Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W55: Topological and Correlated States in Twisted Transition Metal DichalcogenidesRecordings Available

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Sponsoring Units: DCMP Chair: Luis Ruiz Pestana, University of Miami Room: Hyatt Regency Hotel Adler 
Thursday, March 17, 2022 3:00PM  3:12PM 
W55.00001: Correlated States in Bilayer WSe_{2}/WS_{2} Moiré Heterostructures Yinjie Guo, Jordan Pack, Qianhui Shi, Yusong Bai, Song Liu, Kenji Watanabe, Takashi Taniguchi, Xiaoyang Zhu, James C Hone, Cory R Dean Twisted transition metal dichalcogenides heterostructures provide a platform for studying strong electron interactions since flat bands can be introduced by the moiré pattern in a system. Dozens of correlated insulating states have been observed in transition metal dichalcogenide moiré heterostructures. Here, we will present transport and capacitance measurements in bilayer WSe_{2}/WS_{2 }structures. The density of the two layers in natural bilayer WSe2 can be controlled independently in a dual gate device. Therefore, a twocomponent moiré system could be established in such structures which might give rise to interesting phenomenon such as exciton condensate in zero field. 
Thursday, March 17, 2022 3:12PM  3:24PM 
W55.00002: Quantum melting of charge ordered states in Transition Metal Dichalcogenide moire systems Yiqing Zhou, Donna Sheng, EunAh Kim TMD moire systems form a platform for rich interplays between geometric frustration, strong correlation, and quantum fluctuation. Recent experiments [1] at half band filling presented a tantalizing observation of continuous metalinsulator transition (MIT) driven by quantum fluctuation instead of disorder. Motivated by these experiments, we have recently carried out DMRG calculations at 1/2 filling [2] and found a chiral spin liquid phase accompanies the MIT. Here, we turn to fractional filling away from halffilling, where one expects classical chargeordered states driven by strong furtherrange interactions[3]. Specifically, we study the quantum melting of the chargeordered states at 1/3 fillings under the interplay of the strong correlations and the quantum fluctuations using DMRG. 
Thursday, March 17, 2022 3:24PM  3:36PM Withdrawn 
W55.00003: Spintextured Chern bands in ABstacked transition metal dichalcogenide bilayers Yang Zhang, Trithep Devakul, Liang Fu While transitionmetal dichalcogenide (TMD)–based moire materials have been shown to host various correlated electronic phenomena, topological states have not been experimentally observed until now. In this work, using firstprinciple calculations and continuum modeling, we reveal the displacement field–induced topological moire bands ´ in ABstacked TMD heterobilayer MoTe2/WSe2. Valleycontrasting Chern bands with nontrivial spin texture are formed from interlayer hybridization between MoTe2 and WSe2 bands of nominally opposite spins. Our study establishes a recipe for creating topological bands in ABstacked TMD bilayers in general, which provides a highly tunable platform for realizing quantumspin Hall and interactioninduced quantum anomalous Hall effects. 
Thursday, March 17, 2022 3:36PM  3:48PM 
W55.00004: OneDimensional Luttinger Liquids in a TwoDimensional Moiré Lattice Pengjie Wang, Guo Yu, Yves H Kwan, Yanyu Jia, Shiming Lei, Sebastian Klemenz, F. Alexandre Cevallos, Trithep Devakul, Kenji Watanabe, Takashi Taniguchi, Shivaji Sondhi, Robert J Cava, Leslie M Schoop, Siddharth A Parameswaran, Sanfeng Wu The Luttinger liquid (LL) model of onedimensional (1D) electronic systems provides a powerful tool for understanding strongly correlated physics including phenomena such as spincharge separation. Substantial theoretical efforts have attempted to extend the LL phenomenology to two dimensions (2D), especially in models of closely packed perfect arrays of 1D quantum wires, each being described as a LL. For instance, such coupledwire models have been successfully used to construct 2D anisotropic nonFermi liquids, various quantum Hall states, topological phases, and quantum spin liquids. Despite these exciting theoretical developments, an experimental demonstration of highquality arrays of 1D LLs suitable for realizing these models remains absent. In this talk, I will present our recent experimental realization of 2D arrays of 1D LLs with crystalline quality in a moiré superlattice made of twisted bilayer tungsten ditelluride (tWTe_{2}). Originating from the anisotropic lattice of the monolayer, the moiré pattern of tWTe_{2} hosts identical, parallel 1D electronic channels, separated by a fixed nanoscale distance, which is tunable by the twist angle between layers. At a twist angle of ~ 5 degrees, we find that holedoped tWTe_{2} exhibits exceptionally large transport anisotropy with a resistance ratio of ~ 1000, suggesting the formation of 1D channels. The conductance measurement reveals a powerlaw scaling behavior, consistent with the formation of a 2D anisotropic phase that resembles an array of LLs. Our results open the door for realizing a variety of 2D correlated and topological quantum phases based on coupledwire models and LL physics. 
Thursday, March 17, 2022 3:48PM  4:00PM 
W55.00005: Characterization and Quantum Transport of SmallAngle Twisted Bilayer WTe_{2} Guo Yu, Pengjie Wang, Yanyu Jia, Xin Gui, Ratnadwip Singha, Kenji Watanabe, Takashi Taniguchi, Leslie M Schoop, Robert J Cava, Sanfeng Wu Monolayer WTe_{2} hosts various interesting quantum states, including superconductivity, quantum spin Hall states and a novel insulating state with excitonic pairing. Recent experiments have further shown that twisted bilayer WTe_{2} (tWTe_{2}, i.e. a stack of two monolayer WTe_{2} at a specific twist angle) provides another platform for observing novel physics that goes beyond what is seen in the monolayer, including a highly anisotropic 2D phase that mimics a Luttinger liquid. The fascinating physics of tWTe_{2} is originated from its rich, highly tunable Moire lattice structure and strong electron correlations. In this talk, I will present our recent progress in the characterization and transport studies of tWTe_{2} devices, with a focus on those with small twist angles. 
Thursday, March 17, 2022 4:00PM  4:12PM Withdrawn 
W55.00006: Correlated Quantum States in a LargeAngle Twisted Bilayer WTe_{2} Yanyu Jia, Pengjie Wang, Yue Tang, Guo Yu, Shiming Lei, Sebastian Klemenz, F Alexandre Cevallos, Ratnadwip Singha, Michael Onyszczak, Kenji Watanabe, Takashi Taniguchi, Robert J Cava, Leslie M Schoop, Sanfeng Wu Recent experiments and theories have revealed compelling evidence that monolayer WTe_{2}, a quantum spin Hall insulator, is further an excitonic insulator at charge neutrality (CNP). Naturally exfoliated bilayer WTe_{2} has also been reported as an insulator at CNP. Twisted bilayer WTe_{2} (tWTe_{2}) at small angles (i.e., near 5 degree) develops novel 2D anisotropic behaviors that mimic the Luttinger liquid phase of a 1D system. The measured conductance of such small angle tWTe_{2} follows a power law, i.e., an apparent insulating behavior when the temperature is reduced. In this talk, I will present quantum transport studies of tWTe_{2} at a specific large twist angle, where the insulating behavior is fully suppressed even at CNP, in contrast to all the previously known monolayer and bilayer samples. I will discuss detailed transport characterization of this new phase at CNP in the tWTe_{2} that arises by suppressing the excitonic insulating phase of the underlying monolayer. 
Thursday, March 17, 2022 4:12PM  4:24PM Withdrawn 
W55.00007: Twistangle effects on the quantum transport in twisted bilayer WTe_{2} devices Yue Tang, Yanyu Jia, Pengjie Wang, Guo Yu, Ratnadwip Singha, Xin Gui, Kenji Watanabe, Takashi Taniguchi, Robert J Cava, Leslie M Schoop, Sanfeng Wu Intriguing novel phenomena in 2dimensional (2D) moiré systems continuously challenge and inspire new understanding of quantum physics in solids. Exploring moiré physics that goes beyond the intensively studied twisted graphene and semiconducting transition metal dichalcogenide systems is promising to create new possibilities. In this talk, I will present and contrast the transport properties of twisted bilayer WTe_{2} (tWTe_{2}) with various interlayer twist angles, a highly interesting system that is largely unexplored so far. The improvement of sample quality and device design is key to the exploration of the intrinsic quantum properties of tWTe_{2} induced by the Moiré pattern. I will describe our effort for producing transport devices with a design that minimizes unwanted strain in the twisted bilayer samples. I will also discuss the strong influence of the twist angle on the electronic phases by comparing samples with different twist angles. 
Thursday, March 17, 2022 4:24PM  4:36PM 
W55.00008: Phase diagram of the moiré Hubbard model: an effective model for twisted tungsten diselenide Alexander Wietek, Jiawei Zang, Jie Wang, Jennifer Cano, Antoine Georges, Andrew J Millis Twisted moiré materials are a novel experimental platform to study strongly correlated electron physics. Twisted bilayer tungsten diselenide has recently been shown to exhibit a variety of fascinating properties. Here, we present results on an effective model of the lowenergy degrees of freedom of this system, called the "moiré" Hubbard model. We establish a coarse phase diagram at halffilling and study the effects of holedoping the parent magnetic orders. We discover stripe ordering along with weak superconductivity. Holebinding is shown to be dependent on the flux induced by the displacement field. Our results have been obtained by combining numerical exact diagonalization with the density matrix renormalization group method. 
Thursday, March 17, 2022 4:36PM  4:48PM 
W55.00009: Magicangle Twisted Bilayer Systems with QuadraticBandTouching: Exactly Flat Bands with HighChern Number MingRui Li, Hong Yao, AiLei He Twisted bilayer morie system, mainly the twisted bilayer graphene and twisted transitionmetaldichalcogenide, have provided a tunable platform to study the strongcorrelation effect in flat band materials. Here we propose a new twisted bilayer of 2D systems that feature quadraticbandtouching (QBT) points, we find that exactly flat bands can emerge at the magic angles and each of them has a high Chern number C=±2 which was not realized before. We further explain the origin of the exactly flat band and derive the first magic angle. Finally, we study the Coulomb interactions effect at the magic angle and obtain the ground state at 1/4 filling which supports the quantum anomalous Hall effect quantized Hall conductivity 2e^{2}/hc. 
Thursday, March 17, 2022 4:48PM  5:00PM 
W55.00010: Topological Phases in ABstacked MoTe_{2}/WSe_{2}: Z_{2} Topological Insulators, Chern Insulators, and Topological Charge Density Waves Haining Pan, Fengcheng Wu, Ming Xie We present a theory on the quantum phase diagram of ABstacked MoTe_{2}/WSe_{2} using a selfconsistent HartreeFock calculation performed in the planewave basis without projection to a lattice model, motivated by the observation of topological states in this system. At filling factor ν=2 (two holes per moiré unit cell), Coulomb interaction stabilizes a Z_{2} topological insulator by opening a charge gap. At ν=1, the interaction induces two competing states, a spin density wave Mott insulator and a valley polarized state, which could undergo a firstorder phase transition tuned by an outofplane displacement field. The valley polarized state becomes a Chern insulator for certain displacement fields. Moreover, we predict a topological charge density wave forming a honeycomb lattice with ferromagnetism at ν=2/3. Possible future directions on this type of moiré bilayers hosting a rich set of quantum phases are discussed. 
Thursday, March 17, 2022 5:00PM  5:12PM 
W55.00011: Nematic excitonic insulator near topological phase transitions in transition metal dichalcogenide moire superlattices Ming Xie It was recently demonstrated that ABstacked MoTe2/WSe2 bilayer superlattices exhibit quantum anomalous Hall effect (QAHE) at the filling of one hole per moire unit cell. The bilayer undergoes a topological phase transition as the displacement field is tuned. However, the gap closing feature typical of such transition is not evident in the experiment. Here, by considering the effect of electronelectron interaction, we identify an excitonic insulator phase that can preempt the topological phase transition. The insulating phase breaks C3 rotational symmetry and is disfavored by the interlayer tunneling term with a nontrivial layerpseudospin structure. We discuss the experimental implications of the close competition between the two phases and its relevance in similar moire superlattices with nontrivial topology. 
Thursday, March 17, 2022 5:12PM  5:24PM 
W55.00012: Spontaneous fractional Chern insulators in transition metal dichalcogenide moiré superlattices Heqiu Li, Umesh Kumar, Kai Sun, Shizeng Lin The Moiré superlattice realized in twodimensional heterostructures offers an exciting platform to access 
Thursday, March 17, 2022 5:24PM  5:36PM 
W55.00013: Currentinduced valley polarization switching in magnetic moiré materials Naichao Hu, Allan H MacDonald Large anomalous Hall conductivities have been observed in both transitionmetal dichalcogenide and graphenebased moiré materials, and in both cases are attributed to spontaneous valley polarization. Because carriers in opposite valleys have opposite Hall conductivities, this type of orbital magnetism will lead to pumping between valleys [1,2] along the edges of currentcarrying Hall bars. We will report on a theory of this effect and its influence on collective valley dynamics and the stability of valleypolarization microstructures. We will also comment on the possible observability of related effects in conventional magnetic conductors. 
Thursday, March 17, 2022 5:36PM  5:48PM 
W55.00014: Massive Dirac fermions in moiré superlattices: a route toward correlated Chern insulators Ying Su, Heqiu Li, Chuanwei Zhang, Kai Sun, Shizeng Lin Strong electronic correlation in flat minibands renders moiré superlattices fascinating for accessing novel quantum states. Recently, the Mott insulator and correlated Chern insulator have been reported to coexist in a heterobilayer of transition metal dichalcogenides (TMD) [1] that provides an ideal platform to investigate the interplay between strong correlation and topology. In this work, we demonstrate a generic mechanism to realize topological moiré minibands by considering a massive Dirac fermion moving in a moiré potential, which can be achieved in the TMD heterobilayer. We take the MoTe2/WSe2 heterobilayer as an example and show that the topological phase can be driven by a vertical electric field due to the lattice corrugation. Thus a correlated Chern insulator can be stabilized by the Coulomb interaction that breaks the timereversal symmetry spontaneously. Our work explains the recent experiment on the observation of Chern insulating state in the ABstacked MoTe2/WSe2 and unveils a general strategy to design topological moiré materials. 
Thursday, March 17, 2022 5:48PM  6:00PM 
W55.00015: Why don't the twisted bilayers of Mo and W based transition metal dichalcogenides behave like regular semiconductors? Priya Mahadevan, Sumanti Patra, Poonam Kumari, Sanjukta Paul The Mo and W based transition metal dichalcogenides have been known for more than sixty years as examples of solid state lubricants, possible because of the weak van der Waals interactions between the layers which allows one layer to easily slide over the other. It is only recently that one finds unusual phenomena arising in them on doping holes via gating into twisted bilayers, an aspect that we would associate with correlated materials. The untwisted limit has been long understood as a regular semiconductor whose properties are well described by band theory. 
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