Young Lee

All Publications

• Chemical Design of Spin Frustration to Realize Topological Spin Glasses. Journal of the American Chemical Society Amtry, S. M., Campello, A. C., Tong, C. L., Puggioni, D. S., Rondinelli, J. M., Lee, Y. S., Freedman, D. E. 2024

  Abstract

  Patterning spins to generate collective behavior is at the core of condensed matter physics. Physicists develop techniques, including the fabrication of magnetic nanostructures and precision layering of materials specifically to engender frustrated lattices. As chemists, we can access such exotic materials through targeted chemical synthesis and create new lattice types by chemical design. Here, we introduce a new approach to induce magnetic frustration on a modified honeycomb lattice through a competition of alternating antiferromagnetic (AFM) and ferromagnetic (FM) nearest-neighbor interactions. By subtly modulating these two types of interactions through facile synthetic modifications, we created two systems: (1) a topological spin glass and (2) a frustrated spin-canted magnet with low-temperature exchange bias. To design this unconventional magnetic lattice, we used a metal-organic framework (MOF) platform, Ni3(pymca)3X3 (NipymcaX where pymca = pyrimidine-2-carboxylato and X = Cl, Br). We isolated two MOFs, NipymcaCl and NipymcaBr, featuring canted Ni2+-based moments. Despite this similarity, differences in the single-ion anisotropies of the Ni2+ spins result in distinct magnetic properties for each material. NipymcaCl is a topological spin glass, while NipymcaBr is a rare frustrated magnet with low-temperature exchange bias. Density functional theory calculations and Monte Carlo simulations on the NipymcaX lattice support the presence of magnetic frustration as a result of alternating AFM and FM interactions. Our calculations enabled us to determine the ground-state spin configuration and the distribution of spin-spin correlations relative to paradigmatic kagomé and triangular lattices. This modified honeycomb lattice is similar to the electronic Kekulé-O phase in graphene and provides a highly tunable platform to realize unconventional spin physics.

  View details for DOI 10.1021/jacs.4c10113

  View details for PubMedID 39382197

• Anomalous normal-state gap in an electron-doped cuprate. Science (New York, N.Y.) Xu, K. J., He, J., Chen, S. D., He, Y., Abadi, S. N., Rotundu, C. R., Lee, Y. S., Lu, D. H., Guo, Q., Tjernberg, O., Devereaux, T. P., Lee, D. H., Hashimoto, M., Shen, Z. X. 2024; 385 (6710): 796-800

  Abstract

  In the underdoped n-type cuprate Nd2-xCexCuO4, long-range antiferromagnetic order reconstructs the Fermi surface, resulting in a putative antiferromagnetic metal with small Fermi pockets. Using angle-resolved photoemission spectroscopy, we observe an anomalous energy gap, an order of magnitude smaller than the antiferromagnetic gap, in a wide portion of the underdoped regime and smoothly connecting to the superconducting gap at optimal doping. After considering all the known ordering tendencies in tandem with the phase diagram, we hypothesize that the normal-state gap in the underdoped n-type cuprates originates from Cooper pairing. The high temperature scale of the normal-state gap raises the prospect of engineering higher transition temperatures in the n-type cuprates comparable to those of the p-type cuprates.

  View details for DOI 10.1126/science.adk4792

  View details for PubMedID 39146411

• Tilted stripes origin in La_1.88Sr_0.12CuO₄ revealed by anisotropic next-nearest neighbor hopping COMMUNICATIONS PHYSICS He, W., Wen, J., Jiang, H., Xu, G., Tian, W., Taniguchi, T., Ikeda, Y., Fujita, M., Lee, Y. S. 2024; 7 (1)

  View details for DOI 10.1038/s42005-024-01753-z

  View details for Web of Science ID 001282220500003

• Halide Perovskites Breathe Too: The Iodide-Iodine Equilibrium and Self-Doping in Cs2SnI6. ACS central science Vigil, J. A., Wolf, N. R., Slavney, A. H., Matheu, R., Saldivar Valdes, A., Breidenbach, A., Lee, Y. S., Karunadasa, H. I. 2024; 10 (4): 907-919

  Abstract

  The response of an oxide crystal to the atmosphere can be personified as breathing-a dynamic equilibrium between O2 gas and O2- anions in the solid. We characterize the analogous defect reaction in an iodide double-perovskite semiconductor, Cs2SnI6. Here, I2 gas is released from the crystal at room temperature, forming iodine vacancies. The iodine vacancy defect is a shallow electron donor and is therefore ionized at room temperature; thus, the loss of I2 is accompanied by spontaneous n-type self-doping. Conversely, at high I2 pressures, I2 gas is resorbed by the perovskite, consuming excess electrons as I2 is converted to 2I-. Halide mobility and irreversible halide loss or exchange reactions have been studied extensively in halide perovskites. However, the reversible exchange equilibrium between iodide and iodine [2I-(s) ↔ I2(g) + 2e-] described here has often been overlooked in prior studies, though it is likely general to halide perovskites and operative near room temperature, even in the dark. An analysis of the 2I-(s)/I2(g) equilibrium thermodynamics and related transport kinetics in single crystals of Cs2SnI6 therefore provides insight toward achieving stable composition and electronic properties in the large family of iodide perovskite semiconductors.

  View details for DOI 10.1021/acscentsci.4c00056

  View details for PubMedID 38680557

  View details for PubMedCentralID PMC11046464

• Halide Perovskites Breathe Too: The Iodide-Iodine Equilibrium and Self-Doping in Cs₂SnI₆ ACS CENTRAL SCIENCE Vigil, J. A., Wolf, N. R., Slavney, A. H., Matheu, R., Saldivar Valdes, A., Breidenbach, A., Lee, Y. S., Karunadasa, H. I. 2024

  View details for DOI 10.1021/acscentsci.4c00056

  View details for Web of Science ID 001196483400001

• Influence of the Rare Earth Cation on the Magnetic Properties of Layered 12R-Ba4M4+Mn3O12 (M = Ce, Pr) Perovskites. Chemistry of materials : a publication of the American Chemical Society Dzara, M. J., Campello, A. C., Breidenbach, A. T., Strange, N. A., Park, J. E., Ambrosini, A., Coker, E. N., Ginley, D. S., Lee, Y. S., Bell, R. T., Smaha, R. W. 2024; 36 (6): 2810-2818

  Abstract

  Material design is increasingly used to realize desired functional properties, and the perovskite structure family is one of the richest and most diverse: perovskites are employed in many applications due to their structural flexibility and compositional diversity. Hexagonal, layered perovskite structures with chains of face-sharing transition metal oxide octahedra have attracted great interest as quantum materials due to their magnetic and electronic properties. Ba4MMn3O12, a member of the "12R" class of hexagonal, layered perovskites, contains trimers of face-sharing MnO6 octahedra that are linked by a corner-sharing, bridging MO6 octahedron. Here, we investigate cluster magnetism in the Mn3O12 trimers and the role of this bridging octahedron on the magnetic properties of two isostructural 12R materials by systematically changing the M4+ cation from nonmagnetic Ce4+ (f0) to magnetic Pr4+ (f1). We synthesized 12R-Ba4MMn3O12 (M= Ce, Pr) with high phase purity and characterized their low-temperature crystal structures and magnetic properties. Using substantially higher purity samples than previously reported, we confirm the frustrated antiferromagnetic ground state of 12R-Ba4PrMn3O12 below TN ≈ 7.75 K and explore the cluster magnetism of its Mn3O12 trimers. Despite being atomically isostructural with 12R-Ba4CeMn3O12, the f1 electron associated with Pr4+ causes much more complex magnetic properties in 12R-Ba4PrMn3O12. In 12R-Ba4PrMn3O12, we observe a sharp, likely antiferromagnetic transition at T2 ≈ 12.15 K and an additional transition at T1 ≈ 200 K, likely in canted antiferromagnetic order. These results suggest that careful variation of composition within the family of hexagonal, layered perovskites can be used to tune material properties using the complex role of the Pr4+ ion in magnetism.

  View details for DOI 10.1021/acs.chemmater.3c03014

  View details for PubMedID 38558918

  View details for PubMedCentralID PMC10976642

• Influence of the Rare Earth Cation on the Magnetic Properties of Layered 12R-Ba₄M<SUP>4+</SUP>Mn₃O₁₂ (M = Ce, Pr) Perovskites CHEMISTRY OF MATERIALS Dzara, M. J., Campello, A. C., Breidenbach, A. T., Strange, N. A., Park, J., Ambrosini, A., Coker, E. N., Ginley, D. S., Lee, Y. S., Bell, R. T., Smaha, R. W. 2024

  View details for DOI 10.1021/acs.chemmater.3c03014

  View details for Web of Science ID 001181859100001

• Dynamic magnetic phase transition induced by parametric magnon pumping PHYSICAL REVIEW B Shan, J., Curtis, J. B., Guo, M., Roh, C., Rotundu, C. R., Lee, Y. S., Narang, P., Noh, T., Demler, E., Hsieh, D. 2024; 109 (5)

  View details for DOI 10.1103/PhysRevB.109.054302

  View details for Web of Science ID 001198501400004

• Electrostatic Gating of Spin Dynamics of a Quasi-2D Kagome Magnet. Nano letters Li, Z., Zhang, R., Shan, J., Alahmed, L., Xu, A., Chen, Y., Yuan, J., Cheng, X., Miao, X., Wen, J., Mokrousov, Y., Lee, Y. S., Zhang, L., Li, P. 2024

  Abstract

  Electrostatic gating has emerged as a powerful technique for tailoring the magnetic properties of two-dimensional (2D) magnets, offering exciting prospects including enhancement of magnetic anisotropy, boosting Curie temperature, and strengthening exchange coupling effects. Here, we focus on electrical control of the ferromagnetic resonance of the quasi-2D Kagome magnet Cu(1,3-bdc). By harnessing an electrostatic field through ionic liquid gating, significant shifts are observed in the ferromagnetic resonance field in both out-of-plane and in-plane measurements. Moreover, the effective magnetization and gyromagnetic ratios display voltage-dependent variations. A closer examination reveals that the voltage-induced changes can modulate magnetocrystalline anisotropy by several hundred gauss, while the impact on orbital magnetization remains relatively subtle. Density functional theory (DFT) calculations reveal varying d-orbital hybridizations at different voltages. This research unveils intricate physics within the Kagome lattice magnet and further underscores the potential of electrostatic manipulation in steering magnetism with promising implications for the development of spintronic devices.

  View details for DOI 10.1021/acs.nanolett.4c00034

  View details for PubMedID 38323579

• Local probe investigation of the spin dynamics in the kagome and interlayers of orthorhombic barlowite Cu₄(OD)₆FBr: <SUP>79</SUP>Br and <SUP>63</SUP>Cu NQR study PHYSICAL REVIEW MATERIALS Imai, T., Wang, J., Smaha, R. W., He, W., Wen, J., Lee, Y. S. 2024; 8 (1)

  View details for DOI 10.1103/PhysRevMaterials.8.014403

  View details for Web of Science ID 001147553300005

• Bogoliubov quasiparticle on the gossamer Fermi surface in electron-doped cuprates NATURE PHYSICS Xu, K., Guo, Q., Hashimoto, M., Li, Z., Chen, S., He, J., He, Y., Li, C., Berntsen, M. H., Rotundu, C. R., Lee, Y. S., Devereaux, T. P., Rydh, A., Lu, D., Lee, D., Tjernberg, O., Shen, Z. 2023; 19 (12): 1834-+

  View details for DOI 10.1038/s41567-023-02209-x

  View details for Web of Science ID 001178645300029

• Bogoliubov quasiparticle on the gossamer Fermi surface in electron-doped cuprates NATURE PHYSICS Xu, K., Guo, Q., Hashimoto, M., Li, Z., Chen, S., He, J., He, Y., Li, C., Berntsen, M. H., Rotundu, C. R., Lee, Y. S., Devereaux, T. P., Rydh, A., Lu, D., Lee, D., Tjernberg, O., Shen, Z. 2023

  View details for DOI 10.1038/s41567-023-02209

  View details for Web of Science ID 001071598400003

• Stabilizing Au2+ in a mixed-valence 3D halide perovskite NATURE CHEMISTRY Lindquist, K. P., Eghdami, A., Deschene, C. R., Heyer, A. J., Wen, J., Smith, A. G., Solomon, E. I., Lee, Y. S., Neaton, J. B., Ryan, D. H., Karunadasa, H. I. 2023

  View details for DOI 10.1038/s41557-023-01305

  View details for Web of Science ID 001064800000001

• Stabilizing Au2+ in a mixed-valence 3D halide perovskite. Nature chemistry Lindquist, K. P., Eghdami, A., Deschene, C. R., Heyer, A. J., Wen, J., Smith, A. G., Solomon, E. I., Lee, Y. S., Neaton, J. B., Ryan, D. H., Karunadasa, H. I. 2023

  Abstract

  Although Cu2+ is ubiquitous, the relativistic destabilization of the 5d orbitals makes the isoelectronic Au2+ exceedingly rare, typically stabilized only through Au-Au bonding or by using redox non-innocent ligands. Here we report the perovskite Cs4AuIIAuIII2Cl12, an extended solid with mononuclear Au2+ sites, which is stable to ambient conditions and characterized by single-crystal X-ray diffraction. The 2+ oxidation state of Au was assigned using 197Au Mössbauer spectroscopy, electron paramagnetic resonance, and magnetic susceptibility measurements, with comparison to paramagnetic and diamagnetic analogues with Cu2+ and Pd2+, respectively, as well as to density functional theory calculations. This gold perovskite offers an opportunity to study the optical and electronic transport of the uncommon Au2+/3+ mixed-valence state and the characteristics of the elusive Au2+ ion coordinated to simple ligands. Compared with the perovskite Cs2AuIAuIIICl6, which has been studied since the 1920s, Cs4AuIIAuIII2Cl12 exhibits a 0.7 eV reduction in optical absorption onset and a 103-fold increase in electronic conductivity.

  View details for DOI 10.1038/s41557-023-01305-y

  View details for PubMedID 37640854

  View details for PubMedCentralID 8246118

• Enhanced superconductivity by near-neighbor attraction in the doped extended Hubbard model PHYSICAL REVIEW B Peng, C., Wang, Y., Wen, J., Lee, Y. S., Devereaux, T. P., Jiang, H. 2023; 107 (20)

  View details for DOI 10.1103/PhysRevB.107.L201102

  View details for Web of Science ID 000986820400004

• High-energy spin excitations in the quantum spin liquid candidate Zn-substituted barlowite probed by resonant inelastic x-ray scattering PHYSICAL REVIEW B Smaha, R. W., Pelliciari, J., Jarrige, I., Bisogni, V., Breidenbach, A. T., Jiang, J., Wen, J., Jiang, H., Lee, Y. S. 2023; 107 (6)

  View details for DOI 10.1103/PhysRevB.107.L060402

  View details for Web of Science ID 000938787400002

• Emergence of the spin polarized domains in the kagome lattice Heisenberg antiferromagnet Zn-barlowite (Zn0.95Cu0.05)Cu-3(OD)(6)FBr NPJ QUANTUM MATERIALS Yuan, W., Wang, J., Singer, P. M., Smaha, R. W., Wen, J., Lee, Y. S., Imai, T. 2022; 7 (1)

  View details for DOI 10.1038/s41535-022-00531-w

  View details for Web of Science ID 000905019300001

• Quasi-One-Dimensional Metallicity in Compressed CsSnI3. Journal of the American Chemical Society Ke, F., Yan, J., Matheu, R., Niu, S., Wolf, N. R., Yang, H., Yin, K., Wen, J., Lee, Y. S., Karunadasa, H. I., Mao, W. L., Lin, Y. 2022

  Abstract

  Low-dimensional metal halides exhibit strong structural and electronic anisotropies, making them candidates for accessing unusual electronic properties. Here, we demonstrate pressure-induced quasi-one-dimensional (quasi-1D) metallicity in delta-CsSnI3. With the application of pressure up to 40 GPa, the initially insulating delta-CsSnI3 transforms to a metallic state. Synchrotron X-ray diffraction and Raman spectroscopy indicate that the starting 1D chain structure of edge-sharing Sn-I octahedra in delta-CsSnI3 is maintained in the high-pressure metallic phase while the SnI6 octahedral chains are distorted. Our experiments combined with first-principles density functional theory calculations reveal that pressure induces Sn-Sn hybridization and enhances Sn-I coupling within the chain, leading to band gap closure and formation of conductive SnI6 distorted octahedral chains. In contrast, the interchain I...I interactions remain minimal, resulting in a highly anisotropic electronic structure and quasi-1D metallicity. Our study offers a high-pressure approach for achieving diverse electronic platforms in the broad family of low-dimensional metal halides.

  View details for DOI 10.1021/jacs.2c10884

  View details for PubMedID 36534020

• Cesium-mediated electron redistribution and electron-electron interaction in high-pressure metallic CsPbI3. Nature communications Ke, F., Yan, J., Niu, S., Wen, J., Yin, K., Yang, H., Wolf, N. R., Tzeng, Y., Karunadasa, H. I., Lee, Y. S., Mao, W. L., Lin, Y. 2022; 13 (1): 7067

  Abstract

  Electron-phonon coupling was believed to govern the carrier transport in halide perovskites and related phases. Here we demonstrate that electron-electron interaction enhanced by Cs-involved electron redistribution plays a direct and prominent role in the low-temperature electrical transport of compressed CsPbI3 and renders Fermi liquid (FL)-like behavior. By compressing delta-CsPbI3 to 80GPa, an insulator-semimetal-metal transition occurs, concomitant with the completion of a slow structural transition from the one-dimensional Pnma (delta) phase to a three-dimensional Pmn21 (epsilon) phase. Deviation from FL behavior is observed upon CsPbI3 entering the metallic epsilon phase, which progressively evolves into a FL-like state at 186GPa. First-principles density functional theory calculations reveal that the enhanced electron-electron coupling results from the sudden increase of the 5d state occupation in Cs and I atoms. Our study presents a promising strategy of cationic manipulation for tuning the electronic structure and carrier scattering of halide perovskites at high pressure.

  View details for DOI 10.1038/s41467-022-34786-5

  View details for PubMedID 36400789

• Field-tuned ferroquadrupolar quantum phase transition in the insulator TmVO4. Proceedings of the National Academy of Sciences of the United States of America Massat, P., Wen, J., Jiang, J. M., Hristov, A. T., Liu, Y., Smaha, R. W., Feigelson, R. S., Lee, Y. S., Fernandes, R. M., Fisher, I. R. 2022; 119 (28): e2119942119

  Abstract

  We report results of low-temperature heat-capacity, magnetocaloric-effect, and neutron-diffraction measurements of TmVO4, an insulator that undergoes a continuous ferroquadrupolar phase transition associated with local partially filled 4f orbitals of the thulium (Tm[Formula: see text]) ions. The ferroquadrupolar transition, a realization of Ising nematicity, can be tuned to a quantum critical point by using a magnetic field oriented along the c axis of the tetragonal crystal lattice, which acts as an effective transverse field for the Ising-nematic order. In small magnetic fields, the thermal phase transition can be well described by using a semiclassical mean-field treatment of the transverse-field Ising model. However, in higher magnetic fields, closer to the field-tuned quantum phase transition, subtle deviations from this semiclassical behavior are observed, which are consistent with expectations of quantum fluctuations. Although the phase transition is driven by the local 4f degrees of freedom, the crystal lattice still plays a crucial role, both in terms of mediating the interactions between the local quadrupoles and in determining the critical scaling exponents, even though the phase transition itself can be described via mean field. In particular, bilinear coupling of the nematic order parameter to acoustic phonons changes the spatial and temporal fluctuations of the former in a fundamental way, resulting in different critical behavior of the nematic transverse-field Ising model, as compared to the usual case of the magnetic transverse-field Ising model. Our results establish TmVO4 as a model material and electronic nematicity as a paradigmatic example for quantum criticality in insulators.

  View details for DOI 10.1073/pnas.2119942119

  View details for PubMedID 35787036

• Evidence of Magnon-Mediated Orbital Magnetism in a Quasi-2D Topological Magnon Insulator. Nano letters Alahmed, L., Zhang, X., Wen, J., Xiong, Y., Li, Y., Zhang, L., Lux, F., Freimuth, F., Mahdi, M., Mokrousov, Y., Novosad, V., Kwok, W., Yu, D., Zhang, W., Lee, Y. S., Li, P. 2022

  Abstract

  We explore spin dynamics in Cu(1,3-bdc), a quasi-2D topological magnon insulator. The results show that the thermal evolution of the Lande g factor (g) is anisotropic: gin-plane decreases while gout-of-plane increases with increasing temperature T. Moreover, the anisotropy of the g factor (Deltag) and the anisotropy of saturation magnetization (DeltaMs) are correlated below 4 K, but they diverge above 4 K. We show that the electronic orbital moment contributes to the g anisotropy at lower T, while the topological orbital moment induced by thermally excited spin chirality dictates the g anisotropy at higher T. Our work suggests an interplay among topology, spin chirality, and orbital magnetism in Cu(1,3-bdc).

  View details for DOI 10.1021/acs.nanolett.2c00562

  View details for PubMedID 35699946

• Freezing of the Lattice in the Kagome Lattice Heisenberg Antiferromagnet Zn-Barlowite ZnCu_{3}(OD)_{6}FBr. Physical review letters Wang, J., Yuan, W., Singer, P. M., Smaha, R. W., He, W., Wen, J., Lee, Y. S., Imai, T. 2022; 128 (15): 157202

  Abstract

  We use ^{79}Br nuclear quadrupole resonance (NQR) to demonstrate that ultraslow lattice dynamics set in below the temperature scale set by the Cu-Cu superexchange interaction J (≃160K) in the kagome lattice Heisenberg antiferromagnet Zn-barlowite. The lattice completely freezes below 50K, and ^{79}Br NQR line shapes become twice broader due to increased lattice distortions. Moreover, the frozen lattice exhibits an oscillatory component in the transverse spin echo decay, a typical signature of pairing of nuclear spins by indirect nuclear spin-spin interaction. This indicates that some Br sites form structural dimers via a pair of kagome Cu sites prior to the gradual emergence of spin singlets below 30K. Our findings underscore the significant roles played by subtle structural distortions in determining the nature of the disordered magnetic ground state of the kagome lattice.

  View details for DOI 10.1103/PhysRevLett.128.157202

  View details for PubMedID 35499891

• Charge Reservoirs in an Expanded Halide Perovskite Analog: Enhancing High-Pressure Conductivity through Redox-Active Molecules. Angewandte Chemie (International ed. in English) Matheu, R., Ke, F., Breidenbach, A., Wolf, N., Lee, Y., Liu, Z., Leppert, L., Lin, Y., Karunadasa, H. 2022

  Abstract

  As halide perovskites and their derivatives are being developed for numerous optoelectronic applications, controlling their electronic doping remains a fundamental challenge. Herein, we describe a novel strategy of using redox-active organic molecules as stoichiometric electron acceptors. The cavities in the new expanded perovskite analogs (dmpz)[Sn2X6], (X = Br- ( 1Br ) or I- ( 1I )) are occupied by dmpz2+ (N, N'-dimethylpyrazinium), with the LUMOs lying ca. 1 eV above the valence band maximum (VBM). Compressing the metal-halide framework drives up the VBM in 1I relative to the dmpz LUMO. The electronic conductivity increases by a factor of 105 with pressure, reaching 50(17) S cm-1 at 60 GPa, exceeding the high-pressure conductivities of most halide perovskites. This conductivity enhancement is attributed to an increased hole density created by dmpz2+ reduction. This work elevates the role of organic cations in 3D metal-halides, from templating the structure to serving as charge reservoirs for tuning the carrier concentration.

  View details for DOI 10.1002/anie.202202911

  View details for PubMedID 35421260

• Emergence of spin singlets with inhomogeneous gaps in the kagome lattice Heisenberg antiferromagnets Zn-barlowite and herbertsmithite NATURE PHYSICS Wang, J., Yuan, W., Singer, P. M., Smaha, R. W., He, W., Wen, J., Lee, Y. S., Imai, T. 2021

  View details for DOI 10.1038/s41567-021-01310-3

  View details for Web of Science ID 000681561600001

• Alloying a single and a double perovskite: a Cu+/2+ mixed-valence layered halide perovskite with strong optical absorption. Chemical science Connor, B. A., Smaha, R. W., Li, J., Gold-Parker, A., Heyer, A. J., Toney, M. F., Lee, Y. S., Karunadasa, H. I. 2021; 12 (25): 8689-8697

  Abstract

  Introducing heterovalent cations at the octahedral sites of halide perovskites can substantially change their optoelectronic properties. Yet, in most cases, only small amounts of such metals can be incorporated as impurities into the three-dimensional lattice. Here, we exploit the greater structural flexibility of the two-dimensional (2D) perovskite framework to place three distinct stoichiometric cations in the octahedral sites. The new layered perovskites AI 4[CuII(CuIInIII)0.5Cl8] (1, A = organic cation) may be derived from a CuI-InIII double perovskite by replacing half of the octahedral metal sites with Cu2+. Electron paramagnetic resonance and X-ray absorption spectroscopy confirm the presence of Cu2+ in 1. Crystallographic studies demonstrate that 1 represents an averaging of the CuI-InIII double perovskite and CuII single perovskite structures. However, whereas the highly insulating CuI-InIII and CuII perovskites are colorless and yellow, respectively, 1 is black, with substantially higher electronic conductivity than that of either endmember. We trace these emergent properties in 1 to intervalence charge transfer between the mixed-valence Cu centers. We further propose a tiling model to describe how the Cu+, Cu2+, and In3+ coordination spheres can pack most favorably into a 2D perovskite lattice, which explains the unusual 1 : 2 : 1 ratio of these cations found in 1. Magnetic susceptibility data of 1 further corroborate this packing model. The emergence of enhanced visible light absorption and electronic conductivity in 1 demonstrates the importance of devising strategies for increasing the compositional complexity of halide perovskites.

  View details for DOI 10.1039/d1sc01159f

  View details for PubMedID 34257867

  View details for PubMedCentralID PMC8246118

• Alloying a single and a double perovskite: a Cu+/2+ mixed-valence layered halide perovskite with strong optical absorption CHEMICAL SCIENCE Connor, B. A., Smaha, R. W., Li, J., Gold-Parker, A., Heyer, A. J., Toney, M. F., Lee, Y. S., Karunadasa, H. I. 2021

  View details for DOI 10.1039/d1sc01159f

  View details for Web of Science ID 000655654400001

• Site-specific structure at multiple length scales in kagome quantum spin liquid candidates PHYSICAL REVIEW MATERIALS Smaha, R. W., Boukahil, I., Titus, C. J., Jiang, J., Sheckelton, J. P., He, W., Wen, J., Vinson, J., Wang, S., Chen, Y., Teat, S. J., Devereaux, T. P., Das Pemmaraju, C., Lee, Y. S. 2020; 4 (12)

  View details for DOI 10.1103/PhysRevMaterials.4.124406

  View details for Web of Science ID 000600660300004

• Materializing rival ground states in the barlowite family of kagome magnets: quantum spin liquid, spin ordered, and valence bond crystal states NPJ QUANTUM MATERIALS Smaha, R. W., He, W., Jiang, J., Wen, J., Jiang, Y., Sheckelton, J. P., Titus, C. J., Wang, S., Chen, Y., Teat, S. J., Aczel, A. A., Zhao, Y., Xu, G., Lynn, J. W., Jiang, H., Lee, Y. S. 2020; 5 (1)

  View details for DOI 10.1038/s41535-020-0222-8

  View details for Web of Science ID 000527641300002

• High resolution time- and angle-resolved photoemission spectroscopy with 11 eV laser pulses. The Review of scientific instruments Lee, C., Rohwer, T., Sie, E. J., Zong, A., Baldini, E., Straquadine, J., Walmsley, P., Gardner, D., Lee, Y. S., Fisher, I. R., Gedik, N. 2020; 91 (4): 043102

  Abstract

  Performing time- and angle-resolved photoemission (tr-ARPES) spectroscopy at high momenta necessitates extreme ultraviolet laser pulses, which are typically produced via high harmonic generation (HHG). Despite recent advances, HHG-based setups still require large pulse energies (from hundreds of μJ to mJ) and their energy resolution is limited to tens of meV. Here, we present a novel 11 eV tr-ARPES setup that generates a flux of 5 × 1010 photons/s and achieves an unprecedented energy resolution of 16 meV. It can be operated at high repetition rates (up to 250 kHz) while using input pulse energies down to 3 µJ. We demonstrate these unique capabilities by simultaneously capturing the energy and momentum resolved dynamics in two well-separated momentum space regions of a charge density wave material ErTe3. This novel setup offers the opportunity to study the non-equilibrium band structure of solids with exceptional energy and time resolutions at high repetition rates.

  View details for DOI 10.1063/1.5139556

  View details for PubMedID 32357712

• High resolution time- and angle-resolved photoemission spectroscopy with 11 eV laser pulses REVIEW OF SCIENTIFIC INSTRUMENTS Lee, C., Rohwer, T., Sie, E. J., Zong, A., Baldini, E., Straquadine, J., Walmsley, P., Gardner, D., Lee, Y. S., Fisher, I. R., Gedik, N. 2020; 91 (4)

  View details for DOI 10.1063/1.5139556

  View details for Web of Science ID 000526759500002

• Materializing rival ground states in the barlowite family of kagome magnets: quantum spin liquid, spin ordered, and valence bond crystal states. npj quantum materials Smaha, R. W., He, W., Jiang, J. M., Wen, J., Jiang, Y., Sheckelton, J. P., Titus, C. J., Wang, S. G., Chen, Y., Teat, S. J., Aczel, A. A., Zhao, Y., Xu, G., Lynn, J. W., Jiang, H., Lee, Y. S. 2020; 5 (1)

  Abstract

  The spin- 1 2 kagome antiferromagnet is considered an ideal host for a quantum spin liquid (QSL) ground state. We find that when the bonds of the kagome lattice are modulated with a periodic pattern, new quantum ground states emerge. Newly synthesized crystalline barlowite (Cu4(OH)6FBr) and Zn-substituted barlowite demonstrate the delicate interplay between singlet states and spin order on the spin- 1 2 kagome lattice. Comprehensive structural measurements demonstrate that our new variant of barlowite maintains hexagonal symmetry at low temperatures with an arrangement of distorted and undistorted kagome triangles, for which numerical simulations predict a pinwheel valence bond crystal (VBC) state instead of a QSL. The presence of interlayer spins eventually leads to an interesting pinwheel q = 0 magnetic order. Partially Zn-substituted barlowite (Cu3.44Zn0.56(OH)6FBr) has an ideal kagome lattice and shows QSL behavior, indicating a surprising robustness of the QSL against interlayer impurities. The magnetic susceptibility is similar to that of herbertsmithite, even though the Cu2+ impurities are above the percolation threshold for the interlayer lattice and they couple more strongly to the nearest kagome moment. This system is a unique playground displaying QSL, VBC, and spin order, furthering our understanding of these highly competitive quantum states.

  View details for PubMedID 33072886

• Site-Specific Structure at Multiple Length Scales in Kagome Quantum Spin Liquid Candidates. Physical review materials Smaha, R. W., Boukahil, I., Titus, C. J., Jiang, J. M., Sheckelton, J. P., He, W., Wen, J., Vinson, J., Wang, S. G., Chen, Y. S., Teat, S. J., Devereaux, T. P., Pemmaraju, C. D., Lee, Y. S. 2020; 4 (12)

  Abstract

  Realizing a quantum spin liquid (QSL) ground state in a real material is a leading issue in condensed matter physics research. In this pursuit, it is crucial to fully characterize the structure and influence of defects, as these can significantly affect the fragile QSL physics. Here, we perform a variety of cutting-edge synchrotron X-ray scattering and spectroscopy techniques, and we advance new methodologies for site-specific diffraction and L-edge Zn absorption spectroscopy. The experimental results along with our first-principles calculations address outstanding questions about the local and long-range structures of the two leading kagome QSL candidates, Zn-substituted barlowite (Cu3Zn x Cu1-x (OH)6FBr) and herbertsmithite (Cu3Zn(OH)6Cl2). On all length scales probed, there is no evidence that Zn substitutes onto the kagome layers, thereby preserving the QSL physics of the kagome lattice. Our calculations show that antisite disorder is not energetically favorable and is even less favorable in Zn-barlowite compared to herbertsmithite. Site-specific X-ray diffraction measurements of Zn-barlowite reveal that Cu2+ and Zn2+ selectively occupy distinct interlayer sites, in contrast to herbertsmithite. Using the first measured Zn L-edge inelastic X-ray absorption spectra combined with calculations, we discover a systematic correlation between the loss of inversion symmetry from pseudo-octahedral (herbertsmithite) to trigonal prismatic coordination (Zn-barlowite) with the emergence of a new peak. Overall, our measurements suggest that Zn-barlowite has structural advantages over herbertsmithite that make its magnetic properties closer to an ideal QSL candidate: its kagome layers are highly resistant to nonmagnetic defects while the interlayers can accommodate a higher amount of Zn substitution.

  View details for DOI 10.1103/physrevmaterials.4.124406

  View details for PubMedID 34095744

  View details for PubMedCentralID PMC8174140

• Quantum magnetism enabled by chemistry: From quantum spin liquids to topological spin waves Lee, Y. AMER CHEMICAL SOC. 2019

  View details for Web of Science ID 000525061501486

• Fermi surface reconstruction in electron-doped cuprates without antiferromagnetic long-range order. Proceedings of the National Academy of Sciences of the United States of America He, J., Rotundu, C. R., Scheurer, M. S., He, Y., Hashimoto, M., Xu, K., Wang, Y., Huang, E. W., Jia, T., Chen, S., Moritz, B., Lu, D., Lee, Y. S., Devereaux, T. P., Shen, Z. 2019; 116 (9): 3449–53

  Abstract

  Fermi surface (FS) topology is a fundamental property of metals and superconductors. In electron-doped cuprate Nd2-x Ce x CuO4 (NCCO), an unexpected FS reconstruction has been observed in optimal- and overdoped regime (x = 0.15-0.17) by quantum oscillation measurements (QOM). This is all the more puzzling because neutron scattering suggests that the antiferromagnetic (AFM) long-range order, which is believed to reconstruct the FS, vanishes before x = 0.14. To reconcile the conflict, a widely discussed external magnetic-field-induced AFM long-range order in QOM explains the FS reconstruction as an extrinsic property. Here, we report angle-resolved photoemission (ARPES) evidence of FS reconstruction in optimal- and overdoped NCCO. The observed FSs are in quantitative agreement with QOM, suggesting an intrinsic FS reconstruction without field. This reconstructed FS, despite its importance as a basis to understand electron-doped cuprates, cannot be explained under the traditional scheme. Furthermore, the energy gap of the reconstruction decreases rapidly near x = 0.17 like an order parameter, echoing the quantum critical doping in transport. The totality of the data points to a mysterious order between x = 0.14 and 0.17, whose appearance favors the FS reconstruction and disappearance defines the quantum critical doping. A recent topological proposal provides an ansatz for its origin.

  View details for PubMedID 30808739

• Fermi surface reconstruction in electron-doped cuprates without antiferromagnetic long-range order PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA He, J., Rotundu, C. R., Scheurer, M. S., He, Y., Hashimoto, M., Xu, K., Wang, Y., Huang, E. W., Jia, T., Chen, S., Moritz, B., Lu, D., Lee, Y. S., Devereaux, T. P., Shen, Z. 2019; 116 (9): 3449–53

  View details for DOI 10.1073/pnas.1816121116

  View details for Web of Science ID 000459694400024

• Interconnected Signatures of Quantum Spin Liquid Physics Across the Barlowite Family of Quantum Magnets Smaha, R., He, W., Jiang, J., Titus, C., Wen, J., Lee, Y. INT UNION CRYSTALLOGRAPHY. 2019: A122

  View details for DOI 10.1107/S0108767319098775

  View details for Web of Science ID 000549524100123

• Enhancement and destruction of spin-Peierls physics in a one-dimensional quantum magnet under pressure PHYSICAL REVIEW B Rotundu, C. R., Wen, J., He, W., Choi, Y., Haskel, D., Lee, Y. S. 2018; 97 (5)

  View details for DOI 10.1103/PhysRevB.97.054415

  View details for Web of Science ID 000425090200002

• Synthesis dependent properties of barlowite and Zn-substituted barlowite Journal of Solid State Chemistry Smaha, R. W., He, W., Sheckelton, J. P., Wen, J., Lee, Y. S. 2018; 268: 123-129

  View details for DOI 10.1016/j.jssc.2018.08.016

• Infrared phonons as a probe of spin-liquid states in herbertsmithite ZnCu3(OH)(6)Cl-2 JOURNAL OF PHYSICS-CONDENSED MATTER Sushkov, A. B., Jenkins, G. S., Han, T., Lee, Y. S., Drew, H. D. 2017; 29 (9)

  Abstract

  We report on temperature dependence of the infrared reflectivity spectra of a single crystalline herbertsmithite in two polarizations-parallel and perpendicular to the kagome plane of Cu atoms. We observe anomalous broadening of the low frequency phonons possibly caused by fluctuations in the exotic dynamical magnetic order of the spin liquid.

  View details for DOI 10.1088/1361-648X/aa5566

  View details for Web of Science ID 000394595900001

• Correlated impurities and intrinsic spin-liquid physics in the kagome material herbertsmithite PHYSICAL REVIEW B Han, T., Norman, M. R., Wen, J., Rodriguez-Rivera, J. A., Helton, J. S., Broholm, C., Lee, Y. S. 2016; 94 (6)

  View details for DOI 10.1103/PhysRevB.94.060409

  View details for Web of Science ID 000381599300001

• Do quantum spin liquids exist? PHYSICS TODAY Imai, T., Lee, Y. S. 2016; 69 (8): 30-36

  View details for Web of Science ID 000382449500016

• Evidence for a gapped spin-liquid ground state in a kagome Heisenberg antiferromagnet SCIENCE Fu, M., Imai, T., Han, T., Lee, Y. S. 2015; 350 (6261): 655-658

  Abstract

  The kagome Heisenberg antiferromagnet is a leading candidate in the search for a spin system with a quantum spin-liquid ground state. The nature of its ground state remains a matter of active debate. We conducted oxygen-17 single-crystal nuclear magnetic resonance (NMR) measurements of the spin-1/2 kagome lattice in herbertsmithite [ZnCu3(OH)6Cl2], which is known to exhibit a spinon continuum in the spin excitation spectrum. We demonstrated that the intrinsic local spin susceptibility χ(kagome), deduced from the oxygen-17 NMR frequency shift, asymptotes to zero below temperatures of 0.03J, where J ~ 200 kelvin is the copper-copper superexchange interaction. Combined with the magnetic field dependence of χ(kagome) that we observed at low temperatures, these results imply that the kagome Heisenberg antiferromagnet has a spin-liquid ground state with a finite gap.

  View details for DOI 10.1126/science.aab2120

  View details for Web of Science ID 000364162800044

  View details for PubMedID 26542565

• Thermal Hall Effect of Spin Excitations in a Kagome Magnet PHYSICAL REVIEW LETTERS Hirschberger, M., Chisnell, R., Lee, Y. S., Ong, N. P. 2015; 115 (10)

  Abstract

  At low temperatures, the thermal conductivity of spin excitations in a magnetic insulator can exceed that of phonons. However, because they are charge neutral, the spin waves are not expected to display a thermal Hall effect. However, in the kagome lattice, theory predicts that the Berry curvature leads to a thermal Hall conductivity κ_{xy}. Here we report observation of a large κ_{xy} in the kagome magnet Cu(1-3, bdc) which orders magnetically at 1.8 K. The observed κ_{xy} undergoes a remarkable sign reversal with changes in temperature or magnetic field, associated with sign alternation of the Chern flux between magnon bands. The close correlation between κ_{xy} and κ_{xx} firmly precludes a phonon origin for the thermal Hall effect.

  View details for DOI 10.1103/PhysRevLett.115.106603

  View details for Web of Science ID 000360528200005

  View details for PubMedID 26382691