Academic Appointments


2019-20 Courses


Stanford Advisees



  • Alfred Cheung
  • Doctoral Dissertation Reader (AC)
    Tao Jia, Ilkyu Lee, Sam Mumford, Joshua Straquadine, Chao Wang, Patrick Worasaran
  • Postdoctoral Faculty Sponsor
    Shannon Harvey, Seung Sae Hong, Woojin Kim, Denver Li, Ruijuan Xu
  • Doctoral Dissertation Advisor (AC)
    Varun Harbola, Kyuho Lee, Motoki Osada, Prastuti Singh, Bai Yang Wang, Hyeok Yoon
  • Doctoral (Program)
    Tudor Giurgica-Tiron, Omer Hazon, YoungJu Jo, Kuan-Yu Li, Megan Nantel, John Roberts, Wen Wang, Ryotatsu Yanagimoto
  • Postdoctoral Research Mentor
    Shannon Harvey, Woojin Kim, Denver Li, Ruijuan Xu

All Publications


  • 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

  • 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)
  • 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 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., Du, K., Ren, T., Tian, H., Zhang, Z., Hwang, H. Y., Xie, Y. 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

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

  • 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
  • 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)
  • 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
  • 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
  • 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)
  • 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

  • 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
  • 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

  • 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

  • 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., Marshall, A. F., Hikita, Y., Cui, Y., 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)
  • 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)
  • 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)
  • 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)
  • 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
  • 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

  • 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-?
  • 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)
  • 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)
  • 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
  • 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

  • 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 PubMedID 26502952

  • 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-?

    View details for DOI 10.1038/ncomms7991

    View details for PubMedID 25947630

  • 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

  • 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

  • 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-?

    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

  • 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

  • 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
  • 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
  • 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)
  • 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)
  • 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)
  • 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

  • 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)
  • 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)
  • 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

  • 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)
  • 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)
  • 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
  • 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)
  • 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)
  • 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

  • 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)
  • 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
  • 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)
  • 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)
  • 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-?
  • 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)
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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