Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session L57: 2D Materials: Metals, Superconductors, and Correlated Materials - 1Focus Live
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Sponsoring Units: DMP Chair: Darius Torchinsky, Temple University |
Wednesday, March 17, 2021 8:00AM - 8:12AM Live |
L57.00001: Visualizing the multifractal wavefunctions of a disordered two-dimensional electron gas Berthold Jaeck, Fabian Zinser, Elio Koenig, Sune N.P Wissing, Anke B. Schmidt, Markus Horst Donath, Klaus Kern, Christian Ast The wavefunctions of a disordered two-dimensional (2D) electron gas near the quantum-critical Anderson transition are predicted to exhibit multifractal scaling in their real space amplitude. We experimentally investigate the appearance of these characteristics in the spatially resolved local density of states (LDOS) of the 2D mixed surface alloy BixPb(1−x)/Ag(111), combining scanning tunneling microscopy (STM) with spin- and angle-resolved inverse-photoemission experiments [1]. Our detailed knowledge of the mixed surface alloy’s electronic band and lattice structure enables us to construct a realistic Anderson tight binding model and to directly compare the calculated LDOS characteristics with those from STM measurements. The analyses of these 2D LDOS maps reveal their log-normal distributions and multifractal scaling characteristics of the underlying wavefunctions with a finite anomalous scaling exponent. Finally, our experimental results confirm predictions of an exact scaling symmetry for Anderson quantum phase transitions in the Wigner-Dyson classes. |
Wednesday, March 17, 2021 8:12AM - 8:24AM Live |
L57.00002: A higher-dimensional electronic modulation in a misfit chalcogenide (Pb1-xSnx)1+δ(TiSe2)2 Yuhki Kohsaka, Masaharu Shirata, Teppei Ueno, Tadashi Machida, Tetsuo Hanaguri, Kaya Kobayashi We show spectroscopic imaging scanning tunneling microscopy of a misfit chalcogenide (Pb1-xSnx)1+δ(TiSe2)2. The crystal structure of this material consists of a rocksalt-type Pb1-xSnxSe double-layer and two hexagonal TiSe2 tri-layers, namely, layered stacking of rectangular and triangular sublattices. Despite this unique crystal structure, the low-energy electronic states of this material have been suggested to resemble those of 1T-TiSe2. As a result of termination-resolved imaging spectroscopy, we find that the coupling between the two sublattices completely suppresses the charge density wave of 1T-TiSe2 and induces a new electronic modulation characterized by a higher-dimensional description. Our finding not only provides an insight into the origin of the charge density wave of 1T-TiSe2 but also suggests that the interplay between hexagonal and tetragonal sublattices is an interesting avenue for realizing novel electronic matters. |
Wednesday, March 17, 2021 8:24AM - 8:36AM Live |
L57.00003: Terahertz response of monolayer and few-layer WTe2 at the nanoscale Ran Jing, Yinming Shao, Zaiyao Fei, Chiu Fan Bowen Lo, Francesco Ruta, John Staunton, Alexander McLeod, Zhiyuan Sun, Bor-Yuan Jiang, Xinzhong Chen, Michael Fogler, Mengkun Liu, David Cobden, Xiaodong Xu, Dimitri N Basov Tungsten ditelluride (WTe2) is a transition metal dichalcogenide whose physical properties depend critically on the number of layers. We use apertureless scattering-type near-field optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to identify distinct THz range electromagnetic behavior of WTe2 mono-, bi- and tri-layer terraces in the same micro-crystals. We observed clear metallic behavior of the near-field signal on tri-layer regions. Our data are consistent with the existence of surface plasmon polaritons (SPP) in the THz range confined to tri-layer terraces in our specimens. The near-field signal on bi-layer regions surprisingly shows moderately metallicity, but with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations considering thermally activated carriers favor the semimetal scenario over the semiconductor scenario for bi-layer WTe2. THz images for monolayer terraces uncovered weakly insulating behavior consistent with transport data. |
Wednesday, March 17, 2021 8:36AM - 8:48AM Live |
L57.00004: Evidence for a Monolayer Excitonic Insulator Yanyu Jia, Pengjie Wang, Cheng Li Chiu, Zhida Song, Guo Yu, Berthold Jaeck, Shiming Lei, Sebastian Klemenz, F Alexandre Cevallos, Michael Onyszczak, Nadezhda Fishchenko, Xiaomeng Liu, Gelareh Farahi, Fang Xie, Yuanfeng Xu, Kenji Watanabe, Takashi Taniguchi, Andrei B Bernevig, Robert Cava, Leslie M Schoop, Ali Yazdani, Sanfeng Wu The excitonic insulator (EI) phase, arising from the spontaneous formation of electron-hole bound states in semimetals or small gap semiconductors, has been predicted for decades but its experimental observation remains elusive. In this talk, we report systematic studies of WTe2 monolayer insulator and uncover evidence supporting this monolayer crystal as an EI. Our experiments on high-quality transport devices reveal the presence of an intrinsic insulating state at the charge neutrality point (CNP) in clean samples. The state exhibits both a strong sensitivity to the electric displacement field and a Hall anomaly that are consistent with the excitonic pairing. We confirm the correlated nature of the charge-neutral insulator by tunneling spectroscopy. The results support the existence of an EI phase in the clean limit and rule out alternative scenarios of a band insulator or a localized insulator. These observations lay the foundation for understanding a new class of correlated insulators with nontrivial topology and identify monolayer WTe2 as a promising candidate for exploring quantum phases of ground-state excitons. |
Wednesday, March 17, 2021 8:48AM - 9:00AM Live |
L57.00005: Photocurrent imaging of Multi-Memristive Charge Density Wave Switching in Two-Dimensional 1T-TaS2 Tarun Patel, Junichi Okamoto, Tina Dekker, Bowen Yang, Jingjing Gao, Xuan Luo, Wenjian Lu, Yuping Sun, Adam W Tsen
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Wednesday, March 17, 2021 9:00AM - 9:12AM Live |
L57.00006: Reentrant Superconductivity in Strong Magnetic Field: The Case for Moir{\'e} Superconductors Gaurav Chaudhary, Michael Ray Norman Based on the experimental Landau fan and Shubnikov-de Haas data of twisted bilayer graphene (tBG) and twisted double bilayer graphene (tDBG), we study a simple continuum model under perpendicular magnetic field and short range attractive electron-electron interactions. We show that the moir{\'e} based superconductors are ideal platform to observe reentrant superconductivity in Landau level regime; a phase long proposed but never observed experimentally. We first generalize the theory of reentrant superconductivity to a multiband case and include the effect of Landau level broadening due to disorder and periodic potential, which best represents the experimental data and then show that the parameter range in tBG and tDBG is ideal to observe reentrant superconductivity. We further comment on experimental implications and other possible systems for this phase. |
Wednesday, March 17, 2021 9:12AM - 9:24AM Live |
L57.00007: Probing the anisotropic properties of low-dimensional purple bronze Na0.9Mo6O17 in its natural cleavage plane Mohammad Ahmadi, Maureen Reedyk The low dimensional molybdenum oxide bronzes exhibit interesting behaviors including superconductivity, charge-density-wave (CDW) states associated with Fermi surface nesting, metal-insulator transitions, and non-linear transport due to sliding of the CDW at low temperature. There is still no firm consensus regarding a theory to explain their unique features. In the purple bronze family A0.9Mo6O17 A=Li is quasi 1-D while A=Na,K are reported to be quasi-2D compounds. We have carefully investigated the low-dimensional properties of sodium purple bronze. Optical spectroscopy measurements showed a clear anisotropy in the crystal's natural cleavage (a-b) plane. The ac- and dc-resistivity measured along the different crystallographic directions showed two distinct metallic axes perpendicular to each other in the a-b plane. These two in-plane metallic axes undergo CDW transitions at different temperatures, which suggests a distinction in electronic and phononic subsystems along each axis. By investigating the optical and electrical properties of both in-plane axes, below and above the Peierls transition temperature, we can gain further insight into the nature of the CDW formation in this compound. |
Wednesday, March 17, 2021 9:24AM - 10:00AM Live |
L57.00008: Imaging Electronic States in Two-Dimensional Materials Invited Speaker: Abhay Narayan Over the past 15 years, the class of two-dimensional materials has emerged as a new playground to realize quantum phenomena. This class of materials began with graphene, but has quickly blossomed to include two-dimensional semiconductors, insulators, topological insulators, superconductors and magnets. Individual atomic layers of these materials can now be assembled nearly at will into unique multilayer structures by the simple process of "putting things on top of other things". The two-dimensional structure of these materials is perfect for exploration by scanned probes, which have played an important role in elucidating their electronic structure and phases. I will discuss some of the recent developments in this field that now allow us to realize many unique electronic structures using basic building blocks that are individual atomic sheets, and the new quantum phases that emerge in these structures. |
Wednesday, March 17, 2021 10:00AM - 10:12AM Live |
L57.00009: Quantum capacitance measurement in monolayer WTe2 Mina Rashetnia, Xiong Huang, Paul Malinowski, Jiun-Haw Chu, Yongtao Cui Monolayer WTe2 has been demonstrated to be a two-dimensional topological insulator with a bulk gap and conducting edge states. However, the mechanism for the bulk gap formation has not been well understood. To address this question, we perform quantum capacitance measurement on monolayer WTe2 devices, using a highly sensitive capacitance bridge based on GaAs high electron mobility transistors (HEMT). The quantum capacitance is related to the density of states in the 2D sample. We will present the gate and magnetic field dependence of the quantum capacitance and discuss its relation with the bulk electronic structure as well as edge conduction in monolayer WTe2. |
Wednesday, March 17, 2021 10:12AM - 10:24AM Live |
L57.00010: Spatial imaging of electron flow in Corbino geometry Chandan Kumar, John Birkbeck, David Perello, Takashi Taniguchi, Kenji Watanabe, Andre Geim, Shahal Ilani, Joseph Sulpizio Electron flow is classified into three distinct transport regimes: diffusive, ballistic and hydrodynamic. Each regime exhibits a unique spatial profile for the potential of flowing electrons. The Corbino disk geometry, in which current is driven from a circular central contact to an outer ring, ideally highlights the differences between these spatial flow profiles. Unlike a channel geometry, the Corbino disk has no edges between the source and drain electrodes, and therefore electrons can only transfer momentum to impurities, phonons, or other electrons. Amazingly, in the hydrodynamic regime - which is dominated by momentum conserving electron-electron interactions- the electric field is predicted to be expelled entirely from the bulk of the disk leading to a spatially flat electrostatic potential, resulting in a lower resistance than the fully ballistic regime. Here, we present our results on imaging the potential profiles in different transport regimes in a graphene Corbino disk. Using a scanning nanotube single-electron transistor as an ultrasensitive charge detector, we spatially image the potential of flowing electrons in corbino disk. By varying the carrier density and temperature, the imaged potential profiles clearly distinguish the different transport regimes. |
Wednesday, March 17, 2021 10:24AM - 10:36AM Live |
L57.00011: Exotic electronic structure of monolayer 1T phase NbSe2 Mengke Liu, Jia Yu, Joshua A Leveillee, Hyunsue Kim, Keji Lai, Feliciano Giustino, Chih-Kang Shih Group V 1T phase transition metal dichalcogenides (TMDs) have received significant interest due to the strong electron correlation. Naturally occurring bulk NbSe2 is a 2H phase, however, recently it is shown that a monolayer 1T polytype can be grown by MBE. In this work, we report a structural phase control of monolayer NbSe2 grown by MBE. Our scanning tunneling microscopy and spectroscopy reveal the exotic electronic structure on monolayer 1T-NbSe2, which is further supported by our density functional theory calculation results. |
Wednesday, March 17, 2021 10:36AM - 10:48AM Live |
L57.00012: Landau Quantization and Highly Mobile Fermions in an Insulator Pengjie Wang, Guo Yu, Yanyu Jia, Michael Onyszczak, Shiming Lei, Sebastian Klemenz, F Alexandre Cevallos, Kenji Watanabe, Takashi Taniguchi, Robert Cava, Leslie M Schoop, Sanfeng Wu In strongly correlated materials, quasiparticle excitations can carry fractional quantum numbers. An intriguing possibility is the formation of fractionalized, charge-neutral fermions, e.g., spinons and fermionic excitons, that result in neutral Fermi surfaces and Landau quantization in an insulator. While previous experiments in quantum spin liquids, topological Kondo insulators, and quantum Hall systems have hinted at charge-neutral Fermi surfaces, evidence for their existence remains far from conclusive. Here we report experimental observation of Landau quantization in a 2D insulator, i.e., monolayer WTe2, a large gap topological insulator. Using a detection scheme that avoids edge contributions, we uncover strikingly large quantum oscillations in the monolayer insulator's magnetoresistance, with an onset field as small as ~ 0.5 tesla. Despite the huge resistance, the oscillation profile, which exhibits many periods, mimics the Shubnikov-de Haas oscillations in metals. Remarkably, at ultralow temperatures the observed oscillations evolve into discrete peaks near 1.6 tesla, above which the Landau quantized regime is fully developed. Our experiments call for further investigation of the highly unusual ground state of the WTe2 monolayer. |
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