Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session S55: Electronic Properties of Twisted Transition Metal DichalcogenidesRecordings Available
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Sponsoring Units: DCMP Chair: Ryan Muzzio Room: Hyatt Regency Hotel -Adler |
Thursday, March 17, 2022 8:00AM - 8:12AM |
S55.00001: Moire minibands in twisted MoS2 homobilayers Nikhil M Tilak, Guohong Li, Kenji Watanabe, Takashi Taniguchi, Eva Y Andrei Twisted bilayers of semiconducting Group VI Transition Metal Dichalcogenides are predicted to host multiple moire minibands bands near the Valence Band Maximum (VBM). Due to the absence of sublattice symmetry, twist angles close to 0 degrees and those near 60 degrees are inequivalent and are predicted to present substantial differences due to differences in lattice relaxation. We study twisted homobilayers of MoS2 at various twist angles using Scanning Tunneling Microscopy and Spectroscopy. STM topography reveals a triangular moire pattern while spectroscopy shows additional peaks near the VBM which are absent in monolayers or Bernal bilayers. Stacking dependent spectra reveal a periodic modulation of the band gap which enables us to estimate the strength of the moiré potential. We also discuss preliminary electrostatic gating experiments on these devices. |
Thursday, March 17, 2022 8:12AM - 8:24AM |
S55.00002: Magic angles for topological surface states in twisted heterostructures Aaron P Dunbrack, Jennifer Cano Twisted bilayer graphene has been a center of recent theoretical and experimental interest due to the emergence of magic angles and the resulting tunable interaction-driven phases induced in the flat bands. Motivated by these results, we study a related class of systems: twisted interfaces between Dirac materials in which one layer is the surface state of a topological insulator (TI). Using both perturbation theory and global calculations of the band structure, we study two specific scenarios: the interface between two 3D TIs and the interface between graphene and a 3D TI. We derive the conditions to realize a magic angle where the velocity of the TI surface state vanishes. However, in the simplest models, flat bands only result if the interlayer hopping includes spin-flipping terms. We discuss the possibility of realizing magic angles in spite of this constraint. |
Thursday, March 17, 2022 8:24AM - 8:36AM |
S55.00003: STM spectroscopy measurements of R-type and H-type twisted MoSe2/WSe2 heterostructures Rachel L Nieken, Anna Roche, Fateme Mahdikhanysarvejahany, Takashi Taniguchi, Kenji Watanabe, Micheal R Koehler, David G Mandrus, John Schaibley, Brian J LeRoy When semiconducting transition metal dichalcogenides heterostructures are stacked, the twist angle and lattice mismatch leads to a periodic moiré potential. As the angle between the layers changes, so do the electronic properties. As the angle approaches 0- or 60-degrees, interesting characteristics and properties occur. Here we report low temperature scanning tunneling microscopy and spectroscopy measurements on the band gaps and band modulations in MoSe2/WSe2 heterostructures with near 0 degree rotation (R-type) and near 60 degree rotation (H-type). We show that the modulation of the band edge for the R-type is isolated to the valence band while the modulation for H-type occurs at both the conduction and valence band edges. The modulation in the bands and the band gaps for the R-type are overall larger than that of the H-type. Local density of states images reveal that electrons are localized differently at the valence band and conduction band edges of the H-type device. |
Thursday, March 17, 2022 8:36AM - 8:48AM |
S55.00004: ARPES and STM study on twisted transition metal dichalcogenides Ding Pei, Binbin Wang, Zishu Zhou, Zhihai He, Liheng An, Shanmei He, Cheng Chen, Yiwei Li, Zhongkai Liu, Jianpeng Liu, Hongming Weng, Ning Wang, Jiamin Xue, Yulin Chen Twisted van der Waals heterostructures have recently been proposed as a condensed-matter platform for realizing controllable quantum models due to the low-energy moiré bands with specific charge distributions in moiré superlattices. Here, using angle-resolved photoemission spectroscopy with sub-micron spatial resolution (μ-ARPES) and scanning tunneling microscopy (STM), we performed a systematic investigation on the electronic structure of twisted bilayer WSe2. Consistent evidence for moiré bands in both real and momentum space have been observed, which confirms twisted transition metal dichalcogenides as a versatile platform to investigate strongly correlated physics. |
Thursday, March 17, 2022 8:48AM - 9:00AM |
S55.00005: Fractal energy gaps and topological invariants in hBN/graphene/hBN double moiré systems Hiroki Oka, Mikito Koshino We calculate the electronic structure in quasiperiodic double moiré systems of graphene sandwiched by hexagonal boron nitride (hBN) and identify the topological integers of energy gaps. We find that the electronic spectrum contains a number of minigaps, and they exhibit a recursive fractal structure similar to the Hofstadter butterfly when plotted against the twist angle. Each of the energy gaps can be characterized by a set of six integers, which are associated with an area in momentum space. The corresponding area is geometrically interpreted as a quasi-Brillouin zone, which is a polygon enclosed by multiple Bragg planes of the composite periods and can be uniquely specified by the plain wave projection in the weak-potential limit. These sets of integers can be expressed by the second Chern numbers by using a formal mapping to 4D quantum Hall effect. |
Thursday, March 17, 2022 9:00AM - 9:12AM |
S55.00006: First-principles calculations of minimally twisted MoSe2/WSe2 bilayers Madeleine Phillips, C. Stephen Hellberg We present density functional theory (DFT) calculations of MoSe2/WSe2 bilayers twisted a small angle (~ 3o-5o) away from the commensurate 2H stacking. As the twist angle decreases, our calculations show the emergence of flat bands in both conduction and valence bands. The degeneracies of the flat bands suggest their origins in features of the 2H band structure. We also analyze the atomic relaxation of the twisted MoSe2/WSe2 bilayer away from the rigid moiré structure. While full reconstruction is not achieved at the angles we study, we begin to see the increase in area of regions of low energy stacking. We show that the states associated with the conduction and valence band flat bands are localized in different regions of local stacking. The localization of states is pronounced even though reconstruction is incomplete. |
Thursday, March 17, 2022 9:12AM - 9:24AM |
S55.00007: Interlayer exciton structure and moiré-localization revealed by time- and angle-resolved photoemission spectroscopy Ouri Karni, Elyse Barre, Vivek Pareek, Michael K Man, Johnathan D Georgaras, Chakradhar Sahoo, David R Bacon, Xing Zhu, Henrique B Ribeiro, Aidan O'Beirne, Jenny Hu, Abdullah Al-Mahboob, Mohamed M M. M. Abdelrasoul, Nicholas S Chan, Arka Karmakar, Andrew Winchester, Bumho Kim, Kenji Watanabe, Takashi Taniguchi, Katayun Barmak, Julien Madeo, Felipe H da Jornada, Tony F Heinz, Keshav M Dani Heterobilayers made by stacking two monolayers of transition-metal dichalcogenide semiconductors feature a moiré pattern that spatially modulates the electronic band-structure, and with it the energies of interlayer excitons (ILXs) – electron-hole pairs bound across the interlayer gap. It is thus expected to confine ILXs in the moiré potential wells. In this talk, I will present a quantitative analysis of this effect in the MoS2/WSe2 heterobilayer based on time- and angle-resolved photoemission spectroscopy measurements that reveal separately momentum-images of the electron and hole components of the ILX. Aided by theoretical analysis of the photoemission signatures, we extract the real-space structure of the ILX wavefunction, including both its size and its surprisingly strong center-of-mass localization by the moiré potential. This information is fundamental to understanding interactions between moiré-localized ILXs and their potential as single-photon emitters. |
Thursday, March 17, 2022 9:24AM - 9:36AM |
S55.00008: Flat topological bands in twisted van der Waals homobilayers Haonan Wang, Li Yang We construct a continuum model Hamiltonian to describe electronic structures of Moiré superlattices made by twisted van der Waals homobilayers. The calculated band structures exhibit isolated flat bands at a small twist angle. We further find that the different interlayer couplings from different stacking regions facilitate the emergence of topological phases. Finally, we obtain the phase diagram of these isolated bands by tuning the twist angle, staggered layer potential, and magnetic field, which make the twisted homobilayers a promising platform for realizing rich topological phases. |
Thursday, March 17, 2022 9:36AM - 9:48AM |
S55.00009: Interplay between structural and excited-state properties of twisted TMDC moiré heterostructures Felipe H da Jornada The synthesis of quasi-two-dimensional materials, such as monolayer transition metal dichalcogenides (TMDCs), opened the door to studying new classes of systems with nanoscale dimensionality confinement and weak electronic screening, leading to strongly enhanced electron interactions. More recently, there has been a growing interest in studying the electronic and optical properties in heterobilayers of such materials, which support moiré structures and interlayer excitons. However, the interplay between the structural and excited-state properties of such materials is poorly understood, often relying on empirically fitted continuum models. In this talk, we present results obtained from recent formalisms and methods we developed to bridge these two phenomena. First, derive the time-resolved angle-resolved photoemission spectroscopy (TR-ARPES) signature of moiré excitons. Our calculations are in good agreement with recent experiments, and show that moiré excitons can be localized to a surprising extent in the moiré cell, being well-localized even for relatively large twist angles (~2°) – associated with a moiré lattice parameter of ~ 6 nm. We also comment on a new method to extract accurate moiré potentials from first principles, and show why previous ab initio approaches often yield potential depths that are significantly smaller than those deduced from experiment. We also show how such realistically deduced potentials lead to moiré-modulated interlayer excitons with qualitatively different characters. |
Thursday, March 17, 2022 9:48AM - 10:00AM |
S55.00010: Minibands in Short-Wavelength Graphene/WSe2 Moiré Superlattices Eli Gerber, Saien Xie, Brendan D Faeth, Yanhao Tang, Lizhong Li, Christopher T Parzyck, Debanjan Chowdhury, Yahui Zhang, Christopher Jozwiak, Aaron Bostwick, Eli Rotenberg, Jie Shan, Kin Fai Mak, Eun-Ah Kim, Kyle M Shen Moiré superlattices comprised of stacked two-dimensional materials present a versatile platform for engineering and investigating new emergent quantum states of matter. At present, the vast majority of investigated systems have long moiré wavelengths, but investigating these effects at shorter, incommensurate wavelengths remains a challenge. How can the electronic features of the superlattice be described when the continuum limit is no longer applicable? Do minibands still form in such short wavelength systems? We demonstrate that a relatively simple tight-binding model treating the effect of the moiré superlattice as an imposed, external potential can accurately describe the electronic structure of new short-wavelength minibands, and that our findings are supported by ab initio calculations. We present our results in tandem with angle-resolved photoemission spectroscopy (ARPES) measurements studying a short moiré wavelength of 0.5 nm for twisted graphene/WSe2, in which distinct minibands formed by the moiré potential are observed despite the sub-nanometer moiré wavelength. |
Thursday, March 17, 2022 10:00AM - 10:12AM |
S55.00011: Theoretical study of twisted bilayer Bi2(Te,Se)3 Ikuma Tateishi, Motoaki Hirayama Recently, Moiré superlattice systems, such as twisted bilayer graphene, have been very actively studied. |
Thursday, March 17, 2022 10:12AM - 10:24AM |
S55.00012: Imaging 2D Wigner Crystals Hongyuan Li, Shaowei Li, Emma Regan, Danqing Wang, Wenyu Zhao, Salman Kahn, Kentaro Yumigeta, Mark Blei, Takashi Taniguchi, Kenji Watanabe, Sefaattin Tongay, Alex K Zettl, Michael F Crommie, feng wang The Wigner crystal has fascinated condensed matter physicists for nearly 90 years. Direct observation of the 2D Wigner crystal lattice in real space, however, has remained an outstanding challenge. Conventional scanning tunneling microscopy (STM) has sufficient spatial resolution but induces perturbations that can potentially alter this fragile state. Here we demonstrate real-space imaging of 2D Wigner crystals in WSe2/WS2 moiré heterostructures using a specially designed non-invasive STM spectroscopy technique that employs a graphene sensing layer. Different Wigner crystal lattice configurations at fractional electron fillings of n = 1/3, 1/2, and 2/3, where n is the electron number per site, are directly visualized. Particularly, the n = 1/2 state spontaneously breaks the original C3 symmetry and forms a stripe phase. Our study lays a solid foundation for understanding Wigner crystal states in moiré heterostructures and provides an approach that is generally applicable for imaging novel correlated electron lattices in other systems. |
Thursday, March 17, 2022 10:24AM - 10:36AM |
S55.00013: PSHG vs 4D STEM imaging of twist angle in 2D transition metal dichalcogenide bilayers Emmanuel Stratakis, S Psilodimitrakopoulos, A Orekhov, Leonidas Mouchliadis, Daen Jannis, Giorgos Maragkakis, G Kourmoulakis, Nicolas Gauquelin, George Kioseoglou, Johan Verbeeck Atomically thin two-dimensional materials can be vertically stacked with van der Waals bonds that enable interlayer coupling. In the particular case of transition metal dichalcogenide (TMD) bilayers, the relative orientation between the two monolayers, i.e., the twist-angle, modifies the crystal symmetry and creates a superlattice with exciting properties. In this work, we demonstrate an all-optical, high resolution method for pixel-by-pixel mapping of the twist-angle, via polarization-resolved second harmonic generation (PSHG) microscopy and compare it with four-dimensional scanning transmission electron microscopy (4D STEM). It is found that the twist-angle imaging of WS2 bilayers, using the PSHG technique is in excellent agreement with that obtained using electron diffraction. The main advantages of the nonlinear optical method are that the characterization is performed on the same substrate that the bilayer is created on and that it is three orders of magnitude faster than the 4D STEM. |
Thursday, March 17, 2022 10:36AM - 10:48AM |
S55.00014: Feshbach resonances and polaron physics in bilayer semiconductors Clemens Kuhlenkamp, Michael Knap, Marcel Wagner, Richard Schmidt, Atac Imamoglu Bilayer structures of two dimensional materials have been proven to be invaluable platforms to study correlated quantum matter. Semiconductors such as transition metal dichalcogenides (TMDs) are particularly interesting, as they host strongly interacting electrons as well as tightly bound composite bosons in the form of excitons. In this talk we show how scattering between excitons and electrons can be enhanced in bilayer TMDs, by using a solid-state analogue of a Feshbach resonance. Similar to its counterpart in ultracold atomic systems, this resonance allows for precise control of interactions, which facilitates the realization of strongly coupled Bose-Fermi mixtures. As a first step we analyze the properties of a Fermi polaron which forms when a single exciton is resonantly coupled to a Fermi sea of electrons. Experimental signatures and opportunities for the generation of degenerate Bose-Fermi mixtures are also discussed. |
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