Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session K21: Moire Beyond Graphene I |
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Sponsoring Units: DCMP Chair: Mit Naik, University of California, Berkeley Room: Room 213 |
Tuesday, March 7, 2023 3:00PM - 3:12PM |
K21.00001: Topological and magnetic states in AB-stacked transition metal dichalcogenide moiré bilayers Naichao Hu, Nemin Wei, Yongxin Zeng, Chao Lei, Allan H MacDonald Narrowband transition metal dichalcogenide heterobilayer moiré materials with hole filling factor u=1 have Mott insulating ground states with three-sublattice magnetic order. Recent experiments have shown that a surprising quantum anomalous Hall appears when the displacement field is close to the value at which the sense of layer polarization changes. We have performed mean-field theory calculations to determine whether or not this behavior can be explained in terms of broken time-reversal states with spontaneous layer polarization that appear when the highest energy valence subbands of the two layers are close to degeneracy. This scenario requires either a first-order phase transition between the quantum anomalous Hall state and the ordinary Mott insulator states on either side when the displacement field is varied, or a displacement field at which the gap vanishes. We will discuss the phase diagram predicted by this scenario over the full range of displacement field needed to reverse the sense of layer polarization, and compare with other proposals for the origin of the quantum anomalous Hall states. |
Tuesday, March 7, 2023 3:12PM - 3:24PM |
K21.00002: Twisted network model for emergent moire sliding Luttinger liquid Yichen Hu, Biao Lian Recent experiments on twisted WTe2 materials have revealed the existence of novel anisotropic non-Fermi liquid physics. Modeled after the hole-doped regime with quasi one-dimensional chains of Tungsten atoms, we present a two-dimensional network model consisting of two arrays of quantum wires aligned with small angles. Using transfer matrix method, by varying twisting angles, we show how nearly flat bands emerges and their contributions to effective sliding Luttinger liquid physics. |
Tuesday, March 7, 2023 3:24PM - 3:36PM |
K21.00003: Valley physics in twisted bilayer BC3 Toshikaze Kariyado Systems where atomically thin materials are artificially assembled attract huge attention as a platform to explore exotic phenomena. Especially, in a system with slight mismatches between adjacent layers, there arises a nanoscale moiré pattern, which is regarded as a key to manipulate electronic band structure. Then, a proposal for a new stacking design leading to novel phenomena is now highly demanded. |
Tuesday, March 7, 2023 3:36PM - 3:48PM |
K21.00004: Coupling of excitons and electric polarization in a moiré TMD homobilayer William G Holtzmann, Eric Anderson, Feng-ren Fan, Takashi Taniguchi, Kenji Watanabe, Wang Yao, Xiaodong Xu Transition metal dichalcogenide (TMD) moiré systems host a variety of strongly correlated electronic states and display novel light-matter interactions. TMD hetero- and homo-bilayers have distinct crystal symmetries, with homobilayers exhibiting higher symmetry at the expense of an indirect bandgap in most cases. However, twisted MoTe2 (tMoTe2) retains its direct bandgap and strong optical response. Using various optical probes, we show intertwined doping and electric field effects on excitons in tMoTe2 arising from control over the layer symmetry. We also report on optical signatures of the crossover regime between the layer-degenerate and layer-polarized states of the system. This study establishes tMoTe2 system as a promising system to explore excitonic physics within a highly tunable layer-degenerate moiré system. |
Tuesday, March 7, 2023 3:48PM - 4:00PM |
K21.00005: Dipole Ladders with Giant Hubbard U in a Moiré Exciton Lattice Heonjoon Park, Jiayi Zhu, Xi Wang, Yingqi Wang, William G Holtzmann, Kyle Hwangbo, Takashi Taniguchi, Kenji Watanabe, Jiaqiang Yan, Liang Fu, Ting Cao, Di Xiao, Daniel R Gamelin, Hongyi Yu, Wang Yao, Xiaodong Xu Two-dimensional semiconductor moiré superlattices have emerged as a powerful platform for engineering correlated electronic phenomena. Moreover, optical excitation creates charge neutral interlayer excitons with an out-of-plane electric dipole. Strong onsite dipole-dipole interaction promises the formation of correlated bosonic states, akin to the Mott states of electrons, but its observation remains elusive. Here, we report a giant exciton Hubbard U and consequent dipole ladders with spin- and electron-filling dependence in WSe2/WS2 moiré superlattices. By measuring interlayer exciton photoluminescence as a function of excitation intensity, we identify successive new peaks emerge with an energy separation of ~34 meV above the ground state. This corresponds to the sequential injection of excitons into a single site with an energy cost to overcome the remarkably large exciton Hubbard U, forming a dipole ladder. By measuring time-resolved photoluminescence, we also find that the interlayer exciton decay dynamics display a sequential cascaded decay down the dipole ladder. Based on findings of local magnetic moments at two holes per moiré cell, we show that excitons can also fill a second moiré orbital, establishing the two-orbital nature of the moiré potential landscape. Our results pave the way for investigating the Bose-Hubbard model with possible exciton crystal phases in interacting opto-moiré quantum matter. |
Tuesday, March 7, 2023 4:00PM - 4:12PM |
K21.00006: Excitonic insulator in semiconducting TMD moire heterojunction (Part 2) Xiong Huang, Zhen Lian, Dongxue Chen, Ying Su, Mina Rashetnia, Mark Blei, Sefaattin Tongay, Yan Li, Lei Ma, Dmitry Smirnov, Li Xiang, Zenghui Wang, Chuanwei Zhang, Yongtao Cui, Sufei Shi, Takashi Taniguchi, Kenji Watanabe Here we report the observation of an excitonic insulator, a correlated state with strongly bound electrons and holes, in an angle-aligned 1L-WS2/2L-WSe2 moiré superlattice. The moiré coupling induces a flat miniband on the valence-band side only in the 1st WSe2 layer interfacing WS2, barely affecting the 2nd WSe2 layer. The electrostatically introduced holes first fill this miniband and form a Mott insulator with a total carrier density corresponding to one hole per moiré supercell. By applying a vertical electric field, the valence band in the 2nd WSe2 layer can be tuned to overlap with the moiré miniband in the 1st WSe2 layer, realizing the coexistence of electrons and holes at equilibrium, as confirmed by optical spectroscopy measurements. The electron-hole pairs are bound as excitons due to a strong Coulomb interaction. We use microwave impedance microscopy to confirm the insulating nature of this exotic state in a dual-gate device geometry with monolayer graphene as top-gate. The microwave detection of embedded moiré heterojunction is enabled by the reduced conductivity in the graphene top-gate with applied magnetic field. This insulating state has also been observed in a back-gate only device geometry with finite electric field during hole doping. The excitonic insulator has a transition temperature as high as 90 K. Our study demonstrates a moiré system for the study of correlated many-body physics in two dimensions. |
Tuesday, March 7, 2023 4:12PM - 4:24PM |
K21.00007: Excitonic insulator in semiconducting TMD moiré heterojunction (Part 1) Zhen Lian, Dongxue Chen, Xiong Huang, Ying Su, Mina Rashetnia, Lei Ma, Li Yan, Mark Blei, Li Xiang, Sefaattin Tongay, Zenghui Wang, Chuanwei Zhang, Dmitry Smirnov, Yongtao Cui, Sufei Shi The moiré superlattices of 2D semiconductors exhibit many novel physics phenomena including moiré excitons and correlated insulating states. In this work, we report the evidence of an excitonic insulator in angle-aligned 1L/2L WS2/WSe2 moiré heterostructure. First, we investigate reflectance contrast spectra of WS2/WSe2 moiré heterostructure with varying WSe2 layer number. The layer degree of freedom strongly modifies the resonant energies of the moiré excitons. In addition, the 1L/2L and 1L/3L WS2/WSe2 show an extra excitonic peak close to the original WSe2 A-exciton resonance, a result of the interfacial nature of the moiré coupling. Further, we show that the valence bands of the 1st and the 2nd WSe2 layer in 1L/2L WS2/WSe2 can be tuned to overlap by applying electric field. Holes can partially populate the 2nd WSe2 layer at a total filling of one hole per moiré supercell, which is evidenced by the emergence of exciton-polarons from the 2nd WSe2 layer. The abrupt changes of peak positions and intensities of moiré excitons suggest that the system is in an insulating state at n=-1. From particle-hole transformation, electron-like carriers reside in the moiré miniband of the 1st WSe2 layer and bound with holes in the 2nd WSe2 layer due to strong Coulomb interaction, forming an excitonic insulator phase. |
Tuesday, March 7, 2023 4:24PM - 4:36PM |
K21.00008: Proximity-induced moiré exciton in a transition metal dichalcogenide monolayer Sudipta Kundu, Felipe H da Jornada Moiré patterns in two-dimensional materials, formed due to a lattice mismatch or a relative rotation between the layers, have gained research interest as a platform to study exotic electronic and excitonic phases. The moiré potential responsible for the emergent phenomena is induced by the atomic reconstructions and the variation of interlayer interaction upon creating a moiré pattern. Using first-principles calculations, we explore a noninvasive way to induce a spatially varying moiré potential in the target monolayer transition metal dichalcogenide (TMD) by placing it on another multilayer material with domains of alternative electric polarization, for example, a parallel-stacked bilayer hexagonal boron nitride with a small interlayer twist. Our first-principles calculations reveal novel excitons with spatial localization and excited-state series that differ from those possible in regular stackings of twisted bilayer TMDs and suggest an experimental route for the engineering of tunable moiré potential post material synthesis. |
Tuesday, March 7, 2023 4:36PM - 4:48PM |
K21.00009: Nano-Raman and photoluminescence imaging of moiré domains in twisted TMDs Thomas P Darlington, Yinjie Guo, Emanuil S Yanev, Xuehao Wu, James C Hone, Abhay N Pasupathy, Cory R Dean, James Schuck When monolayer transition metal dichalcogenides are vertically stacked, lattice mismatch and non-zero lead to the formation of moiré superlattices. This imposed periodic potential modifies the optoelectronic landscape, leading to the formation of novel phononic and excitonic states. For twisted TMDs, visualization of the moiré domains has typically been done with scanning probe imaging techniques such as atomic force or scanning tunneling microscopy, or electron microscopy, each of which reveal substantial heterogeneity in low twist angle homo and hetero-bilayers due to intrinsic strain and lattice reconstruction. However, because of the diffraction limit, this variation can cofound experimental comparisons with optical probes. In the presentation I will show nano-Raman and nano-photoluminescence imaging of reconstructed twisted TMD bilayers. Our results so a strong optical intensity in the Raman and PL signals from the moiré domain walls. Comparing with AFM imaging, our results show exact agreement of with boundaries of the moiré cells. Further, nano-Raman spectroscopy of the domain walls provides a direct optical of the relative strain. |
Tuesday, March 7, 2023 4:48PM - 5:00PM |
K21.00010: Network model for periodically strained graphene Christophe De Beule, Eugene J Mele, Phong T Vo The long-wavelength physics of monolayer graphene in the presence of periodic strain fields has a |
Tuesday, March 7, 2023 5:00PM - 5:12PM |
K21.00011: Intrinsic spin-orbit torque-induced magnetic switching in a moiré Chern magnet Evgeny Redekop, Charles L Tschirhart, Trevor B Arp, Lizhong Li, Tingxin Li, Shengwei Jiang, Owen I Sheekey, Takashi Taniguchi, Kenji Watanabe, Kin Fai Mak, Jie Shan, Andrea Young Spin-transfer torques and spin-orbit torques (SOT) are two well-developed techniques for generating magnetic memories in which magnetic structure is controlled by applied electrical currents. Both methods utilize multilayer device geometries in which electrically actuated spin currents in one layer drive the switching of a magnetic ground state in the other. Here, we present an intrinsic spin-orbit torque-induced magnetic memory arising in a single two dimensional layer from the interplay of spontaneous magnetism and Berry curvature. We study AB-stacked MoTe2/WSe2, which supports a Chern magnet state and shows current-driven hysteretic switching of its resistance above a threshold current [1]. Using a nanoscale superconducting quantum interference (nanoSQUID-on-tip) device, we establish that the switching observed in transport is the direct consequence of magnetic domain reversals occurring through an intrinsic analog of a SOT mechanism. The lack of an internal material interface and the presence of high fidelity current-to-spin conversion let this system achieve record switching current density of 10^3 A/cm^2. |
Tuesday, March 7, 2023 5:12PM - 5:24PM |
K21.00012: Tunable Magnetic Interaction in Homobilayer Moiré TMDs Eric Anderson, Xiaodong Xu, Wang Yao, Feng-ren Fan, Takashi Taniguchi, Kenji Watanabe Transition metal dichalcogenide hetero- and homobilayer moiré systems have established themselves as a versatile platform for exploring Hubbard model physics. Their constituent monolayers are notable for a single spin-valley locked pseudospin degree of freedom. However, heterobilayer moiré systems intrinsically break the layer degree of freedom, and most TMDs transition from direct to indirect bandgap semiconductors in the bilayer, complicating homobilayer moiré physics. Here, we employ magneto-optical measurements to probe a near-AA stacked homobilayer MoTe2 moiré system. Doping dependent photoluminescence reveals electric field tunable correlated states, and demonstrates the retention of a direct bandgap at the K and K’ points in bilayer. Additional magnetic circular dichroism measurements show signatures of tunable magnetic interactions. The behavior demonstrated via this suite of optical probes establishes moiré MoTe2 as a promising platform for observing correlated and topological physics. |
Tuesday, March 7, 2023 5:24PM - 5:36PM |
K21.00013: Mott insulator states in twisted MoSe2 homobilayer Zhida Liu, Xiaohui Liu, Di Huang, Yue Ni, Jiamin Quan, Chih-Kang Shih, Yongxin Zeng, Allan H MacDonald, Takashi Taniguchi, Kenji Watanabe, Xiaoqin Elaine Li Moirè superlattices formed by twisted transitional metal dichalcogenide (TMD) layers have been proven an ideal system to study strongly correlated states. Twisted homobilayers exhibit unique properties when the twisted layers are near either R or H stacking as their electronic bands are distinct. Here, we report optical spectroscopy studies of excitons in twisted MoSe2 homobilayers in a device with dual gating. In doping dependent photoluminescence and absorption measurements, we found signatures of Mott insulator states. Such correlated states are high tunable in moirè superlattices. |
Tuesday, March 7, 2023 5:36PM - 5:48PM |
K21.00014: Topological phase transition from periodic edge states in moiré superlattices Haonan Wang, Li Yang Topological mosaic pattern can be formed in two-dimensional moiré superlattices, where periodic edge states are located at the boundary between topologically trivial/nontrivial domains. We construct a minimized continuum model to describe these edge states and find that these edge states can be captured by $p_xpm ip_y$ orbitals on the honeycomb lattice, in which an effective atomic spin-orbit coupling (SOC) emerges. As a result, twisting angle will alter the effective SOC strength and drives an overall topological phase transition. Our work reveals a moiré-induced effective SOC mechanism and provides a phase diagram to manipulate the local and global topological properties in moiré systems. |
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