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"Criticality of Two-Dimensional Disordered Dirac Fermions in the Unitary Class and Universality of the Integer Quantum Hall Transition" — Björn Sbierski: Elizabeth J. Dresselhaus, Joel E. Moore, and Ilya A. Gruzberg; Physical Review Letters, 02/18/21.
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Elizabeth J. Dresselhaus, Joel E. Moore, and Ilya A. Gruzberg
Abstract
Two-dimensional (2D) Dirac fermions are a central paradigm of modern condensed matter physics, describing low-energy excitations in graphene, in certain classes of superconductors, and on surfaces of 3D topological insulators. At zero energy E=0, Dirac fermions with mass m are band insulators, with the Chern number jumping by unity at m=0. This observation lead Ludwig et al. [Phys. Rev. B 50, 7526 (1994)] to conjecture that the transition in 2D disordered Dirac fermions (DDF) and the integer quantum Hall transition (IQHT) are controlled by the same fixed point and possess the same universal critical properties. Given the far-reaching implications for the emerging field of the quantum anomalous Hall effect, modern condensed matter physics, and our general understanding of disordered critical points, it is surprising that this conjecture has never been tested numerically. Here, we report the results of extensive numerics on the phase diagram and criticality of 2D DDF in the unitary class. We find a critical line at m=0, with an energy-dependent localization length exponent. At large energies, our results for the DDF are consistent with state-of-the-art numerical results νIQH=2.56–2.62 from models of the IQHT. At E=0, however, we obtain ν0=2.30–2.36 incompatible with νIQH. This result challenges conjectured relations between different models of the IQHT, and several interpretations are discussed.
"Universal behavior of the bosonic metallic ground state in a two-dimensional superconductor" — Zhuoyu Chen: Bai Yang Wang, Adrian G. Swartz, Hyeok Yoon, Yasuyuki Hikita, Srinivas Raghu & Harold Y. Hwang ; npj Quantum Materials, 02/17/21.
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Bai Yang Wang, Adrian G. Swartz, Hyeok Yoon, Yasuyuki Hikita, Srinivas Raghu & Harold Y. Hwang
Abstract
Anomalous metallic behavior, marked by a saturating finite resistivity much lower than the Drude estimate, has been observed in a wide range of two-dimensional superconductors. Utilizing the electrostatically gated LaAlO3/SrTiO3 interface as a versatile platform for superconductor-metal quantum phase transitions, we probe variations in the gate, magnetic field, and temperature to construct a phase diagram crossing from superconductor, anomalous metal, vortex liquid, to the Drude metal state, combining longitudinal and Hall resistivity measurements. We find that the anomalous metal phases induced by gating and magnetic field, although differing in symmetry, are connected in the phase diagram and exhibit similar magnetic field response approaching zero temperature. Namely, within a finite regime of the anomalous metal state, the longitudinal resistivity linearly depends on the field while the Hall resistivity diminishes, indicating an emergent particle-hole symmetry. The universal behavior highlights the uniqueness of the quantum bosonic metallic state, distinct from bosonic insulators and vortex liquids.
"Doping Quantum Spin Liquids on the Kagome Lattice" — Cheng Peng : Yi‐Fan Jiang, Dong‐Ning Sheng , Hong‐Chen Jiang; Advanced Quantum Technologies, 01/28/21.
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Yi‐Fan Jiang, Dong‐Ning Sheng , Hong‐Chen Jiang
Abstract
The authors review recent density‐matrix renormalization group studies of lightly doped quantum spin liquids (QSLs) on the kagome lattice. While a number of distinct conducting phases, including high temperature superconductivity, have been theoretically anticipated, a tendency toward fractionalized insulating charge‐density‐wave (CDW) states is instead found. In agreement with earlier work [Jiang, Devereaux, and Kivelson, Phys. Rev. Lett. 2017, 119, 067002], results for the t‐J model reveal that, starting from a fully gapped QSL, light doping leads to CDW long‐range order with a pattern that depends on lattice geometry and doping concentration such that there is one doped‐hole per CDW unit cell, while the spin–spin correlations remain short‐ranged. Alternatively, this state can be viewed as a stripe crystal or Wigner crystal of spinless holons, rather than of doped holes. From here, by studying generalized versions of the t‐J model, these results are extended to light doping of other types of QSLs, including critical and chiral QSLs. These results suggest that doping these QSLs also leads to insulating states with long‐range CDW order. While the superconducting correlations are short‐ranged, they can be significantly enhanced by second‐neighbor electron hopping. The relevance of these numerical results to Kagome materials is also discussed.
"An Anode-Free Zn–MnO2 Battery" — Yunpei Zhu: Yi Cui, Husam N. Alshareef ; Nano Letters, 01/20/21.
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Yi Cui, Husam N. Alshareef
Abstract
Aqueous Zn-based batteries are attractive because of the low cost and high theoretical capacity of the Zn metal anode. However, the Zn-based batteries developed so far utilize an excess amount of Zn (i.e., thick Zn metal anode), which decreases the energy density of the whole battery. Herein, we demonstrate an anode-free design (i.e., zero-excess Zn), which is enabled by employing a nanocarbon nucleation layer. Electrochemical studies show that this design allows for uniform Zn electrodeposition with high efficiency and stability over a range of current densities and plating capacities. Using this anode-free configuration, we showcase a Zn–MnO2 battery prototype, showing 68.2% capacity retention after 80 cycles. Our anode-free design opens a new direction for implementing aqueous Zn-based batteries in energy storage systems.

"Carrier-specific dynamics in 2H-MoTe2 observed by femtosecond soft x-ray absorption spectroscopy using an x-ray free-electron laser" — Alexander Britz: Andrew R. Attar, Xiang Zhang, Hung-Tzu Chang, Clara Nyby, Aravind Krishnamoorthy, Sang Han Park, Soonnam Kwon, Minseok Kim, Dennis Nordlund, Sami Sainio, Tony F. Heinz, Stephen R. Leone, Aaron M. Lindenberg, Aiichiro Nakano, Pulickel Ajayan, Priya Vashishta, David Fritz, Ming-Fu Lin, Uwe Bergmann; Structural Dynamics, 01/13/21.
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Andrew R. Attar, Xiang Zhang, Hung-Tzu Chang, Clara Nyby, Aravind Krishnamoorthy, Sang Han Park, Soonnam Kwon, Minseok Kim, Dennis Nordlund, Sami Sainio, Tony F. Heinz, Stephen R. Leone, Aaron M. Lindenberg, Aiichiro Nakano, Pulickel Ajayan, Priya Vashishta, David Fritz, Ming-Fu Lin, Uwe Bergmann
Abstract
Femtosecond carrier dynamics in layered 2H-MoTe2 semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe2, the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d5/2 core level (M5-edge, 572–577eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M5-edge involving partially occupied states of the conduction band. A 400 ±110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the TeM5 edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1–2 ps timescale, which is interpreted as electron–hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.
"Doping evolution of the Mott–Hubbard landscape in infinite-layer nickelates" — Berit H. Goodge: Danfeng Li, Kyuho Lee, Motoki Osada, Bai Yang Wang, George A. Sawatzky,Harold Y. Hwang, and Lena F. Kourkoutis; Proceedings of the National Academy of Sciences, 01/12/21.
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Danfeng Li, Kyuho Lee, Motoki Osada, Bai Yang Wang, George A. Sawatzky,Harold Y. Hwang, and Lena F. Kourkoutis
Abstract
The recent observation of superconductivity in Nd0.8Sr0.2NiO2 has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott–Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally resolved electron energy-loss spectroscopy to directly probe the Mott–Hubbard character of Nd1−xSrxNiO2. Upon doping, we observe emergent hybridization reminiscent of the Zhang–Rice singlet via the oxygen-projected states, modification of the Nd 5d states, and the systematic evolution of Ni 3d hybridization and filling. These experimental data provide direct evidence for the multiband electronic structure of the superconducting infinite-layer nickelates, particularly via the effects of hole doping on not only the oxygen but also nickel and rare-earth bands.
"Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis" — Christoph Baeumer: Jiang Li, Qiyang Lu, Allen Yu-Lun Liang, Lei Jin, Henrique Perin Martins, Tomáš Duchoň, Maria Glöß, Sabrina M. Gericke, Marcus A. Wohlgemuth, Margret Giesen, Emily E. Penn, Regina Dittmann, Felix Gunkel, Rainer Waser, Michal Bajdich, Slavomír Nemšák, J. Tyler Mefford & William C. Chueh ; Nature Materials, 01/11/21.
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Jiang Li, Qiyang Lu, Allen Yu-Lun Liang, Lei Jin, Henrique Perin Martins, Tomáš Duchoň, Maria Glöß, Sabrina M. Gericke, Marcus A. Wohlgemuth, Margret Giesen, Emily E. Penn, Regina Dittmann, Felix Gunkel, Rainer Waser, Michal Bajdich, Slavomír Nemšák, J. Tyler Mefford & William C. Chueh
Abstract
Structure–activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO3 epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150 mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO2 surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations.
"Isotropic Pauli-limited superconductivity in the infinite-layer nickelate Nd0.775Sr0.225NiO2" — Bai Yang Wang: Danfeng Li, Berit H. Goodge, Kyuho Lee, Motoki Osada, Shannon P. Harvey, Lena F. Kourkoutis, Malcolm R. Beasley & Harold Y. Hwang; Nature Physics, 01/04/21.
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Danfeng Li, Berit H. Goodge, Kyuho Lee, Motoki Osada, Shannon P. Harvey, Lena F. Kourkoutis, Malcolm R. Beasley & Harold Y. Hwang
Abstract
The recent observation of superconductivity in thin-film infinite-layer nickelates1,2,3 offers a different angle from which to investigate superconductivity in layered oxides4. A wide range of candidate models have been proposed5,6,7,8,9,10, which emphasize single- or multi-orbital electronic structure, Kondo or Hund’s coupling and analogies to cuprates. Further experimental characterization of the superconducting state is needed to develop a full understanding of the nickelates. Here we use magnetotransport measurements to probe the superconducting anisotropy in Nd0.775Sr0.225NiO2. We find that the upper critical field is surprisingly isotropic at low temperatures despite the layered crystal structure. In a magnetic field, the superconductivity is strongly Pauli-limited, such that the paramagnetic effect dominates over orbital de-pairing. Underlying this isotropic response is a substantial anisotropy in the superconducting coherence length, which is at least four times longer in-plane than out-of-plane. A prominent low-temperature upturn in the upper critical field indicates the presence of an unconventional ground state.
"Visualization of dynamic polaronic strain fields in hybrid lead halide perovskites" — Burak Guzelturk: Thomas Winkler, Tim W. J. Van de Goor, Matthew D. Smith, Sean A. Bourelle, Sascha Feldmann, Mariano Trigo, Samuel W. Teitelbaum, Hans-Georg Steinrück, Gilberto A. de la Pena, Roberto Alonso-Mori, Diling Zhu, Takahiro Sato, Hemamala I. Karunadasa, Michael F. Toney, Felix Deschler & Aaron M. Lindenberg; Nature Materials, 01/04/21.
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Thomas Winkler, Tim W. J. Van de Goor, Matthew D. Smith, Sean A. Bourelle, Sascha Feldmann, Mariano Trigo, Samuel W. Teitelbaum, Hans-Georg Steinrück, Gilberto A. de la Pena, Roberto Alonso-Mori, Diling Zhu, Takahiro Sato, Hemamala I. Karunadasa, Michael F. Toney, Felix Deschler & Aaron M. Lindenberg
Abstract
Excitation localization involving dynamic nanoscale distortions is a central aspect of photocatalysis1, quantum materials2 and molecular optoelectronics3. Experimental characterization of such distortions requires techniques sensitive to the formation of point-defect-like local structural rearrangements in real time. Here, we visualize excitation-induced strain fields in a prototypical member of the lead halide perovskites4 via femtosecond resolution diffuse X-ray scattering measurements. This enables momentum-resolved phonon spectroscopy of the locally distorted structure and reveals radially expanding nanometre-scale strain fields associated with the formation and relaxation of polarons in photoexcited perovskites. Quantitative estimates of the magnitude and shape of this polaronic distortion are obtained, providing direct insights into the dynamic structural distortions that occur in these materials5,6,7,8,9. Optical pump–probe reflection spectroscopy corroborates these results and shows how these large polaronic distortions transiently modify the carrier effective mass, providing a unified picture of the coupled structural and electronic dynamics that underlie the optoelectronic functionality of the hybrid perovskites.
"Time-resolved RIXS experiment with pulse-by-pulse parallel readout data collection using X-ray free electron laser" — H Lu: A. Gauthier, M. Hepting, A. S. Tremsin, A. H. Reid, P. S. Kirchmann, Z. X. Shen, T. P. Devereaux, Y. C. Shao, X. Feng, G. Coslovich, Z. Hussain, G. L. Dakovski, Y. D. Chuang & W. S. Lee; Scientific Reports, 12/17/20.
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A. Gauthier, M. Hepting, A. S. Tremsin, A. H. Reid, P. S. Kirchmann, Z. X. Shen, T. P. Devereaux, Y. C. Shao, X. Feng, G. Coslovich, Z. Hussain, G. L. Dakovski, Y. D. Chuang & W. S. Lee
Abstract
Time-resolved resonant inelastic X-ray scattering (RIXS) is one of the developing techniques enabled by the advent of X-ray free electron laser (FEL). It is important to evaluate how the FEL jitter, which is inherent in the self-amplified spontaneous emission process, influences the RIXS measurement. Here, we use a microchannel plate (MCP) based Timepix soft X-ray detector to conduct a time-resolved RIXS measurement at the Ti L3-edge on a charge-density-wave material TiSe2 . The fast parallel Timepix readout and single photon sensitivity enable pulse-by-pulse data acquisition and analysis. Due to the FEL jitter, low detection efficiency of spectrometer, and low quantum yield of RIXS process, we find that less than 2% of the X-ray FEL pulses produce signals, preventing acquiring sufficient data statistics while maintaining temporal and energy resolution in this measurement. These limitations can be mitigated by using future X-ray FELs with high repetition rates, approaching MHz such as the European XFEL in Germany and LCLS-II in the USA, as well as by utilizing advanced detectors, such as the prototype used in this study.