at small scales. Here, we investigate the fundamental intrinsic size dependence of antiferroelectricity in lead-free NaNbO3 membranes. Via a wide range of experimental and theoretical approaches, we probe an intriguing antiferroelectric-to-ferroelectric transition upon reducing membrane thickness. This size effect leads to a ferroelectric single-phase below 40 nm as well as a mixed-phase state with ferroelectric and antiferroelectric orders coexisting above this critical thickness. Furthermore, we show that the antiferroelectric and ferroelectric orders are electrically switchable. First-principle calculations further reveal the observed transition is driven by the structural distortion arising from the membrane surface. Our work provides direct experimental evidence for intrinsic size-driven scaling in antiferroelectrics and demonstrates enormous potential of utilizing size effects to drive emergent properties in environmentally benign lead-free oxides with the membrane platform. This article is protected by copyright. All rights reserved.

  View details for DOI 10.1002/adma.202210562

  View details for PubMedID 36739113

• Electrostatic gating and intercalation in 2D materials NATURE REVIEWS MATERIALS Wu, Y., Li, D., Wu, C., Hwang, H. Y., Cui, Y. 2022

  View details for DOI 10.1038/s41578-022-00473-6

  View details for Web of Science ID 000847996800001

• Intrinsic magnetism in superconducting infinite-layer nickelates NATURE PHYSICS Fowlie, J., Hadjimichael, M., Martins, M. M., Li, D., Osada, M., Wang, B., Lee, K., Lee, Y., Salman, Z., Prokscha, T., Triscone, J., Hwang, H. Y., Suter, A. 2022

  View details for DOI 10.1038/s41567-022-01684-y

  View details for Web of Science ID 000834751200003

• A broken translational symmetry state in an infinite-layer nickelate NATURE PHYSICS Rossi, M., Osada, M., Choi, J., Agrestini, S., Jost, D., Lee, Y., Lu, H., Wang, B., Lee, K., Nag, A., Chuang, Y., Kuo, C., Lee, S., Moritz, B., Devereaux, T. P., Shen, Z., Lee, J., Zhou, K., Hwang, H. Y., Lee, W. 2022

  View details for DOI 10.1038/s41567-022-01660-6

  View details for Web of Science ID 000829739500004

• Heat Conductor-Insulator Transition in Electrochemically Controlled Hybrid Superlattices. Nano letters Zhou, J., Wu, Y., Kwon, H., Li, Y., Xiao, X., Ye, Y., Ma, Y., Goodson, K. E., Hwang, H. Y., Cui, Y. 2022

  Abstract

  Designing materials with ultralow thermal conductivity has broad technological impact, from thermal protection to energy harvesting. Low thermal conductivity is commonly observed in anharmonic and strongly disordered materials, yet a microscopic understanding of the correlation to atomic motion is often lacking. Here we report that molecular insertion into an existing two-dimensional layered lattice structure creates a hybrid superlattice with extremely low thermal conductivity. Vibrational characterization and ab initio molecular dynamics simulations reveal strong damping of transverse acoustic waves and significant softening of longitudinal vibrations. Together with spectral correlation analysis, we demonstrate that the molecular insertion creates liquid-like atomic motion in the existing lattice framework, causing a large suppression of heat conduction. The hybrid materials can be transformed into solution-processable coatings and used for thermal protection in wearable electronics. Our work provides a generic mechanism for the design of heat insulators and may further facilitate the engineering of heat conduction based on understanding atomic correlations.

  View details for DOI 10.1021/acs.nanolett.2c01407

  View details for PubMedID 35715219

• Theory of superconductivity in doped quantum paraelectrics NPJ QUANTUM MATERIALS Yu, Y., Hwang, H. Y., Raghu, S., Chung, S. 2022; 7 (1)

  View details for DOI 10.1038/s41535-022-00466-2

  View details for Web of Science ID 000811209400002

• Electronic structure of superconducting nickelates probed by resonant photoemission spectroscopy MATTER Chen, Z., Osada, M., Li, D., Been, E. M., Chen, S., Hashimoto, M., Lu, D., Mo, S., Lee, K., Wang, B., Rodolakis, F., McChesney, J. L., Jia, C., Moritz, B., Devereaux, T. P., Hwang, H. Y., Shen, Z. 2022; 5 (6)

  View details for DOI 10.1016/j.matt.2022.01.020

  View details for Web of Science ID 000810939100001

• Observation of an intermediate state during lithium intercalation of twisted bilayer MoS2. Nature communications Wu, Y., Wang, J., Li, Y., Zhou, J., Wang, B. Y., Yang, A., Wang, L., Hwang, H. Y., Cui, Y. 2022; 13 (1): 3008

  Abstract

  Lithium intercalation of MoS2 is generally believed to introduce a phase transition from H phase (semiconducting) to T phase (metallic). However, during the intercalation process, a spatially sharp boundary is usually formed between the fully intercalated T phase MoS2 and non-intercalated H phase MoS2. The intermediate state, i.e., lightly intercalated H phase MoS2 without a phase transition, is difficult to investigate by optical-microscope-based spectroscopy due to the narrow size. Here, we report the stabilization of the intermediate state across the whole flake of twisted bilayer MoS2. The twisted bilayer system allows the lithium to intercalate from the top surface and enables fast Li-ion diffusion by the reduced interlayer interaction. The E2g Raman mode of the intermediate state shows a peak splitting behavior. Our simulation results indicate that the intermediate state is stabilized by lithium-induced symmetry breaking of the H phase MoS2. Our results provide an insight into the non-uniform intercalation during battery charging and discharging, and also open a new opportunity to modulate the properties of twisted 2D systems with guest species doping in the Moire structures.

  View details for DOI 10.1038/s41467-022-30516-z

  View details for PubMedID 35637182

• Disentangling Coexisting Structural Order Through Phase Lock-In Analysis of Atomic-Resolution STEM Data. Microscopy and microanalysis : the official journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada Goodge, B. H., El Baggari, I., Hong, S. S., Wang, Z., Schlom, D. G., Hwang, H. Y., Kourkoutis, L. F. 2022: 1-8

  Abstract

  As a real-space technique, atomic-resolution STEM imaging contains both amplitude and geometric phase information about structural order in materials, with the latter encoding important information about local variations and heterogeneities present in crystalline lattices. Such phase information can be extracted using geometric phase analysis (GPA), a method which has generally focused on spatially mapping elastic strain. Here we demonstrate an alternative phase demodulation technique and its application to reveal complex structural phenomena in correlated quantum materials. As with other methods of image phase analysis, the phase lock-in approach can be implemented to extract detailed information about structural order and disorder, including dislocations and compound defects in crystals. Extending the application of this phase analysis to Fourier components that encode periodic modulations of the crystalline lattice, such as superlattice or secondary frequency peaks, we extract the behavior of multiple distinct order parameters within the same image, yielding insights into not only the crystalline heterogeneity but also subtle emergent order parameters such as antipolar displacements. When applied to atomic-resolution images spanning large (~0.5 * 0.5 mum2) fields of view, this approach enables vivid visualizations of the spatial interplay between various structural orders in novel materials.

  View details for DOI 10.1017/S1431927622000125

  View details for PubMedID 35190012

• Insulator-to-metal crossover near the edge of the superconducting dome in Nd1-xSrxNiO2 PHYSICAL REVIEW RESEARCH Hsu, Y., Wang, B., Berben, M., Li, D., Lee, K., Duffy, C., Ottenbros, T., Kim, W., Osada, M., Wiedmann, S., Hwang, H. Y., Hussey, N. E. 2021; 3 (4)

  View details for DOI 10.1103/PhysRevResearch.3.L042015

  View details for Web of Science ID 000715926700001

• Nickelate Superconductivity without Rare-Earth Magnetism: (La,Sr)NiO2. Advanced materials (Deerfield Beach, Fla.) Osada, M., Wang, B. Y., Goodge, B. H., Harvey, S. P., Lee, K., Li, D., Kourkoutis, L. F., Hwang, H. Y. 2021: e2104083

  Abstract

  The occurrence of unconventional superconductivity in cuprates has long motivated the search for manifestations in other layered transition metal oxides. Recently, superconductivity is found in infinite-layer nickelate (Nd,Sr)NiO2 and (Pr,Sr)NiO2 thin films, formed by topotactic reduction from the perovskite precursor phase. A topic of much current interest is whether rare-earth moments are essential for superconductivity in this system. In this study, it is found that with significant materials optimization, substantial portions of the La1- x Srx NiO2 phase diagram can enter the regime of coherent low-temperature transport (x= 0.14 - 0.20), with subsequent superconducting transitions and a maximum onset of 9 K at x= 0.20. Additionally, the unexpected indication of a superconducting ground state in undoped LaNiO2 is observed, which likely reflects the self-doped nature of the electronic structure. Combining the results of (La/Pr/Nd)1- x Srx NiO2 reveals a generalized superconducting dome, characterized by systematic shifts in the unit cell volume and in the relative electron-hole populations across the lanthanides.

  View details for DOI 10.1002/adma.202104083

  View details for PubMedID 34536042

• Fracture and fatigue of thin crystalline SrTiO3 membranes APPLIED PHYSICS LETTERS Harbola, V., Xu, R., Crossley, S., Singh, P., Hwang, H. Y. 2021; 119 (5)

  View details for DOI 10.1063/5.0060465

  View details for Web of Science ID 000680136900004

• Charge order textures induced by non-linear couplings in a half-doped manganite. Nature communications El Baggari, I., Baek, D. J., Zachman, M. J., Lu, D., Hikita, Y., Hwang, H. Y., Nowadnick, E. A., Kourkoutis, L. F. 2021; 12 (1): 3747

  Abstract

  The self-organization of strongly interacting electrons into superlattice structures underlies the properties of many quantum materials. How these electrons arrange within the superlattice dictates what symmetries are broken and what ground states are stabilized. Here we show that cryogenic scanning transmission electron microscopy (cryo-STEM) enables direct mapping of local symmetries and order at the intra-unit-cell level in the model charge-ordered system Nd1/2Sr1/2MnO3. In addition to imaging the prototypical site-centered charge order, we discover the nanoscale coexistence of an exotic intermediate state which mixes site and bond order and breaks inversion symmetry. We further show that nonlinear coupling of distinct lattice modes controls the selection between competing ground states. The results demonstrate the importance of lattice coupling for understanding and manipulating the character of electronic self-organization and that cryo-STEM can reveal local order in strongly correlated systems at the atomic scale.

  View details for DOI 10.1038/s41467-021-24026-7

  View details for PubMedID 34145244

• Understanding Degradation Mechanisms in SrIrO3 Oxygen Evolution Electrocatalysts: Chemical and Structural Microscopy at the Nanoscale ADVANCED FUNCTIONAL MATERIALS Ben-Naim, M., Liu, Y., Stevens, M., Lee, K., Wette, M. R., Boubnov, A., Trofimov, A. A., Ievlev, A. V., Belianinov, A., Davis, R. C., Clemens, B. M., Bare, S. R., Hikita, Y., Hwang, H. Y., Higgins, D. C., Sinclair, R., Jaramillo, T. F. 2021

  View details for DOI 10.1002/adfm.202101542

  View details for Web of Science ID 000662722100001

• Non-universal current flow near the metal-insulator transition in an oxide interface. Nature communications Persky, E., Vardi, N., Monteiro, A. M., van Thiel, T. C., Yoon, H., Xie, Y., Fauque, B., Caviglia, A. D., Hwang, H. Y., Behnia, K., Ruhman, J., Kalisky, B. 2021; 12 (1): 3311

  Abstract

  In systems near phase transitions, macroscopic properties often follow algebraic scaling laws, determined by the dimensionality and the underlying symmetries of the system. The emergence of such universal scaling implies that microscopic details are irrelevant. Here, we locally investigate the scaling properties of the metal-insulator transition at the LaAlO3/SrTiO3 interface. We show that, by changing the dimensionality and the symmetries of the electronic system, coupling between structural and electronic properties prevents the universal behavior near the transition. By imaging the current flow in the system, we reveal that structural domain boundaries modify the filamentary flow close to the transition point, preventing a fractal with the expected universal dimension from forming.

  View details for DOI 10.1038/s41467-021-23393-5

  View details for PubMedID 34083533

• Stabilization of Sr3Al2O6 Growth Templates for Ex Situ Synthesis of Freestanding Crystalline Oxide Membranes. Nano letters Li, D., Adamo, C., Wang, B. Y., Yoon, H., Chen, Z., Hong, S. S., Lu, D., Cui, Y., Hikita, Y., Hwang, H. Y. 2021

  Abstract

  A new synthetic approach has recently been developed for the fabrication of freestanding crystalline perovskite oxide nanomembranes, which involves the epitaxial growth of a water-soluble sacrificial layer. By utilizing an ultrathin capping layer of SrTiO3, here we show that this sacrificial layer, as grown by pulsed laser deposition, can be stabilized in air and therefore be used as transferrable templates for ex situ epitaxial growth using other techniques. We find that the stability of these templates depends on the thickness of the capping layer. On these templates, freestanding superconducting SrTiO3 membranes were synthesized ex situ using molecular beam epitaxy, enabled by the lower growth temperature which preserves the sacrificial layer. This study paves the way for the synthesis of an expanded selection of freestanding oxide membranes and heterostructures with a wide variety of ex situ growth techniques.

  View details for DOI 10.1021/acs.nanolett.1c01194

  View details for PubMedID 33989008

• Epitaxial Stabilization and Oxygen Evolution Reaction Activity of Metastable Columbite Iridium Oxide ACS APPLIED ENERGY MATERIALS Lee, K., Flores, R. A., Liu, Y., Wang, B., Hikita, Y., Sinclair, R., Bajdich, M., Hwang, H. Y. 2021; 4 (4): 3074-3082

  View details for DOI 10.1021/acsaem.0c02788

  View details for Web of Science ID 000644737800012

• Highly Efficient Surface Charge Transfer in Fe2TiO5 Epitaxial Thin Film Photoanodes ACS APPLIED ENERGY MATERIALS Osada, M., Nishio, K., Lee, K., Colletta, M., Goodge, B. H., Kim, W., Kourkoutis, L. F., Hwang, H. Y., Hikita, Y. 2021; 4 (3): 2098-2106

  View details for DOI 10.1021/acsaem.0c02516

  View details for Web of Science ID 000636714000010

• Electronic Structure Trends Across the Rare-Earth Series in Superconducting Infinite-Layer Nickelates PHYSICAL REVIEW X Been, E., Lee, W., Hwang, H. Y., Cui, Y., Zaanen, J., Devereaux, T., Moritz, B., Jia, C. 2021; 11 (1)

  View details for DOI 10.1103/PhysRevX.11.011050

  View details for Web of Science ID 000627595600002

• Strain Gradient Elasticity in SrTiO3 Membranes: Bending versus Stretching. Nano letters Harbola, V., Crossley, S., Hong, S. S., Lu, D., Birkholzer, Y. A., Hikita, Y., Hwang, H. Y. 2021

  Abstract

  Young's modulus determines the mechanical loads required to elastically stretch a material and also the loads required to bend it, given that bending stretches one surface while compressing the opposite one. Flexoelectric materials have the additional property of becoming electrically polarized when bent. The associated energy cost can additionally contribute to elasticity via strain gradients, particularly at small length scales where they are geometrically enhanced. Here, we present nanomechanical measurements of freely suspended SrTiO3 crystalline membrane drumheads. We observe an unexpected nonmonotonic thickness dependence of Young's modulus upon small deflections. Furthermore, the modulus inferred from a predominantly bending deformation is three times larger than that of a predominantly stretching deformation for membranes thinner than 20 nm. In this regime we extract a strain gradient elastic coupling of 2.2 muN, which could be used in new operational regimes of nanoelectro-mechanics.

  View details for DOI 10.1021/acs.nanolett.0c04787

  View details for PubMedID 33689379

• Universal Bound to the Amplitude of the Vortex Nernst Signal in Superconductors. Physical review letters Rischau, C. W., Li, Y., Fauqué, B., Inoue, H., Kim, M., Bell, C., Hwang, H. Y., Kapitulnik, A., Behnia, K. 2021; 126 (7): 077001

  Abstract

  A liquid of superconducting vortices generates a transverse thermoelectric response. This Nernst signal has a tail deep in the normal state due to superconducting fluctuations. Here, we present a study of the Nernst effect in two-dimensional heterostructures of Nb-doped strontium titanate (STO) and in amorphous MoGe. The Nernst signal generated by ephemeral Cooper pairs above the critical temperature has the magnitude expected by theory in STO. On the other hand, the peak amplitude of the vortex Nernst signal below T_{c} is comparable in both and in numerous other superconductors despite the large distribution of the critical temperature and the critical magnetic fields. In four superconductors belonging to different families, the maximum Nernst signal corresponds to an entropy per vortex per layer of ≈k_{B}ln2.

  View details for DOI 10.1103/PhysRevLett.126.077001

  View details for PubMedID 33666461

• Universal behavior of the bosonic metallic ground state in a two-dimensional superconductor NPJ QUANTUM MATERIALS Chen, Z., Wang, B., Swartz, A. G., Yoon, H., Hikita, Y., Raghu, S., Hwang, H. Y. 2021; 6 (1)

  View details for DOI 10.1038/s41535-021-00312-x

  View details for Web of Science ID 000619217000001

• Universal Bound to the Amplitude of the Vortex Nernst Signal in Superconductors PHYSICAL REVIEW LETTERS Rischau, C., Li, Y., Fauque, B., Inoue, H., Kim, M., Bell, C., Hwang, H. Y., Kapitulnik, A., Behnia, K. 2021; 126 (7)

  View details for DOI 10.1103/PhysRevLett.126.077001

  View details for Web of Science ID 000618650100006

• Doping evolution of the Mott-Hubbard landscape in infinite-layer nickelates. Proceedings of the National Academy of Sciences of the United States of America Goodge, B. H., Li, D., Lee, K., Osada, M., Wang, B. Y., Sawatzky, G. A., Hwang, H. Y., Kourkoutis, L. F. 2021; 118 (2)

  Abstract

  The recent observation of superconductivity in [Formula: see text] has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott-Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally resolved electron energy-loss spectroscopy to directly probe the Mott-Hubbard character of [Formula: see text] Upon doping, we observe emergent hybridization reminiscent of the Zhang-Rice singlet via the oxygen-projected states, modification of the Nd 5d states, and the systematic evolution of Ni 3d hybridization and filling. These experimental data provide direct evidence for the multiband electronic structure of the superconducting infinite-layer nickelates, particularly via the effects of hole doping on not only the oxygen but also nickel and rare-earth bands.

  View details for DOI 10.1073/pnas.2007683118

  View details for PubMedID 33397720

• Isotropic Pauli-limited superconductivity in the infinite-layer nickelate Nd0.775Sr0.225NiO2 NATURE PHYSICS Wang, B., Li, D., Goodge, B. H., Lee, K., Osada, M., Harvey, S. P., Kourkoutis, L. F., Beasley, M. R., Hwang, H. Y. 2021

  View details for DOI 10.1038/s41567-020-01128-5

  View details for Web of Science ID 000604865700004

• Phase diagram of infinite layer praseodymium nickelate Pr1-xSrxNiO2 thin films PHYSICAL REVIEW MATERIALS Osada, M., Wang, B., Lee, K., Li, D., Hwang, H. Y. 2020; 4 (12)

  View details for DOI 10.1103/PhysRevMaterials.4.121801

  View details for Web of Science ID 000600662300001

• Beyond Substrates: Strain Engineering of Ferroelectric Membranes. Advanced materials (Deerfield Beach, Fla.) Pesquera, D., Parsonnet, E., Qualls, A., Xu, R., Gubser, A. J., Kim, J., Jiang, Y., Velarde, G., Huang, Y., Hwang, H. Y., Ramesh, R., Martin, L. W. 2020: e2003780

  Abstract

  Strain engineering in perovskite oxides provides for dramatic control over material structure, phase, and properties, but is restricted by the discrete strain states produced by available high-quality substrates. Here, using the ferroelectric BaTiO3 , production of precisely strain-engineered, substrate-released nanoscale membranes is demonstrated via an epitaxial lift-off process that allows the high crystalline quality of films grown on substrates to be replicated. In turn, fine structural tuning is achieved using interlayer stress in symmetric trilayer oxide-metal/ferroelectric/oxide-metal structures fabricated from the released membranes. In devices integrated on silicon, the interlayer stress provides deterministic control of ordering temperature (from 75to 425°C) and releasing the substrate clamping is shown to dramatically impact ferroelectric switching and domain dynamics (including reducing coercive fields to <10kV cm-1 and improving switching times to <5ns for a 20m diameter capacitor in a 100-nm-thick film). In devices integrated on flexible polymers, enhanced room-temperature dielectric permittivity with large mechanical tunability (a 90% change upon ±0.1%strain application) is demonstrated. This approach paves the way toward the fabrication of ultrafast CMOS-compatible ferroelectric memories and ultrasensitive flexible nanosensor devices, and it may also be leveraged for the stabilization of novel phases and functionalities not achievable via direct epitaxial growth.

  View details for DOI 10.1002/adma.202003780

  View details for PubMedID 32964567

• Superconducting Dome in Nd_{1-x}Sr_{x}NiO_{2} Infinite Layer Films. Physical review letters Li, D., Wang, B. Y., Lee, K., Harvey, S. P., Osada, M., Goodge, B. H., Kourkoutis, L. F., Hwang, H. Y. 2020; 125 (2): 027001

  Abstract

  We report the phase diagram of Nd_{1-x}Sr_{x}NiO_{2} infinite layer thin films grown on SrTiO_{3}. A superconducting dome spanning 0.125

  View details for DOI 10.1103/PhysRevLett.125.027001

  View details for PubMedID 32701320

• Superconducting Dome in Nd1-xSrxNiO2 Infinite Layer Films PHYSICAL REVIEW LETTERS Li, D., Wang, B., Lee, K., Harvey, S. P., Osada, M., Goodge, B. H., Kourkoutis, L. F., Hwang, H. Y. 2020; 125 (2)

  View details for DOI 10.1103/PhysRevLett.125.027001

  View details for Web of Science ID 000545870800006

• A Superconducting Praseodymium Nickelate with Infinite Layer Structure. Nano letters Osada, M., Wang, B. Y., Goodge, B. H., Lee, K., Yoon, H., Sakuma, K., Li, D., Miura, M., Kourkoutis, L. F., Hwang, H. Y. 2020

  Abstract

  A variety of nickel oxide compounds have long been studied for their manifestation of various correlated electron phenomena. Recently, superconductivity was observed in nanoscale infinite layer nickelate thin films of Nd0.8Sr0.2NiO2, epitaxially stabilized on SrTiO3 substrates via topotactic reduction from the perovskite precursor phase. Here, we present the synthesis and properties of PrNiO2 thin films on SrTiO3. Upon doping in Pr0.8Sr0.2NiO2, we observe superconductivity with a transition temperature of 7-12 K and robust critical current density at 2 K of 334 kA/cm2. These findings indicate that superconductivity in the infinite layer nickelates is relatively insensitive to the details of the rare earth 4f configuration. Furthermore, they motivate the exploration of a broader family of compounds based on two-dimensional NiO2 planes, which will enable systematic investigation of the superconducting and normal state properties and their underlying mechanisms.

  View details for DOI 10.1021/acs.nanolett.0c01392

  View details for PubMedID 32574061

• Aspects of the synthesis of thin film superconducting infinite-layer nickelates APL MATERIALS Lee, K., Goodge, B. H., Li, D., Osada, M., Wang, B., Cui, Y., Kourkoutis, L. F., Hwang, H. Y. 2020; 8 (4)

  View details for DOI 10.1063/5.0005103

  View details for Web of Science ID 000526748400002

• Robust dx2-y2-wave superconductivity of infinite-layer nickelates PHYSICAL REVIEW B Wu, X., Di Sante, D., Schwemmer, T., Hanke, W., Hwang, H. Y., Raghu, S., Thomale, R. 2020; 101 (6)

  View details for DOI 10.1103/PhysRevB.101.060504

  View details for Web of Science ID 000515626800002

• Electrochemical generation of liquid and solid sulfur on two-dimensional layered materials with distinct areal capacities. Nature nanotechnology Yang, A. n., Zhou, G. n., Kong, X. n., Vilá, R. A., Pei, A. n., Wu, Y. n., Yu, X. n., Zheng, X. n., Wu, C. L., Liu, B. n., Chen, H. n., Xu, Y. n., Chen, D. n., Li, Y. n., Fakra, S. n., Hwang, H. Y., Qin, J. n., Chu, S. n., Cui, Y. n. 2020

  Abstract

  It has recently been shown that sulfur, a solid material in its elementary form S8, can stay in a supercooled state as liquid sulfur in an electrochemical cell. We establish that this newly discovered state could have implications for lithium-sulfur batteries. Here, through in situ studies of electrochemical sulfur generation, we show that liquid (supercooled) and solid elementary sulfur possess very different areal capacities over the same charging period. To control the physical state of sulfur, we studied its growth on two-dimensional layered materials. We found that on the basal plane, only liquid sulfur accumulates; by contrast, at the edge sites, liquid sulfur accumulates if the thickness of the two-dimensional material is small, whereas solid sulfur nucleates if the thickness is large (tens of nanometres). Correlating the sulfur states with their respective areal capacities, as well as controlling the growth of sulfur on two-dimensional materials, could provide insights for the design of future lithium-sulfur batteries.

  View details for DOI 10.1038/s41565-019-0624-6

  View details for PubMedID 31988508

• Extreme tensile strain states in La0.7Ca0.3MnO3 membranes. Science (New York, N.Y.) Hong, S. S., Gu, M. n., Verma, M. n., Harbola, V. n., Wang, B. Y., Lu, D. n., Vailionis, A. n., Hikita, Y. n., Pentcheva, R. n., Rondinelli, J. M., Hwang, H. Y. 2020; 368 (6486): 71–76

  Abstract

  A defining feature of emergent phenomena in complex oxides is the competition and cooperation between ground states. In manganites, the balance between metallic and insulating phases can be tuned by the lattice; extending the range of lattice control would enhance the ability to access other phases. We stabilized uniform extreme tensile strain in nanoscale La0.7Ca0.3MnO3 membranes, exceeding 8% uniaxially and 5% biaxially. Uniaxial and biaxial strain suppresses the ferromagnetic metal at distinctly different strain values, inducing an insulator that can be extinguished by a magnetic field. Electronic structure calculations indicate that the insulator consists of charge-ordered Mn4+ and Mn3+ with staggered strain-enhanced Jahn-Teller distortions within the plane. This highly tunable strained membrane approach provides a broad opportunity to design and manipulate correlated electron states.

  View details for DOI 10.1126/science.aax9753

  View details for PubMedID 32241944

• Strain-induced room-temperature ferroelectricity in SrTiO3 membranes. Nature communications Xu, R. n., Huang, J. n., Barnard, E. S., Hong, S. S., Singh, P. n., Wong, E. K., Jansen, T. n., Harbola, V. n., Xiao, J. n., Wang, B. Y., Crossley, S. n., Lu, D. n., Liu, S. n., Hwang, H. Y. 2020; 11 (1): 3141

  Abstract

  Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO3 by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO3 with 2.0% uniaxial tensile strain, corroborated by the notable features of 180° ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.

  View details for DOI 10.1038/s41467-020-16912-3

  View details for PubMedID 32561835

• Electrochemical generation of liquid and solid sulfur on two-dimensional layered materials with distinct areal capacities Nature Nanotechnology Yang, A., Zhou, G., et al 2020

  View details for DOI 10.1038/s41565-019-0624-6

• Magnetism and Conductivity Along Structural Domain Walls of SrTiO3 JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM Frenkel, Y., Xie, Y., Hwang, H. Y., Kalisky, B. 2020; 33 (1): 195–97

  View details for DOI 10.1007/s10948-019-05301-7

  View details for Web of Science ID 000512752600001

• Magnetism and Conductivity Along Structural Domain Walls of SrTiO3 (vol 71, pg 451, 2019) JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM Frenkel, Y., Xie, Y., Hwang, H. Y., Kalisky, B. 2020; 33 (1): 199

  View details for DOI 10.1007/s10948-019-05413-0

  View details for Web of Science ID 000512752600002

• Electrotunable liquid sulfur microdroplets. Nature communications Zhou, G. n., Yang, A. n., Wang, Y. n., Gao, G. n., Pei, A. n., Yu, X. n., Zhu, Y. n., Zong, L. n., Liu, B. n., Xu, J. n., Liu, N. n., Zhang, J. n., Li, Y. n., Wang, L. W., Hwang, H. Y., Brongersma, M. L., Chu, S. n., Cui, Y. n. 2020; 11 (1): 606

  Abstract

  Manipulating liquids with tunable shape and optical functionalities in real time is important for electroactive flow devices and optoelectronic devices, but remains a great challenge. Here, we demonstrate electrotunable liquid sulfur microdroplets in an electrochemical cell. We observe electrowetting and merging of sulfur droplets under different potentiostatic conditions, and successfully control these processes via selective design of sulfiphilic/sulfiphobic substrates. Moreover, we employ the electrowetting phenomena to create a microlens based on the liquid sulfur microdroplets and tune its characteristics in real time through changing the shape of the liquid microdroplets in a fast, repeatable, and controlled manner. These studies demonstrate a powerful in situ optical battery platform for unraveling the complex reaction mechanism of sulfur chemistries and for exploring the rich material properties of the liquid sulfur, which shed light on the applications of liquid sulfur droplets in devices such as microlenses, and potentially other electrotunable and optoelectronic devices.

  View details for DOI 10.1038/s41467-020-14438-2

  View details for PubMedID 32001696

• Heteroepitaxial vertical perovskite hot-electron transistors down to the monolayer limit. Nature communications Kim, B. S., Hikita, Y., Yajima, T., Hwang, H. Y. 2019; 10 (1): 5312

  Abstract

  Two-dimensional heterostructures combined with vertical geometries are candidates to probe and utilize the physical properties of atomically-thin materials. The vertical configuration enables a unique form of hot-carrier spectroscopy as well as atomic-scale devices. Here, we present the room-temperature evolution of heteroepitaxial perovskite hot-electron transistors using a SrRuO3 base down to the monolayer limit (4A). As a fundamental electronic probe, we observe an abrupt transition in the hot-electron mean free path as a function of base thickness, coinciding with the thickness-dependent resistive transition. As a path towards devices, we demonstrate the integrated synthesis of perovskite one-dimensional electrical edge contacts using water-soluble and growth-compatibleSr3Al2O6 hard masks. Edge-contacted monolayer-base transistors exhibit on/off ratios reaching 108, complete electrostatic screening by the base manifesting pure hot-electron injection, and excellent scaling of the output current density with device dimensions. These results open new avenues for incorporating emergent phenomena at oxide interfaces and in heterostructures.

  View details for DOI 10.1038/s41467-019-13290-3

  View details for PubMedID 31757949

• Superconductivity in an infinite-layer nickelate. Nature Li, D., Lee, K., Wang, B. Y., Osada, M., Crossley, S., Lee, H. R., Cui, Y., Hikita, Y., Hwang, H. Y. 2019; 572 (7771): 624–27

  Abstract

  The discovery of unconventional superconductivity in (La,Ba)2CuO4 (ref. 1) has motivated the study of compounds with similar crystal and electronic structure, with the aim of finding additional superconductors and understanding the origins of copper oxide superconductivity. Isostructural examples include bulk superconducting Sr2RuO4 (ref. 2) and surface-electron-doped Sr2IrO4, which exhibits spectroscopic signatures consistent with a superconducting gap3,4, although a zero-resistance state has not yet been observed. This approach has also led to the theoretical investigation of nickelates5,6, as well as thin-film heterostructures designed to host superconductivity. One such structure is the LaAlO3/LaNiO3 superlattice7-9, which has been recently proposed for the creation of an artificially layered nickelate heterostructure with a singly occupied [Formula: see text] band. The absence of superconductivity observed in previous related experiments has been attributed, at least in part, to incomplete polarization of the eg orbitals10. Here we report the observation of superconductivity in an infinite-layer nickelate that is isostructural to infinite-layer copper oxides11-13. Using soft-chemistry topotactic reduction14-20, NdNiO2 and Nd0.8Sr0.2NiO2 single-crystal thin films are synthesized by reducing the perovskite precursor phase. Whereas NdNiO2 exhibits a resistive upturn at low temperature, measurements of the resistivity, critical current density and magnetic-field response of Nd0.8Sr0.2NiO2 indicate a superconducting transition temperature of about 9 to 15 kelvin. Because this compound is a member of a series of reduced layered nickelate crystal structures21-23, these results suggest the possibility of a family of nickelate superconductors analogous to copper oxides24 and pnictides25.

  View details for DOI 10.1038/s41586-019-1496-5

  View details for PubMedID 31462797

• Large-Area Crystalline BaSnO3 Membranes with High Electron Mobilities ACS APPLIED ELECTRONIC MATERIALS Singh, P., Swartz, A., Lu, D., Hon, S., Lee, K., Marshall, A. F., Nishio, K., Hikita, Y., Hwang, H. Y. 2019; 1 (7): 1269–74

  View details for DOI 10.1021/acsaelm.9b00215

  View details for Web of Science ID 000496314700027

• Freestanding crystalline YBa2Cu3O7-x heterostructure membranes PHYSICAL REVIEW MATERIALS Chen, Z., Wang, B., Goodge, B. H., Lu, D., Hong, S., Li, D., Kourkoutis, L. F., Hikita, Y., Hwang, H. Y. 2019; 3 (6)

  View details for DOI 10.1103/PhysRevMaterials.3.060801

  View details for Web of Science ID 000473315300001

• Delta-doped SrTiO3 top-gated field effect transistor APPLIED PHYSICS LETTERS Inoue, H., Yoon, H., Merz, T. A., Swartz, A. G., Hong, S., Hikita, Y., Hwang, H. Y. 2019; 114 (23)

  View details for DOI 10.1063/1.5090269

  View details for Web of Science ID 000471696100006

• Oxygen Evolution Reaction Activity in IrOx/SrIrO3 Catalysts: Correlations between Structural Parameters and the Catalytic Activity JOURNAL OF PHYSICAL CHEMISTRY LETTERS Lee, K., Osada, M., Hwang, H. Y., Hikita, Y. 2019; 10 (7): 1516-1522

  View details for DOI 10.1021/acs.jpclett.9b00173

  View details for Web of Science ID 000463960700014

• Oxygen Evolution Reaction Activity in IrO x/SrIrO3 Catalysts: Correlations between Structural Parameters and the Catalytic Activity. The journal of physical chemistry letters Lee, K., Osada, M., Hwang, H. Y., Hikita, Y. 2019: 1516–22

  Abstract

  Understanding how structural properties affect the oxygen evolution reaction (OER) of a catalyst can reveal important information not only on the catalytic mechanism but also on the general design strategy of OER catalysts. We report a variation of 0.15 V in the overpotential of the recently discovered IrO x/SrIrO3 OER catalysts, which directly correlates with the structural parameters of the as-synthesized SrIrO3 epitaxial films. This variation is caused by both extrinsic area enhancement and intrinsic electronic structure modification driven by defect formation. These correlations not only indicate that microscopic film defects play an important role in the activity of the IrO x/SrIrO3 catalyst but also provide readily accessible parameters predictive of the activity post-transformation to IrO x/SrIrO3. Establishing strong associations between the catalytic activity and key structural and electronic parameters, rather than synthetic variables, provides important guidance to control and study these complex catalysts independent of the synthetic technique.

  View details for PubMedID 30883127

• A termination-insensitive and robust electron gas at the heterointerface of two complex oxides. Nature communications Zhang, M. n., Du, K. n., Ren, T. n., Tian, H. n., Zhang, Z. n., Hwang, H. Y., Xie, Y. n. 2019; 10 (1): 4026

  Abstract

  The single-crystal SrTiO3 (001) has two different surface terminations, TiO2 and SrO. One most remarkable observation in previous studies is that only the heterointerfaces with TiO2-terminated SrTiO3, which usually combines with polar oxides such as LaAlO3, host an electron gas. Here we show that a robust electron gas can be generated between a non-polar oxide, CaHfO3, and SrTiO3 (001) with either termination. Unlike the well-known electron gas of LaAlO3/SrTiO3, the present one of CaHfO3/SrTiO3 essentially has no critical thickness of CaHfO3, can survive a long-time oxygen annealing at high temperature, and its transport properties are stable under exposure to water and other polar solvents. By electrostatic gating through CaHfO3, field-effect devices are demonstrated using CaHfO3/SrTiO3 heterointerfaces with both terminations. These results show that the electron gas reported in the present study is unique and promising for applications in oxide electronics.

  View details for DOI 10.1038/s41467-019-12036-5

  View details for PubMedID 31492862

• A Two-Dimensional MoS2 Catalysis Transistor by Solid-State Ion Gating Manipulation and Adjustment (SIGMA). Nano letters Wu, Y. n., Ringe, S. n., Wu, C. L., Chen, W. n., Yang, A. n., Chen, H. n., Tang, M. n., Zhou, G. n., Hwang, H. Y., Chan, K. n., Cui, Y. n. 2019

  Abstract

  A variety of methods including tuning chemical compositions, structures, crystallinity, defects and strain, and electrochemical intercalation have been demonstrated to enhance the catalytic activity. However, none of these tuning methods provide direct dynamical control during catalytic reactions. Here we propose a new method to tune the activity of catalysts through solid-state ion gating manipulation and adjustment (SIGMA) using a catalysis transistor. SIGMA can electrostatically dope the surface of catalysts with a high electron concentration over 5 × 1013 cm-2 and thus modulate both the chemical potential of the reaction intermediates and their electrical conductivity. The hydrogen evolution reaction (HER) on both pristine and defective MoS2 were investigated as model reactions. Our theoretical and experimental results show that the overpotential at 10 mA/cm2 and Tafel slope can be in situ, continuously, dynamically, and reversibly tuned over 100 mV and around 100 mV/dec, respectively.

  View details for DOI 10.1021/acs.nanolett.9b02888

  View details for PubMedID 31499003

• Freestanding Oxide Ferroelectric Tunnel Junction Memories Transferred onto Silicon. Nano letters Lu, D. n., Crossley, S. n., Xu, R. n., Hikita, Y. n., Hwang, H. Y. 2019

  Abstract

  Crystalline oxide ferroelectric tunnel junctions enable persistent encoding of information in electric polarization, featuring nondestructive readout and scalability that can exceed current commercial high-speed, nonvolatile ferroelectric memories. However, the well-established fabrication of epitaxial devices on oxide substrates is difficult to adapt to silicon substrates for integration into complementary metal-oxide-semiconductor electronics. In this work, we report ferroelectric tunnel junctions based on 2.8 nm-thick BaTiO3 films grown epitaxially on SrTiO3 growth substrates, released, and relaminated onto silicon. The performance of the transferred devices is comparable to devices characterized on the oxide substrate, suggesting a viable route toward next-generation nonvolatile memories broadly integrable with different materials platforms.

  View details for DOI 10.1021/acs.nanolett.9b01327

  View details for PubMedID 31136184

• Publisher Correction: Carrier density and disorder tuned superconductor-metal transition in a two-dimensional electron system. Nature communications Chen, Z., Swartz, A. G., Yoon, H., Inoue, H., Merz, T. A., Lu, D., Xie, Y., Yuan, H., Hikita, Y., Raghu, S., Hwang, H. Y. 2018; 9 (1): 4570

  Abstract

  The original HTML version of this Article omitted to list Harold Y. Hwang as a corresponding author and incorrectly listed Adrian G. Swartz as a corresponding author. This has been corrected in the HTML version of the Article. The PDF version was correct from the time of publication.

  View details for PubMedID 30374115

• Carrier density and disorder tuned superconductor-metal transition in a two-dimensional electron system (vol 9, 4008, 2018) NATURE COMMUNICATIONS Chen, Z., Swartz, A. G., Yoon, H., Inoue, H., Merz, T. A., Lu, D., Xie, Y., Yuan, H., Hikita, Y., Raghu, S., Hwang, H. Y. 2018; 9

  View details for DOI 10.1038/s41467-018-06960-1

  View details for Web of Science ID 000448576200001

• Superconducting Tunneling Spectroscopy of Spin-Orbit Coupling and Orbital Depairing in Nb :SrTiO3 PHYSICAL REVIEW LETTERS Swartz, A. G., Cheung, A. C., Yoon, H., Chen, Z., Hikita, Y., Raghu, S., Hwang, H. Y. 2018; 121 (16): 167003

  Abstract

  We have examined the intrinsic spin-orbit coupling and orbital depairing in thin films of Nb-doped SrTiO_{3} by superconducting tunneling spectroscopy. The orbital depairing is geometrically suppressed in the two-dimensional limit, enabling a quantitative evaluation of the Fermi level spin-orbit scattering using Maki's theory. The response of the superconducting gap under in-plane magnetic fields demonstrates short spin-orbit scattering times τ_{so}≤1.1  ps. Analysis of the orbital depairing indicates that the heavy electron band contributes significantly to pairing. These results suggest that the intrinsic spin-orbit scattering time in SrTiO_{3} is comparable to those associated with Rashba effects in SrTiO_{3} interfacial conducting layers and can be considered significant in all forms of superconductivity in SrTiO_{3}.

  View details for PubMedID 30387624

• Carrier density and disorder tuned superconductor-metal transition in a two-dimensional electron system NATURE COMMUNICATIONS Chen, Z., Swartz, A. G., Yoon, H., Inoue, H., Merz, T. A., Lu, D., Xie, Y., Yuan, H., Hikita, Y., Raghu, S., Hwang, H. Y. 2018; 9

  View details for DOI 10.1038/s41467-018-06444-2

  View details for Web of Science ID 000446113000010

• Measurement of elastoresistivity at finite frequency by amplitude demodulation REVIEW OF SCIENTIFIC INSTRUMENTS Hristov, A. T., Palmstrom, J. C., Straquadine, J. W., Merz, T. A., Hwang, H. Y., Fisher, I. R. 2018; 89 (10)

  View details for DOI 10.1063/1.5031136

  View details for Web of Science ID 000449144500294

• Measurement of elastoresistivity at finite frequency by amplitude demodulation. The Review of scientific instruments Hristov, A. T., Palmstrom, J. C., Straquadine, J. A., Merz, T. A., Hwang, H. Y., Fisher, I. R. 2018; 89 (10): 103901

  Abstract

  Elastoresistivity, the relation between resistivity and strain, can elucidate the subtle properties of the electronic structure of a material and is an increasingly important tool for the study of strongly correlated materials. To date, elastoresistivity measurements have predominantly been performed with quasi-static (DC) strain. In this work, we demonstrate a method using AC strain in elastoresistivity measurements. A sample experiencing AC strain has a time-dependent resistivity, which modulates the voltage produced by an AC current; this effect produces time-dependent variations in resistivity that are directly proportional to the elastoresistivity, and which can be measured more quickly, with less strain on the sample, and with less stringent requirements for temperature stability than the previous DC technique. Example measurements between 10 Hz and 3 kHz are performed on a material with a large, well-characterized and temperature dependent elastoresistivity: the representative iron-based superconductor Ba(Fe0.975Co0.025)2As2. These measurements yield a frequency independent elastoresistivity and reproduce results from previous DC elastoresistivity methods to within experimental accuracy. We emphasize that the dynamic (AC) elastoresistivity is a distinct material-specific property that has not previously been considered.

  View details for PubMedID 30399873

• Carrier density and disorder tuned superconductor-metal transition in a two-dimensional electron system. Nature communications Chen, Z., Swartz, A. G., Yoon, H., Inoue, H., Merz, T. A., Lu, D., Xie, Y., Yuan, H., Hikita, Y., Raghu, S., Hwang, H. Y. 2018; 9 (1): 4008

  Abstract

  Quantum ground states that arise at atomically controlled oxide interfaces provide an opportunity to address key questions in condensed matter physics, including the nature of two-dimensional metallic behaviour often observed adjacent to superconductivity. At the superconducting LaAlO3/SrTiO3 interface, a metallic ground state emerges upon the collapse of superconductivity with field-effect gating and is accompanied with a pseudogap. Here we utilize independent control of carrier density and disorder of the interfacial superconductor using dual electrostatic gates, which enables the comprehensive examination of the electronic phase diagram approaching zero temperature. We find that the pseudogap corresponds to precursor pairing, and the onset of long-range phase coherence forms a two-dimensional superconducting dome as a function of the dual-gate voltages. The gate-tuned superconductor-metal transitions are driven by macroscopic phase fluctuations of Josephson coupled superconducting puddles.

  View details for PubMedID 30275443

• Observation of signatures of subresolution defects in two-dimensional superconductors with a scanning SQUID PHYSICAL REVIEW B Noad, H., Watson, C. A., Inoue, H., Kim, M., Sato, H. K., Bell, C., Hwang, H. Y., Kirtley, J. R., Moler, K. A. 2018; 98 (6)

  View details for DOI 10.1103/PhysRevB.98.064510

  View details for Web of Science ID 000450499400004

• Ultralow Damping in Nanometer-Thick Epitaxial Spinel Ferrite Thin Films NANO LETTERS Emori, S., Yi, D., Crossley, S., Wisser, J. J., Balakrishnan, P. P., Khodadadi, B., Shafer, P., Klewe, C., N'Diaye, A. T., Urwin, B. T., Mahalingam, K., Howe, B. M., Hwang, H. Y., Arenholz, E., Suzuki, Y. 2018; 18 (7): 4273–78

  Abstract

  Pure spin currents, unaccompanied by dissipative charge flow, are essential for realizing energy-efficient nanomagnetic information and communications devices. Thin-film magnetic insulators have been identified as promising materials for spin-current technology because they are thought to exhibit lower damping compared with their metallic counterparts. However, insulating behavior is not a sufficient requirement for low damping, as evidenced by the very limited options for low-damping insulators. Here, we demonstrate a new class of nanometer-thick ultralow-damping insulating thin films based on design criteria that minimize orbital angular momentum and structural disorder. Specifically, we show ultralow damping in <20 nm thick spinel-structure magnesium aluminum ferrite (MAFO), in which magnetization arises from Fe3+ ions with zero orbital angular momentum. These epitaxial MAFO thin films exhibit a Gilbert damping parameter of ∼0.0015 and negligible inhomogeneous linewidth broadening, resulting in narrow half width at half-maximum linewidths of ∼0.6 mT around 10 GHz. Our findings offer an attractive thin-film platform for enabling integrated insulating spintronics.

  View details for DOI 10.1021/acs.nanolett.8b01261

  View details for Web of Science ID 000439008300031

  View details for PubMedID 29792812

• Spontaneous Ionic Polarization in Ammonia-Based Ionic Liquid ACS APPLIED ENERGY MATERIALS Kim, K., Yuan, H., Jang, H., Kim, B., Seoung, D., Hikita, Y., Hwang, H. Y., Lee, J. 2018; 1 (6): 2717–20

  View details for DOI 10.1021/acsaem.8b00383

  View details for Web of Science ID 000458705800043

• Synthesis and electronic properties of Fe2TiO5 epitaxial thin films APL MATERIALS Osada, M., Nishio, K., Hwang, H. Y., Hikita, Y. 2018; 6 (5)

  View details for DOI 10.1063/1.5025569

  View details for Web of Science ID 000433944800001

• Tuning of Plasmons in Transparent Conductive Oxides by Carrier Accumulation ACS PHOTONICS Liu, X., Kang, J., Yuan, H., Park, J., Cui, Y., Hwang, H. Y., Brongersma, M. L. 2018; 5 (4): 1493–98

  View details for DOI 10.1021/acsphotonics.7b01517

  View details for Web of Science ID 000430642500044

• Gate-Induced Metal-Insulator Transition in MoS2 by Solid Superionic Conductor LaF3 NANO LETTERS Wu, C., Yuan, H., Li, Y., Gong, Y., Hwang, H. Y., Cui, Y. 2018; 18 (4): 2387–92

  Abstract

  Electric-double-layer (EDL) gating with liquid electrolyte has been a powerful tool widely used to explore emerging interfacial electronic phenomena. Due to the large EDL capacitance, a high carrier density up to 1014 cm-2 can be induced, directly leading to the realization of field-induced insulator to metal (or superconductor) transition. However, the liquid nature of the electrolyte has created technical issues including possible side electrochemical reactions or intercalation, and the potential for huge strain at the interface during cooling. In addition, the liquid coverage of active devices also makes many surface characterizations and in situ measurements challenging. Here, we demonstrate an all solid-state EDL device based on a solid superionic conductor LaF3, which can be used as both a substrate and a fluorine ionic gate dielectric to achieve a wide tunability of carrier density without the issues of strain or electrochemical reactions and can expose the active device surface for external access. Based on LaF3 EDL transistors (EDLTs), we observe the metal-insulator transition in MoS2. Interestingly, the well-defined crystal lattice provides a more uniform potential distribution in the substrate, resulting in less interface electron scattering and therefore a higher mobility in MoS2 transistors. This result shows the powerful gating capability of LaF3 solid electrolyte for new possibilities of novel interfacial electronic phenomena.

  View details for PubMedID 29580055

• Atomically engineered epitaxial anatase TiO2 metal-semiconductor field-effect transistors APPLIED PHYSICS LETTERS Kim, B. Y., Minohara, M., Hikita, Y., Bell, C., Hwang, H. Y. 2018; 112 (13)

  View details for DOI 10.1063/1.5024418

  View details for Web of Science ID 000429072800034

• Strain Tuning in Complex Oxide Epitaxial Films Using an Ultrathin Strontium Aluminate Buffer Layer PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS Lu, D., Hikita, Y., Baek, D. J., Merz, T. A., Sato, H., Kim, B., Yajima, T., Bell, C., Vailionis, A., Kourkoutis, L. F., Hwang, H. Y. 2018; 12 (3)

  View details for DOI 10.1002/pssr.201700339

  View details for Web of Science ID 000426745100002

• Polaronic behavior in a weak-coupling superconductor PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Swartz, A. G., Inoue, H., Merz, T. A., Hikita, Y., Raghu, S., Devereaux, T. P., Johnston, S., Hwang, H. Y. 2018; 115 (7): 1475–80

  Abstract

  The nature of superconductivity in the dilute semiconductor SrTiO3 has remained an open question for more than 50 y. The extremely low carrier densities ([Formula: see text]-[Formula: see text] cm-3) at which superconductivity occurs suggest an unconventional origin of superconductivity outside of the adiabatic limit on which the Bardeen-Cooper-Schrieffer (BCS) and Migdal-Eliashberg (ME) theories are based. We take advantage of a newly developed method for engineering band alignments at oxide interfaces and access the electronic structure of Nb-doped SrTiO3, using high-resolution tunneling spectroscopy. We observe strong coupling to the highest-energy longitudinal optic (LO) phonon branch and estimate the doping evolution of the dimensionless electron-phonon interaction strength ([Formula: see text]). Upon cooling below the superconducting transition temperature ([Formula: see text]), we observe a single superconducting gap corresponding to the weak-coupling limit of BCS theory, indicating an order of magnitude smaller coupling ([Formula: see text]). These results suggest that despite the strong normal state interaction with electrons, the highest LO phonon does not provide a dominant contribution to pairing. They further demonstrate that SrTiO3 is an ideal system to probe superconductivity over a wide range of carrier density, adiabatic parameter, and electron-phonon coupling strength.

  View details for PubMedID 29382769

• Gate-Induced Interfacial Superconductivity in 1T-SnSe2 NANO LETTERS Zeng, J., Liu, E., Fu, Y., Chen, Z., Pan, C., Wang, C., Wang, M., Wang, Y., Xu, K., Cai, S., Yan, X., Wang, Y., Liu, X., Wang, P., Liang, S., Cui, Y., Hwang, H. Y., Yuan, H., Miao, F. 2018; 18 (2): 1410–15

  Abstract

  Layered metal chalcogenide materials provide a versatile platform to investigate emergent phenomena and two-dimensional (2D) superconductivity at/near the atomically thin limit. In particular, gate-induced interfacial superconductivity realized by the use of an electric-double-layer transistor (EDLT) has greatly extended the capability to electrically induce superconductivity in oxides, nitrides, and transition metal chalcogenides and enable one to explore new physics, such as the Ising pairing mechanism. Exploiting gate-induced superconductivity in various materials can provide us with additional platforms to understand emergent interfacial superconductivity. Here, we report the discovery of gate-induced 2D superconductivity in layered 1T-SnSe2, a typical member of the main-group metal dichalcogenide (MDC) family, using an EDLT gating geometry. A superconducting transition temperature Tc ≈ 3.9 K was demonstrated at the EDL interface. The 2D nature of the superconductivity therein was further confirmed based on (1) a 2D Tinkham description of the angle-dependent upper critical field Bc2, (2) the existence of a quantum creep state as well as a large ratio of the coherence length to the thickness of superconductivity. Interestingly, the in-plane Bc2 approaching zero temperature was found to be 2-3 times higher than the Pauli limit, which might be related to an electric field-modulated spin-orbit interaction. Such results provide a new perspective to expand the material matrix available for gate-induced 2D superconductivity and the fundamental understanding of interfacial superconductivity.

  View details for PubMedID 29385803

• Imaging and tuning polarity at SrTiO3 domain walls NATURE MATERIALS Frenkel, Y., Haham, N., Shperber, Y., Bell, C., Xie, Y., Chen, Z., Hikita, Y., Hwang, H. Y., Salje, E. H., Kalisky, B. 2017; 16 (12): 1203-+

  Abstract

  Electrostatic fields tune the ground state of interfaces between complex oxide materials. Electronic properties, such as conductivity and superconductivity, can be tuned and then used to create and control circuit elements and gate-defined devices. Here we show that naturally occurring twin boundaries, with properties that are different from their surrounding bulk, can tune the LaAlO3/SrTiO3 interface 2DEG at the nanoscale. In particular, SrTiO3 domain boundaries have the unusual distinction of remaining highly mobile down to low temperatures, and were recently suggested to be polar. Here we apply localized pressure to an individual SrTiO3 twin boundary and detect a change in LaAlO3/SrTiO3 interface current distribution. Our data directly confirm the existence of polarity at the twin boundaries, and demonstrate that they can serve as effective tunable gates. As the location of SrTiO3 domain walls can be controlled using external field stimuli, our findings suggest a novel approach to manipulate SrTiO3-based devices on the nanoscale.

  View details for PubMedID 28920939

• Two-dimensional limit of crystalline order in perovskite membrane films SCIENCE ADVANCES Hong, S., Yu, J., Lu, D., Marshall, A. F., Hikita, Y., Cui, Y., Hwang, H. Y. 2017; 3 (11)

  View details for DOI 10.1126/sciadv.aao5173

  View details for Web of Science ID 000418002000057

• Gated tuned superconductivity and phonon softening in monolayer and bilayer MoS2 NPJ QUANTUM MATERIALS Fu, Y., Liu, E., Yuan, H., Tang, P., Lian, B., Xu, G., Zeng, J., Chen, Z., Wang, Y., Zhou, W., Xu, K., Gao, A., Pan, C., Wang, M., Wang, B., Zhang, S., Cui, Y., Hwang, H. Y., Miao, F. 2017; 2

  View details for DOI 10.1038/s41535-017-0056-1

  View details for Web of Science ID 000410773600001

• Electrical tuning of a quantum plasmonic resonance NATURE NANOTECHNOLOGY Liu, X., Kang, H., Yuan, H., Park, J., Kim, S., Cui, Y., Hwang, H. Y., Brongersma, M. L. 2017; 12 (9): 866-+

  Abstract

  Surface plasmon (SP) excitations in metals facilitate confinement of light into deep-subwavelength volumes and can induce strong light-matter interaction. Generally, the SP resonances supported by noble metal nanostructures are explained well by classical models, at least until the nanostructure size is decreased to a few nanometres, approaching the Fermi wavelength λF of the electrons. Although there is a long history of reports on quantum size effects in the plasmonic response of nanometre-sized metal particles, systematic experimental studies have been hindered by inhomogeneous broadening in ensemble measurements, as well as imperfect control over size, shape, faceting, surface reconstructions, contamination, charging effects and surface roughness in single-particle measurements. In particular, observation of the quantum size effect in metallic films and its tuning with thickness has been challenging as they only confine carriers in one direction. Here, we show active tuning of quantum size effects in SP resonances supported by a 20-nm-thick metallic film of indium tin oxide (ITO), a plasmonic material serving as a low-carrier-density Drude metal. An ionic liquid (IL) is used to electrically gate and partially deplete the ITO layer. The experiment shows a controllable and reversible blue-shift in the SP resonance above a critical voltage. A quantum-mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red shift.

  View details for PubMedID 28604706

• Mapping cation diffusion through lattice defects in epitaxial oxide thin films on the water-soluble buffer layer Sr3Al2O6 using atomic resolution electron microscopy APL MATERIALS Baek, D. J., Lu, D., Hikita, Y., Hwang, H. Y., Kourkoutis, L. F. 2017; 5 (9)

  View details for DOI 10.1063/1.4994538

  View details for Web of Science ID 000412099100008

• Enhancing the barrier height in oxide Schottky junctions using interface dipoles APPLIED PHYSICS LETTERS Tachikawa, T., Hwang, H. Y., Hikita, Y. 2017; 111 (9)

  View details for DOI 10.1063/1.4991691

  View details for Web of Science ID 000408751500008

• Se. Nature nanotechnology Wu, J., Yuan, H., Meng, M., Chen, C., Sun, Y., Chen, Z., Dang, W., Tan, C., Liu, Y., Yin, J., Zhou, Y., Huang, S., Xu, H. Q., Cui, Y., Hwang, H. Y., Liu, Z., Chen, Y., Yan, B., Peng, H. 2017; 12 (6): 530-534

  Abstract

  High-mobility semiconducting ultrathin films form the basis of modern electronics, and may lead to the scalable fabrication of highly performing devices. Because the ultrathin limit cannot be reached for traditional semiconductors, identifying new two-dimensional materials with both high carrier mobility and a large electronic bandgap is a pivotal goal of fundamental research. However, air-stable ultrathin semiconducting materials with superior performances remain elusive at present. Here, we report ultrathin films of non-encapsulated layered Bi2O2Se, grown by chemical vapour deposition, which demonstrate excellent air stability and high-mobility semiconducting behaviour. We observe bandgap values of ∼0.8 eV, which are strongly dependent on the film thickness due to quantum-confinement effects. An ultrahigh Hall mobility value of >20,000 cm(2) V(-1) s(-1) is measured in as-grown Bi2O2Se nanoflakes at low temperatures. This value is comparable to what is observed in graphene grown by chemical vapour deposition and at the LaAlO3-SrTiO3 interface, making the detection of Shubnikov-de Haas quantum oscillations possible. Top-gated field-effect transistors based on Bi2O2Se crystals down to the bilayer limit exhibit high Hall mobility values (up to 450 cm(2) V(-1) s(-1)), large current on/off ratios (>10(6)) and near-ideal subthreshold swing values (∼65 mV dec(-1)) at room temperature. Our results make Bi2O2Se a promising candidate for future high-speed and low-power electronic applications.

  View details for DOI 10.1038/nnano.2017.43

  View details for PubMedID 28369044

• High electron mobility and quantum oscillations in non-encapsulated ultrathin semiconducting Bi2O2Se NATURE NANOTECHNOLOGY Wu, J., Yuan, H., Meng, M., Chen, C., Sun, Y., Chen, Z., Dang, W., Tan, C., Liu, Y., Yin, J., Zhou, Y., Huang, S., Xu, H. Q., Cui, Y., Hwang, H. Y., Liu, Z., Chen, Y., Yan, B., Peng, H. 2017; 12 (6): 530-+

  View details for DOI 10.1038/NNANO.2017.43

  View details for Web of Science ID 000402769100009

• Ubiquitous strong electron-phonon coupling at the interface of FeSe/SrTiO3 NATURE COMMUNICATIONS Zhang, C., Liu, Z., Chen, Z., Xie, Y., He, R., Tang, S., He, J., Li, W., Jia, T., Rebec, S. N., Ma, E. Y., Yan, H., Hashimoto, M., Lu, D., Mo, S., Hikita, Y., Moore, R. G., Hwang, H. Y., Lee, D., Shen, Z. 2017; 8

  Abstract

  The observation of replica bands in single-unit-cell FeSe on SrTiO3 (STO)(001) by angle-resolved photoemission spectroscopy (ARPES) has led to the conjecture that the coupling between FeSe electrons and the STO phonons are responsible for the enhancement of Tc over other FeSe-based superconductors. However the recent observation of a similar superconducting gap in single-unit-cell FeSe/STO(110) raised the question of whether a similar mechanism applies. Here we report the ARPES study of the electronic structure of FeSe/STO(110). Similar to the results in FeSe/STO(001), clear replica bands are observed. We also present a comparative study of STO(001) and STO(110) bare surfaces, and observe similar replica bands separated by approximately the same energy, indicating this coupling is a generic feature of the STO surfaces and interfaces. Our findings suggest that the large superconducting gaps observed in FeSe films grown on different STO surface terminations are likely enhanced by a common mechanism.

  View details for DOI 10.1038/ncomms14468

  View details for Web of Science ID 000393739700001

  View details for PubMedCentralID PMC5311057

• . Nature communications Zhang, C., Liu, Z., Chen, Z., Xie, Y., He, R., Tang, S., He, J., Li, W., Jia, T., Rebec, S. N., Ma, E. Y., Yan, H., Hashimoto, M., Lu, D., Mo, S., Hikita, Y., Moore, R. G., Hwang, H. Y., Lee, D., Shen, Z. 2017; 8: 14468-?

  Abstract

  The observation of replica bands in single-unit-cell FeSe on SrTiO3 (STO)(001) by angle-resolved photoemission spectroscopy (ARPES) has led to the conjecture that the coupling between FeSe electrons and the STO phonons are responsible for the enhancement of Tc over other FeSe-based superconductors. However the recent observation of a similar superconducting gap in single-unit-cell FeSe/STO(110) raised the question of whether a similar mechanism applies. Here we report the ARPES study of the electronic structure of FeSe/STO(110). Similar to the results in FeSe/STO(001), clear replica bands are observed. We also present a comparative study of STO(001) and STO(110) bare surfaces, and observe similar replica bands separated by approximately the same energy, indicating this coupling is a generic feature of the STO surfaces and interfaces. Our findings suggest that the large superconducting gaps observed in FeSe films grown on different STO surface terminations are likely enhanced by a common mechanism.

  View details for DOI 10.1038/ncomms14468

  View details for PubMedID 28186084

• Ultrathin Epitaxial Barrier Layer to Avoid Thermally Induced Phase Transformation in Oxide Heterostructures ACS APPLIED MATERIALS & INTERFACES Baek, D. J., Lu, D., Hikita, Y., Hwang, H. Y., Kourkoutis, L. F. 2017; 9 (1): 54-59

  Abstract

  Incorporating oxides with radically different physical and chemical properties into heterostructures offers tantalizing possibilities to derive new functions and structures. Recently, we have fabricated freestanding 2D oxide membranes using the water-soluble perovskite Sr3Al2O6 as a sacrificial buffer layer. Here, with atomic-resolution spectroscopic imaging, we observe that direct growth of oxide thin films on Sr3Al2O6 can cause complete phase transformation of the buffer layer, rendering it water-insoluble. More importantly, we demonstrate that an ultrathin SrTiO3 layer can be employed as an effective barrier to preserve Sr3Al2O6 during subsequent growth, thus allowing its integration in a wider range of oxide heterostructures.

  View details for DOI 10.1021/acsami.6b14106

  View details for Web of Science ID 000392037400009

  View details for PubMedID 28006100

• Two-dimensional limit of crystalline order in perovskite membrane films. Science advances Hong, S. S., Yu, J. H., Lu, D. n., Marshall, A. F., Hikita, Y. n., Cui, Y. n., Hwang, H. Y. 2017; 3 (11): eaao5173

  Abstract

  Long-range order and phase transitions in two-dimensional (2D) systems-such as magnetism, superconductivity, and crystallinity-have been important research topics for decades. The issue of 2D crystalline order has reemerged recently, with the development of exfoliated atomic crystals. Understanding the dimensional limit of crystalline phases, with different types of bonding and synthetic techniques, is at the foundation of low-dimensional materials design. We study ultrathin membranes of SrTiO3, an archetypal perovskite oxide with isotropic (3D) bonding. Atomically controlled membranes are released after synthesis by dissolving an underlying epitaxial layer. Although all unreleased films are initially single-crystalline, the SrTiO3 membrane lattice collapses below a critical thickness (5 unit cells). This crossover from algebraic to exponential decay of the crystalline coherence length is analogous to the 2D topological Berezinskii-Kosterlitz-Thouless (BKT) transition. The transition is likely driven by chemical bond breaking at the 2D layer-3D bulk interface, defining an effective dimensional phase boundary for coherent crystalline lattices.

  View details for PubMedID 29167822

  View details for PubMedCentralID PMC5696264

• Orientation-resolved domain mapping in tetragonal SrTiO3 using polarized Raman spectroscopy PHYSICAL REVIEW B Gray, D. J., Merz, T. A., Hikita, Y., Hwang, H. Y., Mabuchi, H. 2016; 94 (21)

  View details for DOI 10.1103/PhysRevB.94.214107

  View details for Web of Science ID 000390248900001

• Variation in superconducting transition temperature due to tetragonal domains in two-dimensionally doped SrTiO3 PHYSICAL REVIEW B Noad, H., Spanton, E. M., Nowack, K. C., Inoue, H., Kim, M., Merz, T. A., Bell, C., Hikita, Y., Xu, R., Liu, W., Vailionis, A., Hwang, H. Y., Moler, K. A. 2016; 94 (17)

  View details for DOI 10.1103/PhysRevB.94.174516

  View details for Web of Science ID 000388815300010

• Dual-Gate Modulation of Carrier Density and Disorder in an Oxide Two-Dimensional Electron System NANO LETTERS Chen, Z., Yuan, H., Xie, Y., Lu, D., Inoue, H., Hikita, Y., Bell, C., Hwang, H. Y. 2016; 16 (10): 6130-6136

  Abstract

  Carrier density and disorder are two crucial parameters that control the properties of correlated two-dimensional electron systems. In order to disentangle their individual contributions to quantum phenomena, independent tuning of these two parameters is required. Here, by utilizing a hybrid liquid/solid electric dual-gate geometry acting on the conducting LaAlO3/SrTiO3 heterointerface, we obtain an additional degree of freedom to strongly modify the electron confinement profile and thus the strength of interfacial scattering, independent from the carrier density. A dual-gate controlled nonlinear Hall effect is a direct manifestation of this profile, which can be quantitatively understood by a Poisson-Schrödinger sub-band model. In particular, the large nonlinear dielectric response of SrTiO3 enables a very wide range of tunable density and disorder, far beyond that for conventional semiconductors. Our study provides a broad framework for understanding various reported phenomena at the LaAlO3/SrTiO3 interface.

  View details for DOI 10.1021/acs.nanolett.6b02348

  View details for Web of Science ID 000385469800023

  View details for PubMedID 27605459

• Synthesis of freestanding single-crystal perovskite films and heterostructures by etching of sacrificial water-soluble layers. Nature materials Lu, D., Baek, D. J., Hong, S. S., Kourkoutis, L. F., Hikita, Y., Hwang, H. Y. 2016

  Abstract

  The ability to create and manipulate materials in two-dimensional (2D) form has repeatedly had transformative impact on science and technology. In parallel with the exfoliation and stacking of intrinsically layered crystals, atomic-scale thin film growth of complex materials has enabled the creation of artificial 2D heterostructures with novel functionality and emergent phenomena, as seen in perovskite heterostructures. However, separation of these layers from the growth substrate has proved challenging, limiting the manipulation capabilities of these heterostructures with respect to exfoliated materials. Here we present a general method to create freestanding perovskite membranes. The key is the epitaxial growth of water-soluble Sr 3Al 2O 6 on perovskite substrates, followed by in situ growth of films and heterostructures. Millimetre-size single-crystalline membranes are produced by etching the Sr 3Al 2O 6 layer in water, providing the opportunity to transfer them to arbitrary substrates and integrate them with heterostructures of semiconductors and layered compounds.

  View details for DOI 10.1038/nmat4749

  View details for PubMedID 27618712

• A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction SCIENCE Seitz, L. C., Dickens, C. F., Nishio, K., Hikita, Y., Montoya, J., Doyle, A., Kirk, C., Vojvodic, A., Hwang, H. Y., Norskov, J. K., Jaramillo, T. F. 2016; 353 (6303): 1011-1014

  Abstract

  Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrOx/SrIrO3) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO3 This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO3 or anatase IrO2 motifs. The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.

  View details for DOI 10.1126/science.aaf5050

  View details for Web of Science ID 000382558900034

  View details for PubMedID 27701108

• Defect Control of Conventional and Anomalous Electron Transport at Complex Oxide Interfaces PHYSICAL REVIEW X Gunkel, F., Bell, C., Inoue, H., Kim, B., Swartz, A. G., Merz, T. A., Hikita, Y., Harashima, S., Sato, H. K., Minohara, M., Hoffmann-Eifert, S., Dittmann, R., Hwang, H. Y. 2016; 6 (3)

  View details for DOI 10.1103/PhysRevX.6.031035

  View details for Web of Science ID 000382177500001

• Evolution of the Valley Position in Bulk Transition-Metal Chalcogenides and Their Monolayer Limit. Nano letters Yuan, H., Liu, Z., Xu, G., Zhou, B., Wu, S., Dumcenco, D., Yan, K., Zhang, Y., Mo, S., Dudin, P., Kandyba, V., Yablonskikh, M., Barinov, A., Shen, Z., Zhang, S., Huang, Y., Xu, X., Hussain, Z., Hwang, H. Y., Cui, Y., Chen, Y. 2016; 16 (8): 4738-4745

  Abstract

  Layered transition metal chalcogenides with large spin orbit coupling have recently sparked much interest due to their potential applications for electronic, optoelectronic, spintronics, and valleytronics. However, most current understanding of the electronic structure near band valleys in momentum space is based on either theoretical investigations or optical measurements, leaving the detailed band structure elusive. For example, the exact position of the conduction band valley of bulk MoS2 remains controversial. Here, using angle-resolved photoemission spectroscopy with submicron spatial resolution (micro-ARPES), we systematically imaged the conduction/valence band structure evolution across representative chalcogenides MoS2, WS2, and WSe2, as well as the thickness dependent electronic structure from bulk to the monolayer limit. These results establish a solid basis to understand the underlying valley physics of these materials, and also provide a link between chalcogenide electronic band structure and their physical properties for potential valleytronics applications.

  View details for DOI 10.1021/acs.nanolett.5b05107

  View details for PubMedID 27357620

• Photoinduced Demagnetization and Insulator-to-Metal Transition in Ferromagnetic Insulating BaFeO3 Thin Films PHYSICAL REVIEW LETTERS Tsuyama, T., Chakraverty, S., Macke, S., Pontius, N., Schuessler-Langeheine, C., Hwang, H. Y., Tokura, Y., Wadati, H. 2016; 116 (25)

  Abstract

  We studied the electronic and magnetic dynamics of ferromagnetic insulating BaFeO_{3} thin films by using pump-probe time-resolved resonant x-ray reflectivity at the Fe 2p edge. By changing the excitation density, we found two distinctly different types of demagnetization with a clear threshold behavior. We assigned the demagnetization change from slow (∼150  ps) to fast (<70  ps) to a transition into a metallic state induced by laser excitation. These results provide a novel approach for locally tuning magnetic dynamics. In analogy to heat-assisted magnetic recording, metallization can locally tune the susceptibility for magnetic manipulation, allowing one to spatially encode magnetic information.

  View details for DOI 10.1103/PhysRevLett.116.256402

  View details for Web of Science ID 000378209900014

  View details for PubMedID 27391735

• Anisotropic Transport at the LaAlO3/SrTiO3 Interface Explained by Microscopic Imaging of Channel-Flow over SrTiO3 Domains ACS APPLIED MATERIALS & INTERFACES Frenkel, Y., Haham, N., Shperber, Y., Bell, C., Xie, Y., Chen, Z., Hikita, Y., Hwang, H. Y., Kalisky, B. 2016; 8 (19): 12514-12519

  Abstract

  Oxide interfaces, including the LaAlO3/SrTiO3 interface, have been a subject of intense interest for over a decade due to their rich physics and potential as low-dimensional nanoelectronic systems. The field has reached the stage where efforts are invested in developing devices. It is critical now to understand the functionalities and limitations of such devices. Recent scanning probe measurements of the LaAlO3/SrTiO3 interface have revealed locally enhanced current flow and accumulation of charge along channels related to SrTiO3 structural domains. These observations raised a key question regarding the role these modulations play in the macroscopic properties of devices. Here we show that the microscopic picture, mapped by scanning superconducting quantum interference device, accounts for a substantial part of the macroscopically measured transport anisotropy. We compared local flux data with transport values, measured simultaneously, over various SrTiO3 domain configurations. We show a clear relation between maps of local current density over specific domain configurations and the measured anisotropy for the same device. The domains divert the direction of current flow, resulting in a direction-dependent resistance. We also show that the modulation can be significant and that in some cases up to 95% of the current is modulated over the channels. The orientation and distribution of the SrTiO3 structural domains change between different cooldowns of the same device or when electric fields are applied, affecting the device behavior. Our results, highlight the importance of substrate physics, and in particular, the role of structural domains, in controlling electronic properties of LaAlO3/SrTiO3 devices. Furthermore, these results point to new research directions, exploiting the STO domains' ability to divert or even carry current.

  View details for DOI 10.1021/acsami.6b01655

  View details for Web of Science ID 000376330800069

  View details for PubMedID 27111600

• Depth resolved domain mapping in tetragonal SrTiO3 by micro-Laue diffraction APPLIED PHYSICS LETTERS Merz, T. A., Noad, H., Xu, R., Inoue, H., Liu, W., Hikita, Y., Vailionis, A., Moler, K. A., Hwang, H. Y. 2016; 108 (18)

  View details for DOI 10.1063/1.4948351

  View details for Web of Science ID 000377023300026

• Magnetic anisotropy, damping, and interfacial spin transport in Pt/LSMO bilayers AIP ADVANCES Lee, H. K., Barsukov, I., Swartz, A. G., Kim, B., Yang, L., Hwang, H. Y., Krivorotov, I. N. 2016; 6 (5)

  View details for DOI 10.1063/1.4950971

  View details for Web of Science ID 000377962500062

• Band Edge Engineering of Oxide Photoanodes for Photoelectrochemical Water Splitting: Integration of Subsurface Dipoles with Atomic-Scale Control ADVANCED ENERGY MATERIALS Hikita, Y., Nishio, K., Seitz, L. C., Chakthranont, P., Tachikawa, T., Jaramillo, T. F., Hwang, H. Y. 2016; 6 (7)

  View details for DOI 10.1002/aenm.201502154

  View details for Web of Science ID 000374704200003

• High Responsivity Phototransistors Based on Few-Layer ReS2 for Weak Signal Detection ADVANCED FUNCTIONAL MATERIALS Liu, E., Long, M., Zeng, J., Luo, W., Wang, Y., Pan, Y., Zhou, W., Wang, B., Hu, W., Ni, Z., You, Y., Zhang, X., Qin, S., Shi, Y., Watanabe, K., Taniguchi, T., Yuan, H., Hwang, H. Y., Cui, Y., Miao, F., Xing, D. 2016; 26 (12): 1938-1944

  View details for DOI 10.1002/adfm.201504408

  View details for Web of Science ID 000373129500008

• Thermodynamic guiding principles in selective synthesis of strontium iridate Ruddlesden-Popper epitaxial films APL MATERIALS Nishio, K., Hwang, H. Y., Hikita, Y. 2016; 4 (3)

  View details for DOI 10.1063/1.4943519

  View details for Web of Science ID 000373685100015

• Electric polarization control of magnetoresistance in complex oxide hetero junctions Swartz, A. G., Inoue, H., Hwang, H. Y., Drouhin, H. J., Wegrowe, J. E., Razeghi, M. SPIE-INT SOC OPTICAL ENGINEERING. 2016

  View details for Web of Science ID 000391482500007

• Tuning Band Alignment Using Interface Dipoles at the Pt/Anatase TiO2 Interface ADVANCED MATERIALS Tachikawa, T., Minohara, M., Hikita, Y., Bell, C., Hwang, H. Y. 2015; 27 (45): 7458-?

  Abstract

  The Schottky barrier heights at the Pt/TiO2 (001) junctions are modulated over 0.8 eV by inserting <1 nm of LaAlO3. The large electric field in the LaAlO3 is stabilized by preserving the continuity of in-plane lattice symmetry at the oxide interface. These results greatly expand the application of dipole engineering to versatile polycrystalline metal/binary oxide functional interfaces.

  View details for DOI 10.1002/adma.201503339

  View details for Web of Science ID 000367833200028

• Tuning Band Alignment Using Interface Dipoles at the Pt/Anatase TiO₂ Interface. Advanced materials (Deerfield Beach, Fla.) Tachikawa, T., Minohara, M., Hikita, Y., Bell, C., Hwang, H. Y. 2015; 27 (45): 7458-61

  Abstract

  The Schottky barrier heights at the Pt/TiO2 (001) junctions are modulated over 0.8 eV by inserting <1 nm of LaAlO3. The large electric field in the LaAlO3 is stabilized by preserving the continuity of in-plane lattice symmetry at the oxide interface. These results greatly expand the application of dipole engineering to versatile polycrystalline metal/binary oxide functional interfaces.

  View details for DOI 10.1002/adma.201503339

  View details for PubMedID 26502952

• Origin of the Magnetoresistance in Oxide Tunnel Junctions Determined through Electric Polarization Control of the Interface PHYSICAL REVIEW X Inoue, H., Swartz, A. G., Harmon, N. J., Tachikawa, T., Hikita, Y., Flatte, M. E., Hwang, H. Y. 2015; 5 (4)

  View details for Web of Science ID 000364413500001

• Inelastic x-ray scattering in heterostructures: electronic excitations in LaAlO3/SrTiO3 JOURNAL OF PHYSICS-CONDENSED MATTER Ruotsalainen, K. O., Sahle, C. J., Ritschel, T., Geck, J., Hosoda, M., Bell, C., Hikita, Y., Hwang, H. Y., Fister, T. T., Gordon, R. A., Hamalainen, K., Hakala, M., Huotari, S. 2015; 27 (33)

  Abstract

  We present an investigation of the valence-electron excitation spectra including the collective plasmon modes of SrTiO3, LaAlO3 and their heterostructures with non-resonant inelastic x-ray scattering. We analyse the spectra using calculations based on first principles and atomic multiplet models. We demonstrate the feasibility of performing valence IXS experiments in a total reflection geometry. Surprisingly, we find that the plasmon, interband and semicore excitations in multilayers are well described as a superposition of bulk-compound spectra even in a superstructure composing of layers of only one atomic layer thickness.

  View details for DOI 10.1088/0953-8984/27/33/335501

  View details for Web of Science ID 000359080100005

  View details for PubMedID 26221981

• Polarization-sensitive broadband photodetector using a black phosphorus vertical p-n junction NATURE NANOTECHNOLOGY Yuan, H., Liu, X., Afshinmanesh, F., Li, W., Xu, G., Sun, J., Lian, B., Curto, A. G., Ye, G., Hikita, Y., Shen, Z., Zhang, S., Chen, X., Brongersma, M., Hwang, H. Y., Cui, Y. 2015; 10 (8): 707-713

  Abstract

  The ability to detect light over a broad spectral range is central to practical optoelectronic applications and has been successfully demonstrated with photodetectors of two-dimensional layered crystals such as graphene and MoS2. However, polarization sensitivity within such a photodetector remains elusive. Here, we demonstrate a broadband photodetector using a layered black phosphorus transistor that is polarization-sensitive over a bandwidth from ∼400 nm to 3,750 nm. The polarization sensitivity is due to the strong intrinsic linear dichroism, which arises from the in-plane optical anisotropy of this material. In this transistor geometry, a perpendicular built-in electric field induced by gating can spatially separate the photogenerated electrons and holes in the channel, effectively reducing their recombination rate and thus enhancing the performance for linear dichroism photodetection. The use of anisotropic layered black phosphorus in polarization-sensitive photodetection might provide new functionalities in novel optical and optoelectronic device applications.

  View details for DOI 10.1038/NNANO.2015.112

  View details for PubMedID 26030655

• Direct Imaging of Nanoscale Conductance Evolution in Ion-Gel-Gated Oxide Transistors. Nano letters Ren, Y., Yuan, H., Wu, X., Chen, Z., Iwasa, Y., Cui, Y., Hwang, H. Y., Lai, K. 2015; 15 (7): 4730-4736

  Abstract

  Electrostatic modification of functional materials by electrolytic gating has demonstrated a remarkably wide range of density modulation, a condition crucial for developing novel electronic phases in systems ranging from complex oxides to layered chalcogenides. Yet little is known microscopically when carriers are modulated in electrolyte-gated electric double-layer transistors (EDLTs) due to the technical challenge of imaging the buried electrolyte-semiconductor interface. Here, we demonstrate the real-space mapping of the channel conductance in ZnO EDLTs using a cryogenic microwave impedance microscope. A spin-coated ionic gel layer with typical thicknesses below 50 nm allows us to perform high resolution (on the order of 100 nm) subsurface imaging, while maintaining the capability of inducing the metal-insulator transition under a gate bias. The microwave images vividly show the spatial evolution of channel conductance and its local fluctuations through the transition as well as the uneven conductance distribution established by a large source-drain bias. The unique combination of ultrathin ion-gel gating and microwave imaging offers a new opportunity to study the local transport and mesoscopic electronic properties in EDLTs.

  View details for DOI 10.1021/acs.nanolett.5b01631

  View details for PubMedID 26061780

• Electrically tunable coherent optical absorption in graphene with ion gel. Nano letters Thareja, V., Kang, J., Yuan, H., Milaninia, K. M., Hwang, H. Y., Cui, Y., Kik, P. G., Brongersma, M. L. 2015; 15 (3): 1570-1576

  Abstract

  We demonstrate electrical control over coherent optical absorption in a graphene-based Salisbury screen consisting of a single layer of graphene placed in close proximity to a gold back reflector. The screen was designed to enhance light absorption at a target wavelength of 3.2 μm by using a 600 nm-thick, nonabsorbing silica spacer layer. An ionic gel layer placed on top of the screen was used to electrically gate the charge density in the graphene layer. Spectroscopic reflectance measurements were performed in situ as a function of gate bias. The changes in the reflectance spectra were analyzed using a Fresnel based transfer matrix model in which graphene was treated as an infinitesimally thin sheet with a conductivity given by the Kubo formula. The analysis reveals that a careful choice of the ionic gel layer thickness can lead to optical absorption enhancements of up to 5.5 times for the Salisbury screen compared to a suspended sheet of graphene. In addition to these absorption enhancements, we demonstrate very large electrically induced changes in the optical absorption of graphene of ∼3.3% per volt, the highest attained so far in a device that features an atomically thick active layer. This is attributable in part to the more effective gating achieved with the ion gel over the conventional dielectric back gates and partially by achieving a desirable coherent absorption effect linked to the presence of the thin ion gel that boosts the absorption by 40%.

  View details for DOI 10.1021/nl503431d

  View details for PubMedID 25671369

• Room temperature optical anisotropy of a LaMnO3 thin-film induced by ultra-short pulse laser APPLIED PHYSICS LETTERS Munkhbaatar, P., Marton, Z., Tsermaa, B., Choi, W. S., Seo, S. S., Kim, J. S., Nakagawa, N., Hwang, H. Y., Lee, H. N., Myung-Whun, K. 2015; 106 (9)

  View details for DOI 10.1063/1.4914094

  View details for Web of Science ID 000351069900045

• X-ray spectroscopic study of BaFeO3 thin films: An Fe4+ ferromagnetic insulator PHYSICAL REVIEW B Tsuyama, T., Matsuda, T., Chakraverty, S., Okamoto, J., Ikenaga, E., Tanaka, A., Mizokawa, T., Hwang, H. Y., Tokura, Y., Wadati, H. 2015; 91 (11)

  View details for DOI 10.1103/PhysRevB.91.115101

  View details for Web of Science ID 000350995400001

• Enhanced Electrical Transparency by Ultrathin LaAlO3 Insertion at Oxide Metal/Semiconductor Heterointerfaces NANO LETTERS Yajima, T., Minohara, M., Bell, C., Kumigashira, H., Oshima, M., Hwang, H. Y., Hikita, Y. 2015; 15 (3): 1622-1626

  Abstract

  We demonstrate that the electrical conductivity of metal/semiconductor oxide heterojunctions can be increased over 7 orders of magnitude by inserting an ultrathin layer of LaAlO3. This counterintuitive result, that an interfacial barrier can be driven transparent by inserting a wide-gap insulator, arises from the large internal electric field between the two polar LaAlO3 surfaces. This field modifies the effective band offset in the device, highlighting the ability to design the electrostatic boundary conditions with atomic precision.

  View details for DOI 10.1021/nl504169m

  View details for Web of Science ID 000351188000027

  View details for PubMedID 25654211

• Optical Study of Tetragonal Domains in LaAlO3/SrTiO3 JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM Erlich, Z., Frenkel, Y., DRORI, J., Shperber, Y., Bell, C., Sato, H. K., Hosoda, M., Xie, Y., Hikita, Y., Hwang, H. Y., Kalisky, B. 2015; 28 (3): 1017-1020

  View details for DOI 10.1007/s10948-014-2714-x

  View details for Web of Science ID 000350360800050

• Controlling band alignments by artificial interface dipoles at perovskite heterointerfaces. Nature communications Yajima, T., Hikita, Y., Minohara, M., Bell, C., Mundy, J. A., Kourkoutis, L. F., Muller, D. A., Kumigashira, H., Oshima, M., Hwang, H. Y. 2015; 6: 6759-?

  Abstract

  The concept 'the interface is the device' is embodied in a wide variety of interfacial electronic phenomena and associated applications in oxide materials, ranging from catalysts and clean energy systems to emerging multifunctional devices. Many device properties are defined by the band alignment, which is often influenced by interface dipoles. On the other hand, the ability to purposefully create and control interface dipoles is a relatively unexplored degree of freedom for perovskite oxides, which should be particularly effective for such ionic materials. Here we demonstrate tuning the band alignment in perovskite metal-semiconductor heterojunctions over a broad range of 1.7 eV. This is achieved by the insertion of positive or negative charges at the interface, and the resultant dipole formed by the induced screening charge. This approach can be broadly used in applications where decoupling the band alignment from the constituent work functions and electron affinities can enhance device functionality.

  View details for DOI 10.1038/ncomms7759

  View details for PubMedID 25849738

  View details for PubMedCentralID PMC4396378

• Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors. Nature communications Liu, E., Fu, Y., Wang, Y., Feng, Y., Liu, H., Wan, X., Zhou, W., Wang, B., Shao, L., Ho, C., Huang, Y., Cao, Z., Wang, L., Li, A., Zeng, J., Song, F., Wang, X., Shi, Y., Yuan, H., Hwang, H. Y., Cui, Y., Miao, F., Xing, D. 2015; 6: 6991-?

  Abstract

  Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼10(7)) and low subthreshold swings (100 mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications.

  View details for DOI 10.1038/ncomms7991

  View details for PubMedID 25947630

  View details for PubMedCentralID PMC4432591

• Pressure induced metallization with absence of structural transition in layered molybdenum diselenide. Nature communications Zhao, Z., Zhang, H., Yuan, H., Wang, S., Lin, Y., Zeng, Q., Xu, G., Liu, Z., Solanki, G. K., Patel, K. D., Cui, Y., Hwang, H. Y., Mao, W. L. 2015; 6: 7312-?

  Abstract

  Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ∼60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.

  View details for DOI 10.1038/ncomms8312

  View details for PubMedID 26088416

• Integrated digital inverters based on two-dimensional anisotropic ReS2 field-effect transistors. Nature communications Liu, E., Fu, Y., Wang, Y., Feng, Y., Liu, H., Wan, X., Zhou, W., Wang, B., Shao, L., Ho, C., Huang, Y., Cao, Z., Wang, L., Li, A., Zeng, J., Song, F., Wang, X., Shi, Y., Yuan, H., Hwang, H. Y., Cui, Y., Miao, F., Xing, D. 2015; 6: 6991-?

  Abstract

  Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼10(7)) and low subthreshold swings (100 mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications.

  View details for DOI 10.1038/ncomms7991

  View details for PubMedID 25947630

  View details for PubMedCentralID PMC4432591

• Pressure induced metallization with absence of structural transition in layered molybdenum diselenide. Nature communications Zhao, Z., Zhang, H., Yuan, H., Wang, S., Lin, Y., Zeng, Q., Xu, G., Liu, Z., Solanki, G. K., Patel, K. D., Cui, Y., Hwang, H. Y., Mao, W. L. 2015; 6: 7312-?

  Abstract

  Layered transition-metal dichalcogenides have emerged as exciting material systems with atomically thin geometries and unique electronic properties. Pressure is a powerful tool for continuously tuning their crystal and electronic structures away from the pristine states. Here, we systematically investigated the pressurized behavior of MoSe2 up to ∼60 GPa using multiple experimental techniques and ab-initio calculations. MoSe2 evolves from an anisotropic two-dimensional layered network to a three-dimensional structure without a structural transition, which is a complete contrast to MoS2. The role of the chalcogenide anions in stabilizing different layered patterns is underscored by our layer sliding calculations. MoSe2 possesses highly tunable transport properties under pressure, determined by the gradual narrowing of its band-gap followed by metallization. The continuous tuning of its electronic structure and band-gap in the range of visible light to infrared suggest possible energy-variable optoelectronics applications in pressurized transition-metal dichalcogenides.

  View details for DOI 10.1038/ncomms8312

  View details for PubMedID 26088416

• Large-Scale Production of Graphene Nanoribbons from Electrospun Polymers JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Liu, N., Kim, K., Hsu, P., Sokolov, A. N., Yap, F. L., Yuan, H., Xie, Y., Yan, H., Cui, Y., Hwang, H. Y., Bao, Z. 2014; 136 (49): 17284-17291

  Abstract

  Graphene nanoribbons (GNRs) are promising building blocks for high-performance electronics due to their high electron mobility and dimensionality-induced bandgap. Despite many past efforts, direct synthesis of GNRs with controlled dimensions and scalability remains challenging. Here we report the scalable synthesis of GNRs using electrospun polymer nanofiber templates. Palladium-incorporated poly(4-vinylphenol) nanofibers were prepared by electrospinning with controlled diameter and orientation. Highly graphitized GNRs as narrow as 10 nm were then synthesized from these templates by chemical vapor deposition. A transport gap can be observed in 30 nm-wide GNRs, enabling them to function as field-effect transistors at room temperature. Our results represent the first success on the scalable synthesis of highly graphitized GNRs from polymer templates. Furthermore, the generality of this method allows various polymers to be explored, which will lead to understanding of growth mechanism and rational control over crystallinity, feature size and bandgap to enable a new pathway for graphene electronics.

  View details for DOI 10.1021/ja509871n

  View details for PubMedID 25407608

• Quantification and impact of nonparabolicity of the conduction band of indium tin oxide on its plasmonic properties APPLIED PHYSICS LETTERS Liu, X., Park, J., Kang, J., Yuan, H., Cui, Y., Hwang, H. Y., Brongersma, M. L. 2014; 105 (18)

  View details for DOI 10.1063/1.4900936

  View details for Web of Science ID 000345000000017

• Quantum longitudinal and Hall transport at the LaAlO3/SrTiO3 interface at low electron densities SOLID STATE COMMUNICATIONS Xie, Y., Bell, C., Kim, M., Inoue, H., Hikita, Y., Hwang, H. Y. 2014; 197: 25-29

  View details for DOI 10.1016/j.ssc.2014.08.006

  View details for Web of Science ID 000343223000006

• Visible-light-enhanced gating effect at the LaAlO3/SrTiO3 interface NATURE COMMUNICATIONS Lei, Y., Li, Y., CHEN, Y. Z., Xie, Y. W., Chen, Y. S., Wang, S. H., Wang, J., Shen, B. G., Pryds, N., Hwang, H. Y., Sun, J. R. 2014; 5

  Abstract

  Electrostatic gating field and light illumination are two widely used stimuli for semiconductor devices. Via capacitive effect, a gate field modifies the carrier density of the devices, while illumination generates extra carriers by exciting trapped electrons. Here we report an unusual illumination-enhanced gating effect in a two-dimensional electron gas at the LaAlO3/SrTiO3 interface, which has been the focus of emergent phenomena exploration. We find that light illumination decreases, rather than increases, the carrier density of the gas when the interface is negatively gated through the SrTiO3 layer, and the density drop can be 20 times as large as that caused by the conventional capacitive effect. This effect is further found to stem from an illumination-accelerated interface polarization, an originally extremely slow process. This unusual effect provides a promising controlling of the correlated oxide electronics in which a much larger gating capacity is demanding due to their intrinsic larger carrier density.

  View details for DOI 10.1038/ncomms6554

  View details for Web of Science ID 000345913600018

  View details for PubMedID 25407837

• Atomically Engineered Metal Insulator Transition at the TiO2/LaAlO3 Heterointerface NANO LETTERS Minohara, M., Tachikawa, T., Nakanishi, Y., Hikita, Y., Kourkoutis, L. F., Lee, J., Kao, C., Yoshita, M., Akiyama, H., Bell, C., Hwang, H. Y. 2014; 14 (11): 6743-6746

  Abstract

  We demonstrate that the atomic boundary conditions of simple binary oxides can be used to impart dramatic changes of state. By changing the substrate surface termination of LaAlO3 (001) from AlO2 to LaO, the room-temperature sheet conductance of anatase TiO2 films are increased by over 3 orders of magnitude, transforming the intrinsic insulating state to a high mobility metallic state, while maintaining excellent optical transparency.

  View details for DOI 10.1021/nl5039192

  View details for Web of Science ID 000345723800113

  View details for PubMedID 25343440

• Generation and electric control of spin-valley-coupled circular photogalvanic current in WSe2 NATURE NANOTECHNOLOGY Yuan, H., Wang, X., Lian, B., Zhang, H., Fang, X., Shen, B., Xu, G., Xu, Y., Zhang, S., Hwang, H. Y., Cui, Y. 2014; 9 (10): 851-857

  Abstract

  The valley degree of freedom in layered transition-metal dichalcogenides provides an opportunity to extend the functionalities of spintronics and valleytronics devices. The achievement of spin-coupled valley polarization induced by the non-equilibrium charge-carrier imbalance between two degenerate and inequivalent valleys has been demonstrated theoretically and by optical experiments. However, the generation of a valley and spin current with the valley polarization in transition-metal dichalcogenides remains elusive. Here we demonstrate a spin-coupled valley photocurrent, within an electric-double-layer transistor based on WSe2, whose direction and magnitude depend on the degree of circular polarization of the incident radiation and can be further modulated with an external electric field. This room-temperature generation and electric control of a valley and spin photocurrent provides a new property of electrons in transition-metal dichalcogenide systems, and thereby enables additional degrees of control for quantum-confined spintronic devices.

  View details for DOI 10.1038/NNANO.2014.183

  View details for Web of Science ID 000343617200028

• Generation and electric control of spin-valley-coupled circular photogalvanic current in WSe2. Nature nanotechnology Yuan, H., Wang, X., Lian, B., Zhang, H., Fang, X., Shen, B., Xu, G., Xu, Y., Zhang, S., Hwang, H. Y., Cui, Y. 2014; 9 (10): 851-857

  Abstract

  The valley degree of freedom in layered transition-metal dichalcogenides provides an opportunity to extend the functionalities of spintronics and valleytronics devices. The achievement of spin-coupled valley polarization induced by the non-equilibrium charge-carrier imbalance between two degenerate and inequivalent valleys has been demonstrated theoretically and by optical experiments. However, the generation of a valley and spin current with the valley polarization in transition-metal dichalcogenides remains elusive. Here we demonstrate a spin-coupled valley photocurrent, within an electric-double-layer transistor based on WSe2, whose direction and magnitude depend on the degree of circular polarization of the incident radiation and can be further modulated with an external electric field. This room-temperature generation and electric control of a valley and spin photocurrent provides a new property of electrons in transition-metal dichalcogenide systems, and thereby enables additional degrees of control for quantum-confined spintronic devices.

  View details for DOI 10.1038/nnano.2014.183

  View details for PubMedID 25194947

• Spin-dependent transport across Co/LaAlO3/SrTiO3 heterojunctions APPLIED PHYSICS LETTERS Swartz, A. G., Harashima, S., Xie, Y., Lu, D., Kim, B., Bell, C., Hikita, Y., Hwang, H. Y. 2014; 105 (3)

  View details for DOI 10.1063/1.4891174

  View details for Web of Science ID 000341152300049

• Spatial density profile of electrons near the LaAlO3/SrTiO3 heterointerface revealed by time-resolved photoluminescence spectroscopy APPLIED PHYSICS LETTERS Yamada, Y., Sato, H. K., Hikita, Y., Hwang, H. Y., Kanemitsu, Y. 2014; 104 (15)

  View details for DOI 10.1063/1.4872171

  View details for Web of Science ID 000335145200022

• In-plane terahertz response of thin film Sr2RuO4 PHYSICAL REVIEW B Takahashi, Y., Chakraverty, S., Kawasaki, M., Hwang, H. Y., Tokura, Y. 2014; 89 (16)

  View details for DOI 10.1103/PhysRevB.89.165116

  View details for Web of Science ID 000336906000007

• Visualizing the interfacial evolution from charge compensation to metallic screening across the manganite metal-insulator transition. Nature communications Mundy, J. A., Hikita, Y., Hidaka, T., Yajima, T., Higuchi, T., Hwang, H. Y., Muller, D. A., Kourkoutis, L. F. 2014; 5: 3464-?

  Abstract

  Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in devices. The nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Here we directly quantify with atomic-resolution the charge distribution for manganite-titanate interfaces traversing the metal-insulator transition. By measuring the concentration and valence of the cations, we find an intrinsic interfacial electronic reconstruction in the insulating films. The total charge observed for the insulating manganite films quantitatively agrees with that needed to cancel the polar catastrophe. As the manganite becomes metallic with increased hole doping, the total charge build-up and its spatial range drop substantially. Direct quantification of the intrinsic charge transfer and spatial width should lay the framework for devices harnessing these unique electronic phases.

  View details for DOI 10.1038/ncomms4464

  View details for PubMedID 24632721

• Detection of Berry's Phase in a Bulk Rashba Semiconductor SCIENCE Murakawa, H., Bahramy, M. S., Tokunaga, M., Kohama, Y., Bell, C., Kaneko, Y., Nagaosa, N., Hwang, H. Y., Tokura, Y. 2013; 342 (6165): 1490-1493

  Abstract

  The motion of electrons in a solid has a profound effect on its topological properties and may result in a nonzero Berry's phase, a geometric quantum phase encoded in the system's electronic wave function. Despite its ubiquity, there are few experimental observations of Berry's phase of bulk states. Here, we report detection of a nontrivial π Berry's phase in the bulk Rashba semiconductor BiTeI via analysis of the Shubnikov-de Haas (SdH) effect. The extremely large Rashba splitting in this material enables the separation of SdH oscillations, stemming from the spin-split inner and outer Fermi surfaces. For both Fermi surfaces, we observe a systematic π-phase shift in SdH oscillations, consistent with the theoretically predicted nontrivial π Berry's phase in Rashba systems.

  View details for DOI 10.1126/science.1242247

  View details for Web of Science ID 000328644300052

  View details for PubMedID 24357313

• Tunable coupling of two-dimensional superconductors in bilayer SrTiO3 heterostructures PHYSICAL REVIEW B Inoue, H., Kim, M., Bell, C., Hikita, Y., Raghu, S., Hwang, H. Y. 2013; 88 (24)

  View details for DOI 10.1103/PhysRevB.88.241104

  View details for Web of Science ID 000328577700001

• Multiple helimagnetic phases and topological Hall effect in epitaxial thin films of pristine and Co-doped SrFeO3 PHYSICAL REVIEW B Chakraverty, S., Matsuda, T., Wadati, H., Okamoto, J., Yamasaki, Y., Nakao, H., Murakami, Y., Ishiwata, S., Kawasaki, M., Taguchi, Y., Tokura, Y., Hwang, H. Y. 2013; 88 (22)

  View details for DOI 10.1103/PhysRevB.88.220405

  View details for Web of Science ID 000328577000001

• Locally enhanced conductivity due to the tetragonal domain structure in LaAlO3/SrTiO3 heterointerfaces. Nature materials Kalisky, B., Spanton, E. M., Noad, H., Kirtley, J. R., Nowack, K. C., Bell, C., Sato, H. K., Hosoda, M., Xie, Y., Hikita, Y., Woltmann, C., Pfanzelt, G., Jany, R., Richter, C., Hwang, H. Y., Mannhart, J., Moler, K. A. 2013; 12 (12): 1091-1095

  Abstract

  The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the {001} interface of the band insulators LaAlO3 and TiO2-terminated SrTiO3 (STO; refs , ). Transport and other measurements in this system show a plethora of diverse physical phenomena. To better understand the interface conductivity, we used scanning superconducting quantum interference device microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed on thermal cycling above the STO cubic-to-tetragonal structural transition temperature, implying that the local conductivity is strongly modified by the STO tetragonal domain structure. The interplay between substrate domains and the interface provides an additional mechanism for understanding and controlling the behaviour of heterostructures.

  View details for DOI 10.1038/nmat3753

  View details for PubMedID 24013791

• Tuning the electrons at the LaAlO3/SrTiO3 interface: From growth to beyond growth CHINESE PHYSICS B Xie Yan-Wu, Harold, H. Y. 2013; 22 (12)

  View details for DOI 10.1088/1674-1056/22/12/127301

  View details for Web of Science ID 000329565500062

• Landau Level Spectroscopy of Dirac Electrons in a Polar Semiconductor with Giant Rashba Spin Splitting PHYSICAL REVIEW LETTERS Bordacs, S., Checkelsky, J. G., Murakawa, H., Hwang, H. Y., Tokura, Y. 2013; 111 (16)

  Abstract

  Optical excitations of BiTeI with large Rashba spin splitting have been studied in an external magnetic field (B) applied parallel to the polar axis. A sequence of transitions between the Landau levels (LLs), whose energies are in proportion to √B were observed, being characteristic of massless Dirac electrons. The large separation energy between the LLs makes it possible to detect the strongest cyclotron resonance even at room temperature in moderate fields. Unlike in 2D Dirac systems, the magnetic field induced rearrangement of the conductivity spectrum is directly observed.

  View details for DOI 10.1103/PhysRevLett.111.166403

  View details for Web of Science ID 000326148000013

  View details for PubMedID 24182286

• Controlled Growth of High-Quality Monolayer WS2 Layers on Sapphire and Imaging Its Grain Boundary ACS NANO Zhang, Y., Zhang, Y., Ji, Q., Ju, J., Yuan, H., Shi, J., Gao, T., Ma, D., Liu, M., Chen, Y., Song, X., Hwang, H. Y., Cui, Y., Liu, Z. 2013; 7 (10): 8963-8971

  Abstract

  Atomically thin tungsten disulfide (WS2), a structural analogue to MoS2, has attracted great interest due to its indirect-to-direct band-gap tunability, giant spin splitting, and valley-related physics. However, the batch production of layered WS2 is underdeveloped (as compared with that of MoS2) for exploring these fundamental issues and developing its applications. Here, using a low-pressure chemical vapor deposition method, we demonstrate that high-crystalline mono- and few-layer WS2 flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm(2). Intriguingly, we show that, with adding minor H2 carrier gas, the shape of monolayer WS2 flakes can be tailored from jagged to straight edge triangles and still single crystalline. Meanwhile, some intersecting triangle shape flakes are concomitantly evolved from more than one nucleus to show a polycrystalline nature. It is interesting to see that, only through a mild sample oxidation process, the grain boundaries are easily recognizable by scanning electron microscopy due to its altered contrasts. Hereby, controlling the initial nucleation state is crucial for synthesizing large-scale single-crystalline flakes. We believe that this work would benefit the controlled growth of high-quality transition metal dichalcogenide, as well as in their future applications in nanoelectronics, optoelectronics, and solar energy conversions.

  View details for DOI 10.1021/nn403454e

  View details for Web of Science ID 000326209100068

  View details for PubMedID 24047054

• BaFeO3 cubic single crystalline thin film: A ferromagnetic insulator APPLIED PHYSICS LETTERS Chakraverty, S., Matsuda, T., Ogawa, N., Wadati, H., Ikenaga, E., Kawasaki, M., Tokura, Y., Hwang, H. Y. 2013; 103 (14)

  View details for DOI 10.1063/1.4824210

  View details for Web of Science ID 000325488500065

• Enhancing Electron Mobility at the LaAlO3 /SrTiO3 Interface by Surface Control. Advanced materials Xie, Y., Bell, C., Hikita, Y., Harashima, S., Hwang, H. Y. 2013; 25 (34): 4735-4738

  Abstract

  Mobility of electrons confined at the LaAlO3 /SrTiO3 interface is significantly enhanced by surface control using surface charges and adsorbates, reaching a low temperature value more than 20 000 cm(2) V(-1) s(-1) . A uniform trend that mobility increases with decreasing sheet carrier density is observed.

  View details for DOI 10.1002/adma.201301798

  View details for PubMedID 23852878

• Transistor operation and mobility enhancement in top-gated LaAlO3/SrTiO3 heterostructures APPLIED PHYSICS LETTERS Hosoda, M., Hikita, Y., Hwang, H. Y., Bell, C. 2013; 103 (10)

  View details for DOI 10.1063/1.4820449

  View details for Web of Science ID 000324389700061

• Coexistence of two-dimensional and three-dimensional Shubnikov-de Haas oscillations in Ar+-irradiated KTaO3 PHYSICAL REVIEW B Harashima, S., Bell, C., Kim, M., Yajima, T., Hikita, Y., Hwang, H. Y. 2013; 88 (8)

  View details for DOI 10.1103/PhysRevB.88.085102

  View details for Web of Science ID 000322777300001

• Titanium d(xy) ferromagnetism at the LaAlO3/SrTiO3 interface NATURE MATERIALS Lee, J., Xie, Y. W., Sato, H. K., Bell, C., Hikita, Y., Hwang, H. Y., Kao, C. 2013; 12 (8): 703-706

  Abstract

  A number of recent transport and magnetization studies have shown signs of ferromagnetism in the LaAlO3/SrTiO3 heterostructure, an unexpected property with no bulk analogue in the constituent materials. However, no experiment thus far has provided direct information on the host of the magnetism. Here we report spectroscopic investigations of the magnetism using element-specific techniques, including X-ray magnetic circular dichroism and X-ray absorption spectroscopy, along with corresponding model calculations. We find direct evidence for in-plane ferromagnetic order at the interface, with Ti(3+) character in the dxy orbital of the anisotropic t2g band. These findings establish a striking example of emergent phenomena at oxide interfaces.

  View details for DOI 10.1038/NMAT3674

  View details for Web of Science ID 000322119100014

  View details for PubMedID 23727948

• Measurement of the Femtosecond Optical Absorption of LaAlO3/SrTiO3 Heterostructures: Evidence for an Extremely Slow Electron Relaxation at the Interface PHYSICAL REVIEW LETTERS Yamada, Y., Sato, H. K., Hikita, Y., Hwang, H. Y., Kanemitsu, Y. 2013; 111 (4)

  Abstract

  The photocarrier relaxation dynamics of an n-type LaAlO_{3}/SrTiO_{3} heterointerface is investigated using femtosecond transient absorption (TA) spectroscopy at low temperatures. In both LaAlO_{3}/SrTiO_{3} heterostructures and electron-doped SrTiO_{3} bulk crystals, the TA spectrum shows a Drude-like free carrier absorption immediately after excitation. In addition, a broad absorption band gradually appears within 40 ps, which corresponds to the energy relaxation of photoexcited free electrons into self-trapped polaron states. We reveal that the polaron formation time is enhanced considerably at the LaAlO_{3}/SrTiO_{3} heterointerface as compared to bulk crystals. Further, we discuss the interface effects on the electron relaxation dynamics in conjunction with the splitting of the t_{2g} subbands due to the interface potential.

  View details for DOI 10.1103/PhysRevLett.111.047403

  View details for Web of Science ID 000322218000013

  View details for PubMedID 23931405

• Stoichiometry control of the electronic properties of the LaAlO3/SrTiO3 heterointerface APPLIED PHYSICS LETTERS Sato, H. K., Bell, C., Hikita, Y., Hwang, H. Y. 2013; 102 (25)

  View details for DOI 10.1063/1.4812353

  View details for Web of Science ID 000321145200025

• Spontaneous B-site order and metallic ferrimagnetism in LaSrVMoO6 grown by pulsed laser deposition APPLIED PHYSICS LETTERS Chakraverty, S., Yu, X. Z., Kawasaki, M., Tokura, Y., Hwang, H. Y. 2013; 102 (22)

  View details for DOI 10.1063/1.4809937

  View details for Web of Science ID 000320621600048

• Strongly Spin-Orbit Coupled Two-Dimensional Electron Gas Emerging near the Surface of Polar Semiconductors PHYSICAL REVIEW LETTERS Sakano, M., Bahramy, M. S., Katayama, A., Shimojima, T., Murakawa, H., Kaneko, Y., Malaeb, W., Shin, S., Ono, K., Kumigashira, H., Arita, R., Nagaosa, N., Hwang, H. Y., Tokura, Y., Ishizaka, K. 2013; 110 (10)

  Abstract

  We investigate the two-dimensional highly spin-polarized electron accumulation layers commonly appearing near the surface of n-type polar semiconductors BiTeX (X=I, Br, and Cl) by angular-resolved photoemission spectroscopy. Because of the polarity and the strong spin-orbit interaction built in the bulk atomic configurations, the quantized conduction-band subbands show giant Rashba-type spin splitting. The characteristic 2D confinement effect is clearly observed also in the valence bands down to the binding energy of 4 eV. The X-dependent Rashba spin-orbit coupling is directly estimated from the observed spin-split subbands, which roughly scales with the inverse of the band-gap size in BiTeX.

  View details for DOI 10.1103/PhysRevLett.110.107204

  View details for Web of Science ID 000315898400006

  View details for PubMedID 23521291

• Compositional and gate tuning of the interfacial conductivity in LaAlO3/LaTiO3/SrTiO3 heterostructures APPLIED PHYSICS LETTERS Hosoda, M., Bell, C., Hikita, Y., Hwang, H. Y. 2013; 102 (9)

  View details for DOI 10.1063/1.4794410

  View details for Web of Science ID 000316085200013

• Shubnikov-de Haas oscillations in the bulk Rashba semiconductor BiTeI PHYSICAL REVIEW B Bell, C., Bahramy, M. S., Murakawa, H., Checkelsky, J. G., Arita, R., Kaneko, Y., Onose, Y., Tokunaga, M., Kohama, Y., Nagaosa, N., Tokura, Y., Hwang, H. Y. 2013; 87 (8)

  View details for DOI 10.1103/PhysRevB.87.081109

  View details for Web of Science ID 000315483300003

• Hot electron transport in a strongly correlated transition-metal oxide SCIENTIFIC REPORTS Rana, K. G., Yajima, T., Parui, S., Kemper, A. F., Devereaux, T. P., Hikita, Y., Hwang, H. Y., Banerjee, T. 2013; 3

  Abstract

  Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant for oxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La₀.₇Sr₀.₃MnO₃ (LSMO)- Nb-doped SrTiO₃ (Nb:STO) interface and find the characteristic hot-electron attenuation length in LSMO to be 1.48 ± 0.10 unit cells (u.c.) at -1.9 V, increasing to 2.02 ± 0.16 u.c. at -1.3 V at room temperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron and polaron scattering. Direct visualization of the local electron transport shows different transmission at the terraces and at the step-edges.

  View details for DOI 10.1038/srep01274

  View details for Web of Science ID 000314864800002

  View details for PubMedID 23429420

  View details for PubMedCentralID PMC3572443

• Intrinsic spin-orbit coupling in superconducting delta-doped SrTiO3 heterostructures PHYSICAL REVIEW B Kim, M., Kozuka, Y., Bell, C., Hikita, Y., Hwang, H. Y. 2012; 86 (8)

  View details for DOI 10.1103/PhysRevB.86.085121

  View details for Web of Science ID 000307505600005

• Gate-tuned superfluid density at the superconducting LaAlO3/SrTiO3 interface PHYSICAL REVIEW B Bert, J. A., Nowack, K. C., Kalisky, B., Noad, H., Kirtley, J. R., Bell, C., Sato, H. K., Hosoda, M., Hikita, Y., Hwang, H. Y., Moler, K. A. 2012; 86 (6)

  View details for DOI 10.1103/PhysRevB.86.060503

  View details for Web of Science ID 000307271400001

• Scanning Probe Manipulation of Magnetism at the LaAlO3/SrTiO3 Heterointerface NANO LETTERS Kalisky, B., Bert, J. A., Bell, C., Xie, Y., Sato, H. K., Hosoda, M., Hikita, Y., Hwang, H. Y., Moler, K. A. 2012; 12 (8): 4055-4059

  Abstract

  Manipulation of magnetism is a longstanding goal of research in exotic materials. In this work, we demonstrate that the small ferromagnetic patches in LaAlO(3)/SrTiO(3) heterostructures can be dramatically changed by in situ contact of a scanning probe. Our results provide a platform for manipulation of small magnets through either a strong magneto-elastic coupling or sensitivity to surface modification. The ability to locally control magnetism is particularly interesting due to the presence of superconductivity with strong spin-orbit coupling in LaAlO(3)/SrTiO(3).

  View details for DOI 10.1021/nl301451e

  View details for Web of Science ID 000307211000030

  View details for PubMedID 22769056

• Effect of anisotropic strain on the charge dynamics of Nd0.5Sr0.5MnO3 thin films SOLID STATE COMMUNICATIONS Munkhbaatar, P., Nakagawa, N., Hwang, H. Y., Kim, J. S., Myung-Whun, K. 2012; 152 (16): 1541-1544

  View details for DOI 10.1016/j.ssc.2012.05.028

  View details for Web of Science ID 000307039700008

• Metal-to-insulator transition in anatase TiO2 thin films induced by growth rate modulation APPLIED PHYSICS LETTERS Tachikawa, T., Minohara, M., Nakanishi, Y., Hikita, Y., Yoshita, M., Akiyama, H., Bell, C., Hwang, H. Y. 2012; 101 (2)

  View details for DOI 10.1063/1.4733724

  View details for Web of Science ID 000306360600037

• Scanning SQUID susceptometry of a paramagnetic superconductor PHYSICAL REVIEW B Kirtley, J. R., Kalisky, B., Bert, J. A., Bell, C., Kim, M., Hikita, Y., Hwang, H. Y., Ngai, J. H., Segal, Y., Walker, F. J., Ahn, C. H., Moler, K. A. 2012; 85 (22)

  View details for DOI 10.1103/PhysRevB.85.224518

  View details for Web of Science ID 000305531200005

• Critical thickness for ferromagnetism in LaAlO3/SrTiO3 heterostructures NATURE COMMUNICATIONS Kalisky, B., Bert, J. A., Klopfer, B. B., Bell, C., Sato, H. K., Hosoda, M., Hikita, Y., Hwang, H. Y., Moler, K. A. 2012; 3

  Abstract

  In LaAlO(3)/SrTiO(3) heterointerfaces, charge carriers migrate from the LaAlO(3) to the interface in an electronic reconstruction. Magnetism has been observed in LaAlO(3)/SrTiO(3), but its relationship to the interface conductivity is unknown. Here we show that reconstruction is necessary, but not sufficient, for the formation of magnetism. Using scanning superconducting quantum interference device microscopy we find that magnetism appears only above a critical LaAlO(3) thickness, similar to the conductivity. We observe no change in ferromagnetism with gate voltage, and detect ferromagnetism in a non-conducting p-type sample. These observations indicate that the carriers at the interface do not need to be itinerant to generate magnetism. The ferromagnetism appears in isolated patches whose density varies greatly between samples. This inhomogeneity strongly suggests that disorder or local strain generates magnetism in a population of the interface carriers.

  View details for DOI 10.1038/ncomms1931

  View details for Web of Science ID 000306099900049

  View details for PubMedID 22735450

• Subband Structure of a Two-Dimensional Electron Gas Formed at the Polar Surface of the Strong Spin-Orbit Perovskite KTaO3 PHYSICAL REVIEW LETTERS King, P. D., He, R. H., Eknapakul, T., Buaphet, P., Mo, S., Kaneko, Y., Harashima, S., Hikita, Y., Bahramy, M. S., Bell, C., Hussain, Z., Tokura, Y., Shen, Z., Hwang, H. Y., Baumberger, F., Meevasana, W. 2012; 108 (11)

  Abstract

  We demonstrate the formation of a two-dimensional electron gas (2DEG) at the (100) surface of the 5d transition-metal oxide KTaO3. From angle-resolved photoemission, we find that quantum confinement lifts the orbital degeneracy of the bulk band structure and leads to a 2DEG composed of ladders of subband states of both light and heavy carriers. Despite the strong spin-orbit coupling, our measurements provide a direct upper bound for the potential Rashba spin splitting of only Δk(parallel)}~0.02 Å(-1) at the Fermi level. The polar nature of the KTaO3(100) surface appears to help mediate the formation of the 2DEG as compared to nonpolar SrTiO3(100).

  View details for DOI 10.1103/PhysRevLett.108.117602

  View details for Web of Science ID 000301478400015

  View details for PubMedID 22540511

• Single-valley quantum Hall ferromagnet in a dilute MgxZn1-xO/ZnO strongly correlated two-dimensional electron system PHYSICAL REVIEW B Kozuka, Y., Tsukazaki, A., Maryenko, D., Falson, J., Bell, C., Kim, M., Hikita, Y., Hwang, H. Y., Kawasaki, M. 2012; 85 (7)

  View details for DOI 10.1103/PhysRevB.85.075302

  View details for Web of Science ID 000299898500005

• Emergent phenomena at oxide interfaces NATURE MATERIALS Hwang, H. Y., Iwasa, Y., Kawasaki, M., Keimer, B., Nagaosa, N., Tokura, Y. 2012; 11 (2): 103-113

  Abstract

  Recent technical advances in the atomic-scale synthesis of oxide heterostructures have provided a fertile new ground for creating novel states at their interfaces. Different symmetry constraints can be used to design structures exhibiting phenomena not found in the bulk constituents. A characteristic feature is the reconstruction of the charge, spin and orbital states at interfaces on the nanometre scale. Examples such as interface superconductivity, magneto-electric coupling, and the quantum Hall effect in oxide heterostructures are representative of the scientific and technological opportunities in this rapidly emerging field.

  View details for DOI 10.1038/NMAT3223

  View details for Web of Science ID 000299428700008

  View details for PubMedID 22270825

• General considerations of the electrostatic boundary conditions in oxide heterostructures MULTIFUNCTIONAL OXIDE HETEROSTRUCTURES Higuchi, T., Hwang, H. Y., Tsymbal, E. Y., Dagotto, E. R., Eom, C. B., Ramesh, R. 2012: 183-213

  View details for Web of Science ID 000399094100008

• Electronic charges and electric potential at LaAlO3/SrTiO3 interfaces studied by core-level photoemission spectroscopy PHYSICAL REVIEW B Takizawa, M., Tsuda, S., Susaki, T., Hwang, H. Y., Fujimori, A. 2011; 84 (24)

  View details for DOI 10.1103/PhysRevB.84.245124

  View details for Web of Science ID 000298561800002

• Control of electronic conduction at an oxide heterointerface using surface polar adsorbates NATURE COMMUNICATIONS Xie, Y., Hikita, Y., Bell, C., Hwang, H. Y. 2011; 2

  Abstract

  The interface between LaAlO(3) and SrTiO(3) possesses a range of intriguing properties, notably a proposed connection between the surface state of the LaAlO(3) and the conductivity buried in the SrTiO(3). Here we study the effect of the surface adsorption of a variety of common laboratory solvents on the conductivity at the interface between LaAlO(3) and SrTiO(3). We show that the application of chemicals such as acetone, ethanol, and water can induce a large change in the conductivity, and, in particular, an insulator to metal transition around the critical LaAlO(3) thickness. This phenomenon is observed only for polar solvents. These data provide experimental evidence for a general polarization-facilitated electronic transfer mechanism.

  View details for DOI 10.1038/ncomms1501

  View details for Web of Science ID 000296787300007

  View details for PubMedID 21988910

• Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface NATURE PHYSICS Bert, J. A., Kalisky, B., Bell, C., Kim, M., Hikita, Y., Hwang, H. Y., Moler, K. A. 2011; 7 (10): 767-771

  View details for DOI 10.1038/nphys2079

  View details for Web of Science ID 000295584200013

• Fermi Surface and Superconductivity in Low-Density High-Mobility delta-Doped SrTiO3 PHYSICAL REVIEW LETTERS Kim, M., Bell, C., Kozuka, Y., Kurita, M., Hikita, Y., Hwang, H. Y. 2011; 107 (10)

  Abstract

  The electronic structure of low-density n-type SrTiO3 δ-doped heterostructures is investigated by angular dependent Shubnikov-de Haas oscillations. In addition to a controllable crossover from a three- to two-dimensional Fermi surface, clear beating patterns for decreasing dopant layer thicknesses are found. These indicate the lifting of the degeneracy of the conduction band due to subband quantization in the two-dimensional limit. Analysis of the temperature-dependent oscillations shows that similar effective masses are found for all components, associated with the splitting of the light electron pocket. The dimensionality crossover in the superconducting state is found to be distinct from the normal state, resulting in a rich phase diagram as a function of dopant layer thickness.

  View details for DOI 10.1103/PhysRevLett.107.106801

  View details for Web of Science ID 000294406100009

  View details for PubMedID 21981518

• Reentrant insulating state in ultrathin manganite films APPLIED PHYSICS LETTERS Kim, B., Kwon, D., Yajima, T., Bell, C., Hikita, Y., Kim, B. G., Hwang, H. Y. 2011; 99 (9)

  View details for DOI 10.1063/1.3628659

  View details for Web of Science ID 000294489300048

• Common Origin of the Circular-Dichroism Pattern in Angle-Resolved Photoemission Spectroscopy of SrTiO3 and CuxBi2Se3 PHYSICAL REVIEW LETTERS Ishida, Y., Kanto, H., Kikkawa, A., Taguchi, Y., Ito, Y., Ota, Y., Okazaki, K., Malaeb, W., Mulazzi, M., Okawa, M., Watanabe, S., Chen, C., Kim, M., Bell, C., Kozuka, Y., Hwang, H. Y., Tokura, Y., Shin, S. 2011; 107 (7)

  Abstract

  Circular dichroism in the angular distribution of photoelectrons from SrTiO(3):Nb and Cu(x)Bi(2)Se(3) is investigated by 7-eV laser angle-resolved photoemission spectroscopy. In addition to the well-known node that occurs in the circular dichroism pattern when the incidence plane matches the mirror plane of the crystal, we show that another type of node occurs when the mirror plane of the crystal is vertical to the incidence plane and the electronic state is two-dimensional. The flower-shaped circular dichroism patterns in the angular distribution occurring around the Fermi level of SrTiO(3):Nb and around the Dirac point of Cu(x)Bi(2)Se(3) are explained on equal footings. We point out that the penetration depth of the topological states of Cu(x)Bi(2)Se(3) depends on momentum.

  View details for DOI 10.1103/PhysRevLett.107.077601

  View details for Web of Science ID 000293705600026

  View details for PubMedID 21902429

• Structural Comparison of n-Type and p-Type LaAlO3/SrTiO3 Interfaces PHYSICAL REVIEW LETTERS Yamamoto, R., Bell, C., Hikita, Y., Hwang, H. Y., Nakamura, H., Kimura, T., Wakabayashi, Y. 2011; 107 (3)

  Abstract

  Using a surface x-ray diffraction technique, we investigated the atomic structure of two types of interfaces between LaAlO3 and SrTiO3, that is, p-type (SrO/AlO2) and n-type (TiO2/LaO) interfaces. Our results demonstrate that the SrTiO3 in the sample with the n-type interface has a large polarized region, while that with the p-type interface has a limited polarized region. In addition, atomic intermixing was observed to extend deeper into the SrTiO3 substrate at the n-type interface compared to the p type. These differences result in distinct degrees of band bending, which likely contributes to the striking contrast in electrical conductivity between the two types of interfaces.

  View details for DOI 10.1103/PhysRevLett.107.036104

  View details for Web of Science ID 000292764600006

  View details for PubMedID 21838380

• Electric field penetration in Au/Nb: SrTiO3 Schottky junctions probed by bias-dependent internal photoemission APPLIED PHYSICS LETTERS Hikita, Y., Kawamura, M., Bell, C., Hwang, H. Y. 2011; 98 (19)

  View details for DOI 10.1063/1.3589375

  View details for Web of Science ID 000290586800026

• Nanometer-scale epitaxial strain release in perovskite heterostructures using "SrAlOx" sliding buffer layers APPLIED PHYSICS LETTERS Sato, H. K., Mundy, J. A., Higuchi, T., Hikita, Y., Bell, C., Muller, D. A., Hwang, H. Y. 2011; 98 (17)

  View details for DOI 10.1063/1.3583459

  View details for Web of Science ID 000290046100012

• Tuning the Electron Gas at an Oxide Heterointerface via Free Surface Charges ADVANCED MATERIALS Xie, Y., Bell, C., Hikita, Y., Hwang, H. Y. 2011; 23 (15): 1744-?

  View details for DOI 10.1002/adma.201004673

  View details for Web of Science ID 000289531800008

  View details for PubMedID 21400617

• A heteroepitaxial perovskite metal-base transistor NATURE MATERIALS Yajima, T., Hikita, Y., Hwang, H. Y. 2011; 10 (3): 198-201

  Abstract

  'More than Moore' captures a concept for overcoming limitations in silicon electronics by incorporating new functionalities in the constituent materials. Perovskite oxides are candidates because of their vast array of physical properties in a common structure. They also enable new electronic devices based on strongly-correlated electrons. The field effect transistor and its derivatives have been the principal oxide devices investigated thus far, but another option is available in a different geometry: if the current is perpendicular to the interface, the strong internal electric fields generated at back-to-back heterojunctions can be used for oxide electronics, analogous to bipolar transistors. Here we demonstrate a perovskite heteroepitaxial metal-base transistor operating at room temperature, enabled by interface dipole engineering. Analysis of many devices quantifies the evolution from hot-electron to permeable-base behaviour. This device provides a platform for incorporating the exotic ground states of perovskite oxides, as well as novel electronic phases at their interfaces.

  View details for DOI 10.1038/NMAT2946

  View details for Web of Science ID 000287517400019

  View details for PubMedID 21258354

• LaVO4:Eu Phosphor films with enhanced Eu solubility APPLIED PHYSICS LETTERS Higuchi, T., Hotta, Y., Hikita, Y., Maruyama, S., Hayamizu, Y., Akiyama, H., Wadati, H., Hawthorn, D. G., Regier, T. Z., Blyth, R. I., Sawatzky, G. A., Hwang, H. Y. 2011; 98 (7)

  View details for DOI 10.1063/1.3554749

  View details for Web of Science ID 000287507200013

• Built-in and induced polarization across LaAlO3/SrTiO3 heterojunctions NATURE PHYSICS Singh-Bhalla, G., Bell, C., Ravichandran, J., Siemons, W., Hikita, Y., Salahuddin, S., Hebard, A. F., Hwang, H. Y., Ramesh, R. 2011; 7 (1): 80-86

  View details for DOI 10.1038/NPHYS1814

  View details for Web of Science ID 000285501100021

• Enhancing the electron mobility via delta-doping in SrTiO3 APPLIED PHYSICS LETTERS Kozuka, Y., Kim, M., Ohta, H., Hikita, Y., Bell, C., Hwang, H. Y. 2010; 97 (22)

  View details for DOI 10.1063/1.3524198

  View details for Web of Science ID 000284965000046

• Microscopic origins for stabilizing room-temperature ferromagnetism in ultrathin manganite layers PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Kourkoutis, L. F., Song, J. H., Hwang, H. Y., Muller, D. A. 2010; 107 (26): 11682-11685

  Abstract

  La(0.7)Sr(0.3)MnO(3) is a conducting ferromagnet at room temperature. Combined with thin SrTiO(3) layers, the resulting heterostructures could be used as highly spin-polarized magnetic-tunnel-junction memories. However, when shrunk to dimensions below an apparent critical thickness, the structures become insulating and ferromagnetic ordering is suppressed. Interface spin and charge modulations are thought to create an interfacial dead layer, thus fundamentally limiting the use of this material in atomic-scale devices. The thickness of this dead layer, and whether it is intrinsic, is still controversial. Here we use atomic-resolution electron spectroscopy to demonstrate that the degradation of the magnetic and transport properties of La(0.7)Sr(0.3)MnO(3)/SrTiO(3) multilayers correlates with atomic intermixing at the interfaces, and the presence of extended two-dimensional cation defects in the La(0.7)Sr(0.3)MnO(3) layers (in contrast to three-dimensional precipitates in thick films). When these extrinsic defects are eliminated, metallic ferromagnetism at room temperature can be stabilized in five-unit-cell-thick manganite layers in superlattices, placing the upper limit for any intrinsic dead layer at two unit cells per interface.

  View details for DOI 10.1073/pnas.1005693107

  View details for Web of Science ID 000279332300008

  View details for PubMedID 20547875

• Atomic-resolution spectroscopic imaging of oxide interfaces PHILOSOPHICAL MAGAZINE Kourkoutis, L. F., Xin, H. L., Higuchi, T., Hotta, Y., Lee, J. H., Hikita, Y., Schlom, D. G., Hwang, H. Y., Muller, D. A. 2010; 90 (35-36): 4731-4749

  View details for DOI 10.1080/14786435.2010.518983

  View details for Web of Science ID 000283318700012

• Two-dimensional normal-state quantum oscillations in a superconducting heterostructure NATURE Kozuka, Y., Kim, M., Bell, C., Kim, B. G., Hikita, Y., Hwang, H. Y. 2009; 462 (7272): 487-490

  Abstract

  Semiconductor heterostructures provide an ideal platform for studying high-mobility, low-density electrons in reduced dimensions. The realization of superconductivity in heavily doped diamond, silicon, silicon carbide and germanium suggests that Cooper pairs eventually may be directly incorporated in semiconductor heterostructures, but these newly discovered superconductors are currently limited by their extremely large electronic disorder. Similarly, the electron mean free path in low-dimensional superconducting thin films is usually limited by interface scattering, in single-crystal or polycrystalline samples, or atomic-scale disorder, in amorphous materials, confining these examples to the extreme 'dirty limit'. Here we report the fabrication of a high-quality superconducting layer within a thin-film heterostructure based on SrTiO(3) (the first known superconducting semiconductor). By selectively doping a narrow region of SrTiO(3) with the electron-donor niobium, we form a superconductor that is two-dimensional, as probed by the anisotropy of the upper critical magnetic field. Unlike in previous examples, however, the electron mobility is high enough that the normal-state resistance exhibits Shubnikov-de Haas oscillations that scale with the perpendicular field, indicating two-dimensional states. These results suggest that delta-doped SrTiO(3) provides a model system in which to explore the quantum transport and interplay of both superconducting and normal electrons. They also demonstrate that high-quality complex oxide heterostructures can maintain electron coherence on the macroscopic scales probed by transport, as well as on the microscopic scales demonstrated previously.

  View details for DOI 10.1038/nature08566

  View details for Web of Science ID 000272144200039

  View details for PubMedID 19940921

• Magnetodielectric coupling in nonmagnetic Au/GaAs:Si Schottky barriers PHYSICAL REVIEW B Tongay, S., Hebard, A. F., Hikita, Y., Hwang, H. Y. 2009; 80 (20)

  View details for DOI 10.1103/PhysRevB.80.205324

  View details for Web of Science ID 000272311400082

• Resonant soft x-ray scattering studies of interface reconstructions in SrTiO3/LaAlO3 superlattices JOURNAL OF APPLIED PHYSICS Wadati, H., Hawthorn, D. G., Geck, J., Higuchi, T., Hikita, Y., Hwang, H. Y., Kourkoutis, L. F., Muller, D. A., Huang, S., Huang, D. J., Lin, H., Schuessler-Langeheine, C., Wu, H., Schierle, E., Weschke, E., Ingle, N. J., Sawatzky, G. A. 2009; 106 (8)

  View details for DOI 10.1063/1.3246788

  View details for Web of Science ID 000271358100051

• Multiple conducting carriers generated in LaAlO3/SrTiO3 heterostructures APPLIED PHYSICS LETTERS Seo, S. S., Marton, Z., Choi, W. S., Hassink, G. W., Blank, D. H., Hwang, H. Y., Noh, T. W., Egami, T., Lee, H. N. 2009; 95 (8)

  View details for DOI 10.1063/1.3213390

  View details for Web of Science ID 000269723200030

• Enhanced lattice polarization in SrTiO3/LaAlO3 superlattices measured using optical second-harmonic generation PHYSICAL REVIEW B Ogawa, N., Miyano, K., Hosoda, M., Higuchi, T., Bell, C., Hikita, Y., Hwang, H. Y. 2009; 80 (8)

  View details for DOI 10.1103/PhysRevB.80.081106

  View details for Web of Science ID 000269639300006

• Mn3O4 precipitates in laser-ablated manganite films APPLIED PHYSICS LETTERS Higuchi, T., Yajima, T., Kourkoutis, L. F., Hikita, Y., Nakagawa, N., Muller, D. A., Hwang, H. Y. 2009; 95 (4)

  View details for DOI 10.1063/1.3193667

  View details for Web of Science ID 000268611900056

• Interface reconstruction in V-oxide heterostructures determined by x-ray absorption spectroscopy APPLIED PHYSICS LETTERS Wadati, H., Hawthorn, D. G., Geck, J., Regier, T. Z., Blyth, R. I., Higuchi, T., Hotta, Y., Hikita, Y., Hwang, H. Y., Sawatzky, G. A. 2009; 95 (2)

  View details for DOI 10.1063/1.3177328

  View details for Web of Science ID 000268089200091

• Thickness dependence of the mobility at the LaAlO3/SrTiO3 interface APPLIED PHYSICS LETTERS Bell, C., Harashima, S., Hikita, Y., Hwang, H. Y. 2009; 94 (22)

  View details for DOI 10.1063/1.3149695

  View details for Web of Science ID 000266674300034

• Enhanced thermodynamic stability of epitaxial oxide thin films ADVANCED MATERIALS Song, J. H., Susaki, T., Hwang, H. Y. 2008; 20 (13): 2528-?

  View details for DOI 10.1002/adma.200701919

  View details for Web of Science ID 000257808700012

• Negative differential resistance induced by Mn substitution at SrRuO3/Nb : SrTiO3 Schottky interfaces PHYSICAL REVIEW B Hikita, Y., Kourkoutis, L. F., Susaki, T., Muller, D. A., Takagi, H., Hwang, H. Y. 2008; 77 (20)

  View details for DOI 10.1103/PhysRevB.77.205330

  View details for Web of Science ID 000256971800086

• Atomic-scale chemical imaging of composition and bonding by aberration-corrected microscopy SCIENCE Muller, D. A., Kourkoutis, L. F., Murfitt, M., Song, J. H., Hwang, H. Y., Silcox, J., Dellby, N., Krivanek, O. L. 2008; 319 (5866): 1073-1076

  Abstract

  Using a fifth-order aberration-corrected scanning transmission electron microscope, which provides a factor of 100 increase in signal over an uncorrected instrument, we demonstrated two-dimensional elemental and valence-sensitive imaging at atomic resolution by means of electron energy-loss spectroscopy, with acquisition times of well under a minute (for a 4096-pixel image). Applying this method to the study of a La(0.7)Sr(0.3)MnO3/SrTiO3 multilayer, we found an asymmetry between the chemical intermixing on the manganese-titanium and lanthanum-strontium sublattices. The measured changes in the titanium bonding as the local environment changed allowed us to distinguish chemical interdiffusion from imaging artifacts.

  View details for DOI 10.1126/science.1148820

  View details for Web of Science ID 000253311700038

  View details for PubMedID 18292338

• Polar discontinuity doping of the LaVO3/SrTiO3 interface PHYSICAL REVIEW LETTERS Hotta, Y., Susaki, T., Hwang, H. Y. 2007; 99 (23)

  Abstract

  We have investigated the transport properties of LaVO_{3}/SrTiO_{3} Mott-insulator-band-insulator heterointerfaces for various configurations. The (001)-oriented n-type VO_{2}/LaO/TiO_{2} polar discontinuity is conducting, exhibiting a LaVO3 thickness-dependent metal-insulator transition and low temperature anomalous Hall effect. The (001) p-type VO_{2}/SrO/TiO_{2} interface, formed by inserting a single layer of bulk metallic SrVO3 or SrO, drives the interface insulating. The (110) heterointerface is also insulating, indicating interface conduction arising from electronic reconstructions.

  View details for DOI 10.1103/PhysRevLett.99.236805

  View details for Web of Science ID 000251451000051

  View details for PubMedID 18233395

• Growth mode control of the free carrier density in SrTiO3-delta films JOURNAL OF APPLIED PHYSICS Ohtomo, A., Hwang, H. Y. 2007; 102 (8)

  View details for DOI 10.1063/1.2798385

  View details for Web of Science ID 000250589300070

• Asymmetric interface profiles in LaVO3/SrTiO3 heterostructures grown by pulsed laser deposition APPLIED PHYSICS LETTERS Kourkoutis, L. F., Muller, D. A., Hotta, Y., Hwang, H. Y. 2007; 91 (16)

  View details for DOI 10.1063/1.2798060

  View details for Web of Science ID 000250295700066

• Characterization of the Schottky barrier in SrRuO3/Nb : SrTiO3 junctions APPLIED PHYSICS LETTERS Hikita, Y., Kozuka, Y., Susaki, T., Takagi, H., Hwang, H. Y. 2007; 90 (14)

  View details for DOI 10.1063/1.2719157

  View details for Web of Science ID 000245512200104

• Electronic structure of the Mott insulator LaVO3 in a quantum well geometry APPLIED PHYSICS LETTERS Hotta, Y., Wadati, H., Fujimori, A., Susaki, T., Hwang, H. Y. 2006; 89 (25)

  View details for DOI 10.1063/1.2422898

  View details for Web of Science ID 000243415200040

• Growth and epitaxial structure of LaVOx films APPLIED PHYSICS LETTERS Hotta, Y., Mukunoki, Y., Susaki, T., Hwang, H. Y., Fitting, L., Muller, D. A. 2006; 89 (3)

  View details for DOI 10.1063/1.2227786

  View details for Web of Science ID 000239174100044

• Why some interfaces cannot be sharp NATURE MATERIALS Nakagawa, N., Hwang, H. Y., Muller, D. A. 2006; 5 (3): 204-209

  View details for DOI 10.1038/nmat1569

  View details for Web of Science ID 000235707900019

• Atomically flat (110) SrTiO3 and heteroepitaxy APPLIED PHYSICS LETTERS Mukunoki, Y., Nakagawa, N., Susaki, T., Hwang, H. Y. 2005; 86 (17)

  View details for DOI 10.1063/1.1920415

  View details for Web of Science ID 000229185500020

• Magnetocapacitance and exponential magnetoresistance in manganite-titanate heterojunctions APPLIED PHYSICS LETTERS Nakagawa, N., Asai, M., Mukunoki, Y., Susaki, T., Hwang, H. Y. 2005; 86 (8)

  View details for DOI 10.1063/1.1868882

  View details for Web of Science ID 000227609000045

• Atomic-scale imaging of nanoengineered oxygen vacancy profiles in SrTiO3 NATURE Muller, D. A., Nakagawa, N., Ohtomo, A., Grazul, J. L., Hwang, H. Y. 2004; 430 (7000): 657-661

  Abstract

  At the heart of modern oxide chemistry lies the recognition that beneficial (as well as deleterious) materials properties can be obtained by deliberate deviations of oxygen atom occupancy from the ideal stoichiometry. Conversely, the capability to control and confine oxygen vacancies will be important to realize the full potential of perovskite ferroelectric materials, varistors and field-effect devices. In transition metal oxides, oxygen vacancies are generally electron donors, and in strontium titanate (SrTiO3) thin films, oxygen vacancies (unlike impurity dopants) are particularly important because they tend to retain high carrier mobilities, even at high carrier densities. Here we report the successful fabrication, using a pulsed laser deposition technique, of SrTiO3 superlattice films with oxygen doping profiles that exhibit subnanometre abruptness. We profile the vacancy concentrations on an atomic scale using annular-dark-field electron microscopy and core-level spectroscopy, and demonstrate absolute detection sensitivities of one to four oxygen vacancies. Our findings open a pathway to the microscopic study of individual vacancies and their clustering, not only in oxides, but in crystalline materials more generally.

  View details for DOI 10.1038/nature02756

  View details for Web of Science ID 000223085400040

  View details for PubMedID 15295595

• Active mode locking of broadband quantum cascade lasers IEEE JOURNAL OF QUANTUM ELECTRONICS Soibel, A., Capasso, F., Gmachl, C., Peabody, M. L., Sergent, A. M., Paiella, R., Hwang, H. Y., Sivco, D. L., Cho, A. Y., Liu, H. C., Jirauschek, C., Kartner, F. X. 2004; 40 (7): 844-851

  View details for DOI 10.1109/JQE.2004.830186

  View details for Web of Science ID 000222277900002

• High-mobility electrons in SrTiO3 heterostructures PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES Hwang, H. Y., Ohtomo, A., Nakagawa, N., Muller, D. A., Grazul, J. L. 2004; 22 (1-3): 712-716

  View details for DOI 10.1016/j.physe.2003.12.106

  View details for Web of Science ID 000221140800172

• Surface depletion in doped SrTiO3 thin films APPLIED PHYSICS LETTERS Ohtomo, A., Hwang, H. Y. 2004; 84 (10): 1716-1718

  View details for DOI 10.1063/1.1668329

  View details for Web of Science ID 000189384900032

• A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface NATURE Ohtomo, A., Hwang, H. Y. 2004; 427 (6973): 423-426

  Abstract

  Polarity discontinuities at the interfaces between different crystalline materials (heterointerfaces) can lead to nontrivial local atomic and electronic structure, owing to the presence of dangling bonds and incomplete atomic coordinations. These discontinuities often arise in naturally layered oxide structures, such as the superconducting copper oxides and ferroelectric titanates, as well as in artificial thin film oxide heterostructures such as manganite tunnel junctions. If polarity discontinuities can be atomically controlled, unusual charge states that are inaccessible in bulk materials could be realized. Here we have examined a model interface between two insulating perovskite oxides--LaAlO3 and SrTiO3--in which we control the termination layer at the interface on an atomic scale. In the simple ionic limit, this interface presents an extra half electron or hole per two-dimensional unit cell, depending on the structure of the interface. The hole-doped interface is found to be insulating, whereas the electron-doped interface is conducting, with extremely high carrier mobility exceeding 10,000 cm2 V(-1) s(-1). At low temperature, dramatic magnetoresistance oscillations periodic with the inverse magnetic field are observed, indicating quantum transport. These results present a broad opportunity to tailor low-dimensional charge states by atomically engineered oxide heteroepitaxy.

  View details for DOI 10.1038/nature02308

  View details for Web of Science ID 000188470500037

  View details for PubMedID 14749825

• Free-space mid-infrared optical links using quantum cascade lasers Conference on Free-Space Laser Communication Technologies XV Martini, R., Bethea, C., Capasso, F., Gmachl, C., Paiella, R., Whittaker, E. A., Hwang, H. Y., Sivco, D. L., Baillargeon, J. N., Cho, A. Y. SPIE-INT SOC OPTICAL ENGINEERING. 2003: 20–24

  View details for Web of Science ID 000184963500004

• Artificial charge-modulation in atomic-scale perovskite titanate superlattices NATURE Ohtomo, A., Muller, D. A., Grazul, J. L., Hwang, H. Y. 2002; 419 (6905): 378-380

  Abstract

  The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi(3+)O(3) embedded in SrTi(4+)O(3). Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO(3) layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.

  View details for DOI 10.1038/nature00977

  View details for Web of Science ID 000178195400040

  View details for PubMedID 12353030

• Epitaxial growth and electronic structure of LaTiOx films APPLIED PHYSICS LETTERS Ohtomo, A., Muller, D. A., Grazul, J. L., Hwang, H. Y. 2002; 80 (21): 3922-3924

  View details for DOI 10.1063/1.1481767

  View details for Web of Science ID 000175709000014

• Quantum cascade lasers with double metal-semiconductor waveguide resonators APPLIED PHYSICS LETTERS Unterrainer, K., Colombelli, R., Gmachl, C., Capasso, F., Hwang, H. Y., Sergent, A. M., Sivco, D. L., Cho, A. Y. 2002; 80 (17): 3060-3062

  View details for DOI 10.1063/1.1469657

  View details for Web of Science ID 000175144300010

• Recent results in quantum cascade lasers and intersubband transitions in GaN/AlGaN multiple quantum wells PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES Gmachl, C., Ng, H. M., Paiella, R., Martini, R., Hwang, H. Y., Sivco, D. L., Capasso, F., Cho, A. Y., Frolov, S. V., Chu, S. N., Liu, H. C. 2002; 13 (2-4): 823-828

  View details for Web of Science ID 000176869100170

• High-frequency modulation without the relaxation oscillation resonance in quantum cascade lasers APPLIED PHYSICS LETTERS Paiella, R., Martini, R., Capasso, F., Gmachl, C., Hwang, H. Y., Sivco, D. L., Baillargeon, J. N., Cho, A. Y., Whittaker, E. A., Liu, H. C. 2001; 79 (16): 2526-2528

  View details for Web of Science ID 000171466200008

• Quantum cascade lasers with low-loss chalcogenide lateral waveguides IEEE PHOTONICS TECHNOLOGY LETTERS Gmachl, C., Hwang, H. Y., Paiella, R., Sivco, D. L., Baillargeon, J. N., Capasso, F., Cho, A. Y. 2001; 13 (3): 182-184

  View details for Web of Science ID 000167872700004

• Monolithic active mode locking of quantum cascade lasers APPLIED PHYSICS LETTERS Paiella, R., Capasso, F., Gmachl, C., Hwang, H. Y., Sivco, D. L., Hutchinson, A. L., Cho, A. Y., Liu, H. C. 2000; 77 (2): 169-171

  View details for Web of Science ID 000087914100005

• Magnetic anisotropy of doped manganite thin films and crystals 7th Joint Magnetism and Magnetic Materials / International Magnetics Conference Suzuki, Y., Hwang, H. Y., Cheong, S. W., Siegrist, T., van Dover, R. B., Asamitsu, A., Tokura, Y. AMER INST PHYSICS. 1998: 7064–66

  View details for Web of Science ID 000077795500280

• Magnetic anisotropy and lattice distortions in the doped perovskite manganites Symposium on Metallic Magnetic Oxides at the Materials-Research-Society Fall Meeting Suzuki, Y., Hwang, H. Y., Cheong, S. W., van Dover, R. B., Asamitsu, A., Tokura, Y. MATERIALS RESEARCH SOCIETY. 1998: 41–51

  View details for Web of Science ID 000073253400007

• Ln(3)Cu(4)P(4)O(2): A new lanthanide transition metal Pnictide oxide structure type Symposium on Solid-State Chemistry of Inorganic Materials Cava, R. J., Zandbergen, H. W., Krajewski, J. J., Siegrist, T., Hwang, H. Y., Ramirez, A. P., Batlogg, B. MATERIALS RESEARCH SOCIETY. 1997: 305–310

  View details for Web of Science ID A1997BH92W00044

• The effects of strain on the magnetic anisotropy of doped manganite thin films Symposium on Epitaxial Oxide Thin Films III, at the 1997 MRS Spring Meeting Suzuki, Y., Tsai, C., Hwang, H. Y., Cheong, S. W., VANDOVER, R. B. MATERIALS RESEARCH SOCIETY. 1997: 205–210

  View details for Web of Science ID A1997BJ69L00029

• ORDERING OF CHARGE-CARRIERS IN (LA,SR)2NIO4 International Conference on Strongly Correlated Electron Systems (SCES 93) Cheong, S. W., Chen, C. H., Hwang, H. Y., Batlogg, B., RUPP, L. W., Cooper, A. S. ELSEVIER SCIENCE BV. 1994: 659–661

  View details for Web of Science ID A1994NV53800223