Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session U57: Charge Density Waves, Excitonic and Correlated States in Two-Dimensional Materials and SemimetalsFocus
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Sponsoring Units: DMP Chair: Frederic Joucken, University of California, Santa Cruz Room: Mile High Ballroom 3A |
Thursday, March 5, 2020 2:30PM - 2:42PM |
U57.00001: Mott insulating behaviors in few-layer 1T-TaSe2 Siqi Wang, Sui Yang, Kenji Watanabe, Takashi Taniguchi, Kai Rossnagel, Xiang Zhang Material systems with strong electron correlation may exhibit exotic properties in stark contrast to conventional Fermi liquid. Among them, Mott insulators continues to attract tremendous research interest due to its close relation with unconventional superconductors and quantum spin liquids. Recent advances in layered van der Waals materials provide a novel material family which intrinsically has great in-situ tunablility and therefore may act as a versatile quantum simulator for complicated systems. While the low temperature insulating phase with commensurate charge density wave (CCDW) order of the two-dimensional (2D) material 1T-TaS2 has been ascribed to a Mott insulator, recent work casts doubt on this identification by considering the vertical stacking order of 2D CDWs. On the other hand, the CCDW phase in question was reported to be fragile towards mechanical exfoliation, hindering direct characterization in atomic-layer limit. Here we present electrical transport characterization of few-layer 1T-TaSe2, a ‘sibling’ material with simpler phase diagram and more robust CCDW order. Our results provide evidence of Mott insulating behaviors in this long-thought trivial metal, thus hinting at an ideal platform for the investigation of strongly correlated physics in 2D systems. |
Thursday, March 5, 2020 2:42PM - 2:54PM |
U57.00002: Excitonic insulator in MoS2 under pressure Samaneh Ataei, Daniele Varsano, Davide Sangalli, Elisa Molinari, Massimo Rontani Among correlated insulators, a fascinating paradigm applies to narrow-gap semiconductors, whose low-energy excitations are pairs of electron (e) and hole (h) bound by Coulomb attraction, the excitons. If the binding energy overcomes the gap, then excitons spontaneously condense at thermodynamic equilibrium—similarly to Cooper pairs in a superconductor—giving rise to the ‘excitonic insulator’ (EI). Crucially, breakthrough reports [1] of the EI in transition metal dichalcogenides could not assess at which extent the condensation of eh pairs was due to the formation of bound excitons or to the softening of the phonon responsible of the observed structural change. Here, by means of many-body perturbation theory from first principles, we demonstrate that MoS2 at high pressure is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular. The self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an in-plane anti-ferroelectric texture. At the onset of the EI phase, those optical phonons that share the exciton momentum are folded to the zone center, providing a unique Raman fingerprint that has been observed but not explained yet. |
Thursday, March 5, 2020 2:54PM - 3:06PM |
U57.00003: Geometric Frustration in a Monolayer TMD Alloy Mehmet Dogan, Amin Azizi, Jeffrey D. Cain, Rahmatollah Eskandari, Xuanze Yu, Emily C Glazer, Alex Zettl, Marvin L Cohen Geometric frustration occurs when the constituent interactions of a system cannot be simultaneously satisfied due to the underlying geometry. In an idealized frustrated system, long-range order is prevented, and degenerate ground-states with short-range order are observed, leading to extensive entropy at 0 K. In real materials, higher-order effects can lead to relieving of frustration, allowing long-range order to appear. Here, we directly observe frustrated ordering of the atomic species in a monolayer transition metal dichalcogenide (TMD) alloy, using scanning transmission electron microscopy (STEM). We find that this system is analogous to the 2D Ising model of antiferromagnetic spins in a triangular lattice, and long-range order is thoroughly suppressed. Using density functional theory, we build a lattice model that closely matches the statistical properties of the experimental atomic distribution. We also demonstrate the effects of atomic ordering on the band structure. We predict that the degree of ordering can be controlled by the growth temperature, allowing the tuning of the electronic, optical and thermal properties. |
Thursday, March 5, 2020 3:06PM - 3:18PM |
U57.00004: van der Waals tunneling spectroscopy of WTe2 monolayer Yanyu Jia, Pengjie Wang, Guo Yu, Michael Onyszczak, F. Alexandre Cevallos, Shiming Lei, Sebastian Klemenz, Berthold Jaeck, Kenji Watanabe, Takashi Taniguchi, Leslie Schoop, Robert J. Cava, Ali Yazdani, Sanfeng Wu The discovery of superconducting and quantum spin Hall (QSH) states in the single layer WTe2 opens up new possibilities of engineering correlated topological quantum matter based on 2D crystals. In this talk, I will introduce a tunneling device geometry based on van der Waals heterostructures to perform spectroscopic studies of monolayer WTe2. The visualization of gate-induced changes in the monolayer’s electronic structure will be discussed. |
Thursday, March 5, 2020 3:18PM - 3:30PM |
U57.00005: Anisotropic conductivity in monolayer WTe2 Bosong Sun, Paul Malinowski, Zaiyao Fei, Wenjin Zhao, Tauno Palomaki, Xiong Huang, Elliott Runburg, Yongtao Cui, Jiun-Haw Chu, Xiaodong Xu, David Cobden The layered semimetal WTe2 behaves in the monolayer limit as a topological insulator but the nature of the insulating state in the interior bulk is unclear. We study its conductivity as a function of gate doping, temperature, and current direction. Care is needed to eliminate helical edge conduction from the measurements, including along cracks, which we locate by microwave impedance microscopy. The conductivity is found to be highly anisotropic for hole doping and much less so for electron doping. Surprisingly, for hole doping the conductivity is about three times lower along the a-axis, the direction of the tungsten chains, than along the b-axis, perpendicular to the chains. We consider the implications of this observation for the possibility that the state is a kind of excitonic insulator. |
Thursday, March 5, 2020 3:30PM - 3:42PM |
U57.00006: Probing correlated states in WTe2 devices Pengjie Wang, Guo Yu, Yanyu Jia, Michael Onyszczak, Shiming Lei, Sebastian Klemenz, F. Alexandre Cevallos, Kenji Watanabe, Takashi Taniguchi, Robert J. Cava, Leslie Schoop, Sanfeng Wu Interesting behaviors, such as quantum spin Hall effect, superconductivity, non-linear Hall effect, and ferroelectricity have been observed in atomically thin tungsten ditelluride (WTe2). We introduce a new device geometry to further probe the intrinsic electronic response of two-dimensional WTe2. In this talk, we will report our recent transport studies on these new devices, in search of exotic correlated states. |
Thursday, March 5, 2020 3:42PM - 4:18PM |
U57.00007: Monopole Superconductivity and Density-Wave Order in Weyl Semi-metals Invited Speaker: Yi Li We introduce a novel topological class of superconducting and density-wave states which exhibit monopole harmonic symmetry. This so-called “monopole harmonic order” can be realized in interacting Weyl semimetal materials. Generally, when two Fermi surfaces carry different Chern numbers, the many-body ordering between these two Fermi surfaces can exhibit a non-trivial Berry phase inherited from band structure topology. In contrast to other known many-body order, the total vorticity of the ordering gap nodes in momentum space is determined by the monopole charge of the pair Berry phase and is independent of specific ordering mechanisms. Connections to experimental realizations and signatures of monopole harmonic superconductivity and monopole harmonic charge-density-wave states are proposed. |
Thursday, March 5, 2020 4:18PM - 4:30PM |
U57.00008: Layer-dependent charge density wave in few layer VSe2 films Byoung Ki Choi, Ganbat Duvjir, Søren Ulstrup, In Hak Lee, Hyuk Jin Kim, Ly Thi Trinh, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Jungdae Kim, Young Jun Chang VSe2 monolayer (ML) has recently attracted attention about charge ordering property in two-dimensional limit. Researches have been discussing the origin of charge ordering characteristic of VSe2 ML and there is an inconsistent argument. We have reported two types of charge-ordering transitions in VSe2 ML on graphene substrates. One is enhanced charge density wave (CDW) originated from bulk VSe2 and one is metal-insulator transition (MIT) caused by interface effect between graphene and VSe2 ML. In this presentation, we present layer dependent CDW properties in VSe2 1ML, 2ML and 3ML on graphene substrates. We will show depressed CDW effects with increasing number of layers. We will also discuss the MIT property of VSe2 monolayer changing substrates and implications of interface effect between substrate and VSe2 ML. |
Thursday, March 5, 2020 4:30PM - 4:42PM |
U57.00009: Evidence of charge density wave with anisotropic gap in monolayer VTe2 film Yuan Wang, Junhai Ren, Jiaheng Li, Yujia Wang, Huining Peng, Pu Yu, Wenhui Duan, Shuyun Zhou We report experimental evidence of charge density wave (CDW) transition in monolayer 1T-VTe2 film. Low energy electron diffraction (LEED) measurements reveal 4×4 reconstruction peaks below transition temperature. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal arc-like pockets with anisotropic CDW gaps up to 50 meV. The anisotropic CDW gap is attributed to the imperfect nesting of the CDW wave vector, and first-principles calculations reveal phonon softening at the same vector. These results suggest the important roles of both Fermi surface nesting and electron-phonon interaction in the CDW mechanism of monolayer VTe2 film. |
Thursday, March 5, 2020 4:42PM - 4:54PM |
U57.00010: Ultrafast Electron Diffraction in Charge Density Wave State of TiSe2 Paul Xhori, Anton Anikin, Jacob Bolduc, Shalin Patel, Maher Harb, Goran Karapetrov We probe the ultrafast laser response in thin TiSe2 single crystals below and above the CDW transition temperature using ultrafast electron diffraction. The electron-phonon dynamics is initiated by 150 fs pulses centered at 400 nm. While we observe a comparably fast initial diffraction intensity decay at room temperature, we do not observe any bi-exponential dynamics that was previously reported in the dynamics of <110> peaks at fluences above 1 mJ/cm2 with 800 nm centered excitation. On the other hand, at temperatures below the CDW transition temperature bi-exponential dynamics is observed above the threshold fluence of 0.5 mJ/cm2. This threshold fluence is more than two times larger than the threshold fluence for CDW melting with an 800 nm pump. Bi-exponential behaviour is linked to a strong electron-phonon coupling process followed by a slower electron-lattice equilibration. This shows the importance of the Jahn-Teller mechanism for charge density wave formation in TiSe2. We will also present ultrafast electron diffraction results on CuxTiSe2 single crystals. |
Thursday, March 5, 2020 4:54PM - 5:06PM |
U57.00011: The current-induced metastable CDW heterostructure in 2D 1T-TaS2 Masaro Yoshida, Takuro Sato, Fumitaka Kagawa, Yoshihiro Iwasa The layered 1T-TaS2 is a showcase of electronic phases including complex charge density waves (CDWs), superconductivity, Mott insulator, and spin liquid. In addition to such thermodynamically stable phases, thermally inaccessible, persistent metastable metallic states can be induced by means of current [e.g.1]. We investigated the current-induced phase transition to the persistent metastable state by performing noise spectroscopy measurements [2]. We observed the emergence of a broadband noise at the transition, indicating the sliding motion of CDWs. Surprisingly, the dynamical property of the CDW was found to be preserved after the injection of current, although the electronic properties are significantly modified by the injection. It is likely that the CDW sheets slide to change their 3D structure, the stacking way, followed by the considerable change in electronic structure. We suggest that after the current injection, the CDW sheets stack in a metastable way that yields metallic electronic structure. Our discovery emphasizes the importance of the stacking degree of freedom, the concept of Van der Waals heterostructure, to create exotic electronic states in 2D material. |
Thursday, March 5, 2020 5:06PM - 5:18PM |
U57.00012: Doping a Mott insulator at the lateral junctions of single layer 1H-1T NbSe2 Lian Li, Huimin Zhang, Liwei Liu, Zhuozhi Ge, Michael Weinert At the single layer limit, transition metal dichalcogenides can adopt a variety of structural polymorphs with significantly different electronic properties. Here we report the selective growth of single layer 1T- and 1H-NbSe2 on epitaxial graphene/SiC substrates by molecular beam epitaxy. Using a combination of scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we show that the 1H-NbSe2 is metallic and exhibits a 3 by 3 charge density wave (CDW) at below 20K, while 1T-NbSe2 is a Mott-insulator exhibiting a root 13 by root 13 CDW at room temperature. Furthermore, at the 1H-1T lateral junction, we observe a one-dimensional channel that is characterized by a V-shaped gap in tunneling spectroscopy. The spectra can be fitted by power law, similar to other substitutionally doped Mott insulators. Those finding suggests that lateral heterojunctions of different transition metal dichalcogenide polymorphs present an opportunity to engineer new quantum phases with emergent properties. |
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U57.00013: Infrared spectroscopy of few-layer excitonic insulator candidate Ta2NiSe5 Chaoyu Song, Chong Wang, Shenyang Huang, hugen Yan Ta2NiSe5 is proposed as an excitonic insulator, which has narrow direct bandgap and high transition temperature. In this talk, we will present our infrared spectroscopy study of few-layer Ta2NiSe5. In addition to the large anisotropic absorption peak at about 0.4 eV reported by many previous studies, a weak but clear excitation was newly found at lower frequency. It is possibly a signature of the electron correlation or from another electronic transition. |
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