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 Y23: Electronic Effects of Twisted 2D Heterostructures |
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Sponsoring Units: DCMP Chair: Cameron Chaffey, University of California, Davis Room: Room 215 |
Friday, March 10, 2023 8:00AM - 8:12AM |
Y23.00001: Addressing spin order of magic-angle graphene through edge state equilibration Jesse Hoke, Yifan Li, Julian May-Mann, Kenji Watanabe, Takashi Taniguchi, Barry Bradlyn, Taylor L Hughes, Ben Feldman
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Friday, March 10, 2023 8:12AM - 8:24AM |
Y23.00002: Domain-dependent surface adhesion in twisted few-layer graphene Valerie Hsieh, Dorri Halbertal, Nathan R Finney, Ziyan Zhu, Eli Gerber, Michele Pizzochero, Emine Kucukbenli, Gabriel R Schleder, Mattia Angeli, Kenji Watanabe, Takashi Taniguchi, Eun-Ah Kim, Efthimios Kaxiras, James C Hone, Cory R Dean, Dmitri N Basov Twisted van der Waals heterostructures are a highly tunable platform due to the many degrees of freedom available for controlling their electronic and chemical properties. Here, we focus on the local stacking order of low-degree twisted graphene heterostructures as a platform for manipulating the surface chemistry of this class of materials. We report the emergence and engineering of stacking domain-dependent surface adhesion in twisted few-layer graphene. Minimally twisted double bi- and tri-layer graphene heterostructures were fabricated and imaged using mid-infrared near-field optical microscopy and atomic force microscopy to identify rhombohedral and Bernal stacking domains. We then observed that metallic nanoparticles and liquid water exhibit a domain-selective adhesion on these heterostructures, with preference for the rhombohedral stacking configurations. Finally, we used an atomic force microscope to manipulate nanoparticles located at certain stacking domains, resulting in a local reconfiguration of the moiré superlattice near the nanoparticles at the μm-scale. Our findings establish a new approach to controlling moiré chemistry and nanoengineering. |
Friday, March 10, 2023 8:24AM - 8:36AM |
Y23.00003: Truncated Atomic Plane Wave Method for the Subband Structure Calculations ofMoire Systems Wangqian Miao, Chu Li, Xu Han, Ding Pan, Xi Dai We propose a highly efficient and accurate numerical scheme named Truncated Atomic Plane Wave (TAPW) method to determine the subband structure of Twisted Bilayer Graphene (TBG) inspired by the Bistritzer-MacDonald (BM) model. Our method utilizes real space information of carbon atoms in the moir'e unit cell and projects the full tight binding Hamiltonian into a much smaller subspace using atomic plane waves. Using our new method, we are able to present accurate electronic band structures of TBG in a wide range of twist angles together with detailed moir'e potential and screened Coulomb interaction at the first magic angle. Furthermore, we generalize our formalism to solve the problem of low frequency moir'e phonons in TBG. |
Friday, March 10, 2023 8:36AM - 8:48AM |
Y23.00004: 3D nonlinear optical metamaterials from twisted 2D van der Waals interfaces Bumho Kim, Jicheng Jin, Zhi Wang, Li He, Thomas Christensen, Eugene J Mele, Bo Zhen Metamaterials for nonlinear optics are created by organizing structural units (meta-atoms) which are typically on the scale of about a hundred nanometers. However, truly altering atomic symmetry and enabling new nonlinear responses requires control at the atomic scale, down to a few angstroms. Here, we report the discovery of 3D nonlinear optical van der Waals (vdW) metamaterials realized by precise control of individual atomic-scale interfaces. We theoretically show and experimentally demonstrate that adding a screw axis symmetry up to a twisted eight-layer WS2 stack entirely alters the allowed nonlinear susceptibility components, as new nonlinear susceptibility components of interfaces can be enabled while those of individual layers can be forbidden. We further show that the interfacial nonlinear responses that do not exist in natural WS2 can be enhanced by increasing the number of constituent layers of the metamaterials. Our findings suggest a new approach to reconfiguring the allowed nonlinear susceptibility components of vdW materials via electronic wavefunction symmetry engineering. |
Friday, March 10, 2023 8:48AM - 9:00AM |
Y23.00005: Emergent moiré phonons due to zone folding in WSe2-WS2 van der Waals heterostructures Hsun-Jen Chuang, Madeleine Phillips, Kathleen M McCreary, Darshana Wickramaratne, Matthew R Rosenberger, Vladimir P Oleshko, Nicholas V Proscia, Mark I Lohmann, Dante J O’Hara, Paul D Cunningham, C Stephen Hellberg, Berend T Jonker Bilayers of 2D materials offer opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs) where the constituent monolayers have different lattice constants, a moiré superlattice forms with a length scale larger than the lattice constant of either constituent material regardless of twist angle. Here, we report the appearance of moiré Raman modes from nearly aligned WSe2-WS2 vdWHs in the range of 240 cm-1 -260 cm-1, which are absent in both monolayers and homobilayers of WSe2 and WS2 and in largely misaligned WSe2-WS2 vdWHs. Using first-principles calculations and geometric arguments we show that these moiré Raman modes are a consequence of the large moiré length scale which results in zone-folded phonon modes that are Raman active. These modes are sensitive to changes in twist angle, but notably, they occur at identical frequencies for a given small twist angle away from either the 0-degree or 60-degree aligned heterostructure. Our measurements also show a strong Raman intensity modulation in the frequency range of interest, with near 0 and near 60-degree vdWHs exhibiting a markedly different dependence on excitation energy. In near 0-degree aligned WSe2-WS2 vdWHs, a nearly complete suppression of both the moiré modes and the WSe2 A1g Raman mode (~250 cm-1) is observed when exciting with 532 nm CW laser at room temperature. Temperature-dependent reflectance contrast measurements demonstrate the significant Raman intensity modulation arises from resonant Raman effects. |
Friday, March 10, 2023 9:00AM - 9:12AM |
Y23.00006: Layer Dependence of Moiré Excitons and Correlated Electronic States in WSe2/WS2 Moiré Superlattices Dongxue Chen, Zhen Lian, Xiong Huang, Ying Su, Zenghui Wang, Chuanwei Zhang, Yongtao Cui, Sufei Shi Moiré coupling in transition metal dichalcogenides (TMDCs) superlattices introduces flat minibands that enable strong electronic correlation and fascinating correlated states, and it also modifies the strong Coulomb-interaction-driven excitons and gives rise to moiré excitons. Here, we introduce the layer degree of freedom to the WSe2/WS2 moiré superlattice by changing WSe2 from monolayer to bilayer and trilayer. We observe systematic changes of optical spectra of the moiré excitons, which directly confirm the highly interfacial nature of moiré coupling at the WSe2/WS2 interface. In addition, the energy resonances of moiré excitons are strongly modified, with their separation significantly increased in multilayer WSe2/monolayer WS2 moiré superlattice. The additional WSe2 layers also modulate the strong electronic correlation strength, evidenced by the reduced Mott transition temperature with added WSe2 layer(s). The layer dependence of both moiré excitons and correlated electronic states can be well described by our theoretical model. Our study presents a new method to tune the strong electronic correlation and moiré exciton bands in the TMDCs moiré superlattices, ushering in an exciting platform to engineer quantum phenomena stemming from strong correlation and Coulomb interaction. |
Friday, March 10, 2023 9:12AM - 9:24AM |
Y23.00007: Generating in-situ Heterostrain of Moiré Superlattice Heterostructures Jordan M Fonseca, John Cenker, Ying Xia, Yuzhou Zhao, Jihui Yang, Shuai Zhang, Jun Liu, James J De Yoreo, Di Xiao, Xiaodong Xu Moiré superlattices of van der Waals semiconductors have emerged as a highly tunable material platform to study strong electron correlations and simulate Hubbard model physics in two dimensions. Electrostatic gating enables a high degree of control over electron density and interlayer coupling; however, much of the physics of moiré superlattices is set by the geometric properties of lattice mismatch and relative twist angle, which are selected during device fabrication and cannot generally be altered in situ. In this talk, I will discuss a technique for generating and tuning in situ heterostrain, which enables dramatically altering the moiré wavelength and symmetry by applying different amounts of uniaxial strain to each layer in a transition metal dichalcogenide moiré superlattice. I will describe the experimental apparatus and device fabrication approach we use to achieve heterostrain and show preliminary evidence for the presence of heterostrain obtained with a combination of atomic force microscopy and cryogenic optical measurements. Finally, the possibility of applying heterostrain to a fully dual-gated moiré superlattice and the associated challenges will be covered. |
Friday, March 10, 2023 9:24AM - 9:36AM |
Y23.00008: Electrostatic Potential of hBN Moiré Superlattices Dong Seob Kim, Rigo Mayorga-Luna, Dingyi Ye, Tixuan Tan, Yue Ni, Zhida Liu, Roy C Dominguez, Mitchell T Ford, Frank Y Gao, Saba Arash, Kenji Watanabe, Takashi Taniguchi, Suenne Kim, Chih-Kang Shih, Keji Lai, Wang Yao, Li Yang, Xiaoqin Elaine Li, Yoichi Miyahara Twisted hexagonal boron nitride (hBN) layers have been demonstrated to exhibit ferroelectric domains due to charge redistribution at the interface. Here, we investigate the electrostatic potential at the top surface of twisted hBN bilayers, prepared by folding exfoliated thin flakes. Using Kelvin Probe Force Microscopy measurements, we show that this potential can be engineered by tuning the twist angle and adjusting the thickness of the top hBN layer. Furthermore, we study double moiré superlattices with different twist angles at each interface. These findings can be further extended to twisted multilayers to significantly expand the versatility of hBN and moiré superlattices in engineering material properties. |
Friday, March 10, 2023 9:36AM - 9:48AM |
Y23.00009: Programmable twist angle and strain profile in 2D materials Maëlle A Kapfer, Bjarke S Jessen, Megan Eisele, Dorte R Danielsen, Ariane Marchese, Valerie Hsieh, Kenji Watanabe, Takashi Taniguchi, James C Hone, Peter Bøggild, Cory R Dean Moiré superlattices in twisted two-dimensional materials have generated tremendous excitement as a platform for achieving quantum properties on demand. However, the moiré pattern is highly sensitive to the interlayer atomic registry, and current assembly techniques suffer from imprecise control of the average twist angle, spatial inhomogeneity in the local twist angle, and distortions due to random strain. Here, we demonstrate a new way to manipulate the moiré patterns in hetero- and homo-bilayers through in-plane bending of monolayer ribbons, using the tip of an atomic force microscope. This technique achieves continuous variation of twist angles with improved twist-angle homogeneity and reduced random strain, resulting in moiré patterns with highly tunable wavelength and ultra-low disorder. Our results pave the way for detailed studies of ultra-low disorder moiré systems and the realization of precise strain-engineered devices. |
Friday, March 10, 2023 9:48AM - 10:00AM |
Y23.00010: Cavity vacuum control of topological transition in a full filling moiré superlattice Zuzhang Lin, Wang Yao The interaction between quasiparticles in condensed matter systems induced by exchanging virtual photons has attracted widespread interest because it leads to remarkable and exotic phases of matter. Such virtual-photon-mediated interaction is typically investigated in a strong light-matter coupling regime realized in a metallic split-ring terahertz (THz) electromagnetic resonator. Moiré superlattice embedded in a THz resonator is a suitable system to explore cavity control at frequency down to the Terahertz range. Here we study the topological transition in cavity-embedded moiré superlattice doped at filling factor 1 (one electron/hole per supercell). Within the mean-field approximation, we show that the virtual-photon-mediated interaction can introduce a topological band inversion, namely the electronic system can be in the topological trivial (nontrivial) states in the absence (presence) of the cavity in a vacuum. The cavity vacuum-mediated interaction Hamiltonian provides a topological nontrivial mass term, which is also gate-tunable. Our results point to a promising new toolbox for manipulating topological transition. |
Friday, March 10, 2023 10:00AM - 10:12AM |
Y23.00011: Moiré excitons with Wannier functions Indrajit Maity, Valerio Vitale, Arash A Mostofi, Johannes C Lischner Recent studies in moiré superlattices have demonstrated that many properties of excitons, such as their localization, and lifetimes can be controllably manipulated. Existing theoretical strategies to model moiré excitons, such as those based on effective mass models, neglect several important aspects including atomic reconstructions that take place in a moiré superlattice. On the other hand, the highly accurate first-principles GW-plus-Bethe-Salpeter-Equation (GW+BSE) approach is computationally much more demanding. To make these calculations affordable without compromising accuracy, we develop a new framework to solve the BSE using density-functional-theory calculations in conjunction with Wannier functions. We use the Keldysh potential to describe the screened Coulomb interaction with the parameters obtained from first-principles calculations. Our calculations accurately predict the A and B excitons in semiconducting monolayer transition-metal-dichalcogenides at a fraction of the conventional GW+BSE computational cost. Motivated by recent experiments, we study the twist-angle dependence of the low-energy intralayer and interlayer moiré excitons of the WS2/WSe2 heterobilayer. Our calculations are in good agreement with previous studies. |
Friday, March 10, 2023 10:12AM - 10:24AM |
Y23.00012: Itinerant electron ferromagnetism in moiré materials Pawel Potasz, Allan H MacDonald Moiré materials are artificial two-dimensional crystals with flexibly tunable properties. I will survey some possibilities for realizing flexible itinerant electron ferromagnetism in moiré materials, including single-band Nagaoka-like ferromagnetism, and multi-band ferromagnetism that is analogous to that of transition metal ferromagnets. Our analysis of moiré ferromagnetism is dependent on derivations of effective models that describe the electronic physics accurately at the moiré scale, and on numerical many-body methods used to analyze competitions between different types of electronic order. Emphasis will be placed on using finite-temperature Lanczos methods to estimate the highest possible Curie temperatures that are likely to be achievable in graphene multilayers, transition metal dichalcogenide multilayers, and in other two-dimensional material stacks containing moirés. |
Friday, March 10, 2023 10:24AM - 10:36AM |
Y23.00013: Branched Flow and Stable Superwires in Periodic Two-Dimensional Systems Esa Rasanen, Alvar Daza, Anton Graf, Eric J Heller Branched flow is a common phenomenon in wave dynamics, where deflections of particles, rays or waves due to random fluctuations in the surrounding medium eventually lead to a chaotic arborescent pattern. Branched flow has been reported in a remarkable variety of physical systems covering, e.g., pulsar-generated microwaves, ocean waves, beams of light and two-dimensional (2D) electron gas [1,2]. Here we report unexpected branched flow in electronic wave-packet dynamics of 2D periodic lattices without any randomized disorder [2]. The branching appears at wavelengths shorter than the typical length scale of the ordered periodic structure and for energies above the potential barrier. The branching is qualitatively similar in classical and quantum description. The strongest branches remain stable indefinitely and may create linear dynamical channels. In these channels the waves are not confined directly by potential walls as electrons in ordinary wires but rather, indirectly and more subtly by dynamical stability. We discuss the relevance of these "superwires" in 2D materials, especially in moiré superlattices, where the wavelengths are typically small compared to the lattice constant - thus creating an ideal system for branched flow. [1] For a review, see E. J. Heller, R. Fleischmann, and T. Kramer, Physics Today 74, 12, 44 (2021). [2] A. Daza, Eric J. Heller, A. M. Graf, and E. Rasanen, Proc. Nat. Acad. Sci. 118 (40), e2110285118 (2021). |
Friday, March 10, 2023 10:36AM - 10:48AM |
Y23.00014: Atomic Reconstruction in Trilayer Graphene Moiré Superlatices Isaac M Craig, Madeline Van Winkle, Kaidi Zhang, Catherine Groschner, Nikita Dowlatshahi, Sinead M Griffin, Kwabena Bediako Moiré superlattices formed from independently twisting trilayers of graphene have been proposed as an ideal model for studying electronic correlation. Multilayered moiré systems offer several advantages over their twisted bilayer analogs, including more robust and tunable superconductivity, a wide range of twist angles associated with flat band formation, and larger scale moiré patterns. Atomic reconstruction, which strongly impacts the electronic structure of twisted graphene structures, has been suggested to play a major role in the relative versatility of super-conductivity in trilayers. Despite this, reconstruction in graphene trilayers has been only probed using indirect measurements or those only applicable to exposed samples. Herein, we exploit an inteferometric 4D-STEM approach to obtain displacement and strain maps in a representative range of twisted trilayer graphene structures. This methodology correlates local intensity modulations to stacking order and allows us to selectively probe bilayer interfaces within the material. The resulting mechanism we present informs a more complete understanding of how atomic reconstruction scales with layer number in graphene moirés and modulates symmetries crucial for establishing superconductivity. |
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