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 T21: Moire Beyond Graphene II |
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Sponsoring Units: DCMP Chair: Ziyan Zhu, Stanford University Room: Room 213 |
Thursday, March 9, 2023 11:30AM - 11:42AM |
T21.00001: Analysis of Moiré Phonons in 2D Twisted Bilayers Daniel T Larson, Jonathan Z Lu, Ziyan Zhu, Mattia Angeli, Efthimios Kaxiras Understanding the phonon structure of two-dimensional (2D) twisted bilayers is important both for interpreting experimental |
Thursday, March 9, 2023 11:42AM - 11:54AM |
T21.00002: Imaging In-plane Charge-transfer Excitons in a twisted-WS2 Moiré Superlattice Hongyuan Li, Ziyu Xiang, Mit H Naik, Steven G Louie, Michael F Crommie, feng wang The transition-metal dichalcogenides (TMD) moire superlattice provides a versatile platform to study novel quantum phases, such as strongly correlated behavior and nontrivial topological phases. Exotic elementary excitations such as moire excitons emerge in such TMDC moire superlattices, which has been detected by various optical spectroscopic measurements. The moire potential can significantly modulate how the excited electron and hole bond with each other, while its microscopic nature remains unclear. Previous works mostly attributed it to the Umklapp scattering of the Wannier exciton7-10, while recent studies pointed out that moire potentials could even modify the internal structure of the excitons and yield a new type of in-plane charge transfer (ICT) excitons, where the bound electrons and holes are spatially separated. In this work, we performed a microscopic study of the ICT exciton in a twisted WS2 (t-WS2) moire superlattice combining nanoscale photocurrent measurement and ab initio calculations. Combining laser excitation and scanning tunneling microscopy (STM), photocurrent spectroscopy and mapping with sub-moire spatial resolution were performed. Because of the charge separation in the ICT excitons, the STM tip can selectively tunnel the electron or hole within the exciton depending on its spatial position. Our photocurrent map shows a spatial oscillating photocurrent polarity, providing strong evidence for the existence of ICT exciton in this t-WS2 moire superlattice. This result agrees perfectly with ab initio calculated ICT exciton wavefunction. By investigating the response of the oscillating photocurrent under an in-plane electric field, we further confirm the bound nature of the ICT excitons and rule out the potential electron-hole plasma. |
Thursday, March 9, 2023 11:54AM - 12:06PM |
T21.00003: Strong Correlated States in WSe2/WS2 Moiré Superlattices Yinjie Guo, Jordan Pack, Qianhui Shi, Yusong Bai, Song Liu, Jiaqiang Yan, Kenji Watanabe, Takashi Taniguchi, David G Mandrus, Xiaoyang Zhu, James C Hone, Cory R Dean Transition metal dichalcogenide (TMD) moiré systems provide a highly tunable platform to study strong electronic correlated states. Recently, fractional filling states have been observed in WSe2/WS2 moiré superlattices with unconventional optical techniques, scanning tunneling microscopy (STM) and scanning microwave impedance microscopy (MIM). However, few electrical-transport and capacitance studies have been reported on this system due to the large contact resistance that exists at metal-TMD junctions. Here, we performed measurements on aligned WSe2/WS2 heterostructures. Our study provides a new technique to explore long-range strong interactions in TMD moiré superlattices. |
Thursday, March 9, 2023 12:06PM - 12:18PM |
T21.00004: Moiré phonon in graphene/hexagonal boron nitride moiré superlattice Lukas P. A. Krisna, Mikito Koshino We theoretically study in-plane acoustic phonons of graphene/hexagonal boron nitride moiré superlattice by using a continuum model. The original phonon bands of individual layers are found to be strongly hybridized and reconstructed into moiré phonon bands consisting of dispersive bands and flat bands. The phonon band structure can be effectively described by a spring-mass network model to simulate the motion of moiré domain walls, where the flat-band modes are interpreted as vibrations of independent, decoupled strings. |
Thursday, March 9, 2023 12:18PM - 12:30PM Author not Attending |
T21.00005: Hybridized Excitons in WS2/MoSe2 Heterobilayers Visualized by Time- and Angle-Resolved Photoemission Spectroscopy Alice Kunin, Zachary H Withers, Sergey Chernov, Jin Bakalis, Ziling Li, Shuyu Cheng, Gerd Schönhense, Xu Du, Roland K Kawakami, Thomas K Allison Heterostructures consisting of stacked, twisted bilayers of transition metal dichalcogenides (TMDs) are a unique platform to achieve tunable optoelectronic properties. The twist angle yields a moiré superlattice that spatially modulates the electronic band structure and leads to the formation of interlayer excitons, in which the electron and hole of the exciton occupy different TMD layers. In WS2/MoSe2 twisted bilayers, the conduction bands align nearly degenerately, leading to hybridization between intralayer and interlayer excitons. Here, we present the momentum-resolved ultrafast dynamics of hybrid exciton states in a near-60-degree twisted WS2/MoSe2 heterobilayer. Using time- and angle-resolved photoemission spectroscopy, we directly visualize the lifetimes and energetics of both intralayer and hybridized exciton states in momentum space. We also characterize the temperature- and exciton density-dependence of these dynamics, as well as the interplay of the formation of intervalley momentum-forbidden dark excitons. Our results provide important insights into the effects of the moiré superlattice and band alignment on the ultrafast exciton dynamics. |
Thursday, March 9, 2023 12:30PM - 12:42PM |
T21.00006: Tunable spin and valley excitations of correlated insulators in Γ-valley moiré bands Benjamin Foutty, Jiachen Yu, Trithep Devakul, Carlos R Kometter, Yang Zhang, Kenji Watanabe, Takashi Taniguchi, Liang Fu, Ben Feldman Moiré superlattices formed from transition metal dichalcogenides (TMDs) have been shown to support a variety of quantum electronic phases that are highly tunable using applied electromagnetic fields. In this talk, I will describe a new approach to engineer and study correlations within Γ-valley moiré bands in twisted double bilayer WSe2. Through a combination of local and global electronic compressibility measurements, we identify charge-ordered phases at multiple integer and fractional moiré band fillings ν. By measuring the magnetic field dependence of their energy gaps and the chemical potential upon doping, we reveal spin-polarized ground states with novel spin polaron quasiparticle excitations. In addition, an applied displacement field allows us to realize a new mechanism of metal-insulator transition at ν = -1 driven by tuning between Γ- and K-valley moiré bands. Together, our results demonstrate control over both the spin and valley character of the correlated ground and excited states in this system. |
Thursday, March 9, 2023 12:42PM - 12:54PM |
T21.00007: Electronic and Excitonic Behaviors in WSe2/WS2 Hetero-bilayers as a Function of Moiré Wavelength Birui Yang, William G Holtzmann, Xinyi Xu, Song Liu, Takashi Taniguchi, Kenji Watanabe, James C Hone, James Schuck, Xiaodong Xu, Cory R Dean Moiré patterning of Transitional metal dichalcogenide van der Waals heterostructures provides a way to engineer tunable flat bands that host correlated electronic and excitonic phases. We fabricated “continuously twisted” hetero-bilayers by integrating large area WS2 flakes together with narrow WSe2 ribbons that are bent by a nano-manipulator. Fabrication of heterostructures with continuous variation of twisted angle and moiré wavelength, not only makes a precise control of the twisted angle achievable, but also provides a platform to study the behaviors of electrons and excitons in WSe2/WS2 hetero-bilayers as a function of the moiré wavelength of the superlattice. |
Thursday, March 9, 2023 12:54PM - 1:06PM |
T21.00008: Domain Walls stretching in Graphite Moiré by Scanning Tunneling Microscope based deformation Nirjhar Sarkar, Prabhakar R Bandaru, Robert C Dynes Pressure-induced devices have recently become an interesting tunable parameter for exploring unconventional material properties. In this work, we merge the idea of a long-forgotten deformation technique using Scanning Tunneling Microscopy tip to deform with angstrom resolution the two-dimensional layered surfaces. This helps to image and probe in-situ the gradual nano-scale change in surface structures and consequent properties that cannot be observed in conventional methods of pressure-induced devices. One of the strange behaviors observed was the lateral surface expansion of one-dimensional domain walls of graphitic moire patterns. This showed that the term "domain wall" (between Bernal and rhombohedral graphite domains) is neither a domain nor a wall and thus a misnomer. |
Thursday, March 9, 2023 1:06PM - 1:18PM |
T21.00009: Visualizing the tunable electronic structure of twisted transition metal dichalcogenide heterobilayers Ryan Muzzio, Alfred Jones, Paulina E Majchrzak, Henrique P Martins, Simranjeet Singh, Christopher Jozwiak, Aaron Bostwick, Eli Rotenberg, Philip Hofmann, Søren Ulstrup, Jyoti Katoch Two dimensional (2D) materials are a wonderful template to explore novel quantum phenomena in the ultra thin limit. The addition of a twist angle between the stacked materials, such as two graphene layers twisted with respect to each other, produces a moiré lattice which can lead to drastic ahnges in the composite’s physical properties[1,2,3,4]. Beyond graphene, twist-angle dependent bilayer heterostructures of transition metal dichalcogenides (TMDCs) also display extraordinary novel moire physics[5,6]. In this presentation, we will discuss our nano- and micro-focused angle resolved photoemission spectroscopy (ARPES) studies performed on twisted bilayer TMDC systems. We domeonstrate, over a wide range of twist angles, the effect of the moiré lattice and proximity effects on the band structure by investigating the high symmetry points’ effective masses, band positionings, and location of the moiré bands across four TMDC heterobilayers. Our work demonstrates the tunability of the electronic properties in twisted 2D bilayers and the power of ARPES to provide a momentum-resolved view of their electronic structure. |
Thursday, March 9, 2023 1:18PM - 1:30PM |
T21.00010: Exact diagonalization studies of semiconductor moiré materials Nicolás Morales-Durán, Jie Wang, Pawel Potasz, Cecile Repellin, Jennifer Cano, Allan H MacDonald Semiconductor moiré materials based on transition metal dichalcogenides (TMDs) provide a platform to study the interplay between strong electronic correlations and band topology, promising to reveal new quantum phases of matter. Depending on the TMD monolayer constituents, topologically trivial or non-trivial flat bands, that are well described by low-energy continuum models, are expected in these systems. We use exact diagonalization to study moiré TMD continuum models projected to the topmost flat band. For C = 0 bands, we investigate the correlated insulating states at half- and fractional-fillings, determining their magnetic properties and how phase transitions from antiferromagnetic to ferromagnetic ground states can take place. For C = 1 bands, we study the feasibility of fractional Chern insulating ground states and their instability towards a charge density wave phase. We suggest guiding principles for the phase transitions and discuss possible realizations in experiments. |
Thursday, March 9, 2023 1:30PM - 1:42PM |
T21.00011: In-plane charge-transfer excitons in near-60° twisted bilayer WS2: a first-principles GW-BSE study Mit H Naik, Woochang Kim, Zhenglu Li, Felipe H da Jornada, Steven G Louie Moiré superlattices based on monolayer transition metal dichalcogenides host exotic excitonic states of qualitatively different character from the individual layers. Twisted homobilayer transition metal dichalcogenides are known to host flat electronic bands at the valence and conduction band edges for twist-angles close to 60°, whose spatial modulation can be described by solutions of a particle in a triangular quantum well. Here, we extend the pristine unit-cell matrix projection (PUMP) [1] approach to study the GW-BSE spectrum of a 57.7° twisted bilayer WS2, with 4000 atoms in the moiré unit-cell, including spin-orbit coupling effects. We discover the lowest-energy exciton to be a layer-hybridized exciton with in-plane charge transfer character, with the photoexcited holes derived from the Γ valley and the photoexcited electrons from the K valley in the pristine unit-cell BZ. Furthermore, the higher energy excitonic states possess highly non-trivial characters, being strongly influenced by the spatial modulation of the valence states due to the triangular quantum well potential. |
Thursday, March 9, 2023 1:42PM - 1:54PM |
T21.00012: Kagome flat bands, fragile topology, quantum spin Hall effect, and more in graphene moire materials Michael G Scheer, Biao Lian We present a moire continuum model for graphene modulated by moire interference with another two-dimensional material. Without considering spin-orbit coupling, the model exhibits a rich phase diagram including both fragile topological bands and bands with trivial symmetry indicators. Many elementary and composite band representations of the space group P6mm appear as the symmetry indicators for connected bands near charge neutrality. Examples include one-orbital kagome lattice bands and two-orbital honeycomb lattice bands, both of which include flat bands. If one includes spin-orbit coupling, quantum spin Hall bands emerge with spin Chern numbers up to 3. Experimental realizations of this model may lead to a deeper understanding of correlations in flat bands and topological phases of matter. |
Thursday, March 9, 2023 1:54PM - 2:06PM |
T21.00013: Double moire commensuration strains in twisted trilayer hexagonal boron nitride Youngju Park, Nicolas Leconte, Prathap K Jharapla, Md Shaifullah, Jeil Jung We obtain the double moire commensuration strains in trilayer hexagonal boron nitride (t3BN) systems by relaxing the atomic positions with interatomic forces modeled through pairwise classical force fields using the LAMMPS molecular dynamics code. We can distinguish three relative alignments of the hBN layers for the two interfaces thanks to the polarity of the B and N sublattice atoms. Our lattice relaxation calculations show that when the twist angle of one interface is fixed, the t3BN systems have total energy minima when the two moire patterns are commensurate, i.e. when their angles are aligned and they have the same period. We calculate the sliding energy landscapes for the commensurate moire pattern geometries to predict the most probable local stacking atomic structure in experimental devices. |
Thursday, March 9, 2023 2:06PM - 2:18PM |
T21.00014: Flat conduction and valence bands for interlayer excitons in moiré type-II transition-metal dichalcogenide heterobilayers Sara Conti, Andrey Chaves, Tribhuwan Pandey, Lucian Covaci, Francois M Peeters, David Neilson, Milorad V Milosevic We analyze the flatness of conduction and valence bands of interlayer excitons in MoS2/WSe2 van der Waals heterobilayers, tuned by interlayer twist angle [1], pressure [2,3], and external electric field [4]. We employ an efficient continuum model where the moiré pattern from lattice mismatch and/or twisting is represented by an equivalent mesoscopic periodic potential. We demonstrate that the mismatch moiré potential is too weak to produce significant flattening. Moreover, we draw attention to the fact that the quasiparticle effective masses around the Γ-point and the band flattening are reduced with twisting. As an alternative approach, we show (i) that reducing the interlayer distance by uniform vertical pressure can significantly increase the effective mass of the moiré hole, and (ii) that the moiré depth and its band flattening effects are strongly enhanced by accessible electric gating fields perpendicular to the heterobilayer, with resulting electron and hole effective masses increased by more than an order of magnitude leading to ultra-flat bands. These findings impose boundaries on the commonly generalized benefits of moiré twistronics, while also revealing alternate feasible routes to achieve truly flat electron and hole bands to carry us to strongly-correlated excitonic phenomena on demand. |
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