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 Q40: 2D Moiré Materials: Novel Fabrication and MaterialsFocus
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Sponsoring Units: DMP Chair: Daniel Rhodes, University of Wisconsin - Madison; Shaowei Li, University of California, San Diego Room: Room 232 |
Wednesday, March 8, 2023 3:00PM - 3:36PM |
Q40.00001: Top-down Production of 2D Materials in Macroscopic Sizes Invited Speaker: Fang Liu Scalable and controllable top-down exfoliation processes to obtain 2D materials with sufficient size and high quality are often desired for their mass production and device implementation. We have developed a few metal-assisted methods to exfoliate a variety of van der Waals (vdW) single crystals into 2D flakes with controlled thickness and morphology, and achieved high yield, high quality, and macroscopic dimensions. Examples include single-crystal monolayer transition metal dichalcogenides (TMDCs), few-layer hBN flakes with controlled thickness, and monolayer nanoribbons. The universal top-down approach will gain access to a variety of 2D nanoribbons, analogous to the widely used Scotch tape exfoliation in preparation of monolayers. High-quality and large-area monolayers will allow us to further assemble them into artificial stacks, which has been shown to achieve enhanced responses in high-harmonic generation. The heterobilayer stacks have been used to reveal light-induced interlayer thermal transfer dynamics in ultrafast electron diffraction experiments. Obtaining macroscopic 2D materials with enhanced yield will not only facilitate the fundemental research, but also take us one step closer to mass production and commercialization of the 2D devices in the future. |
Wednesday, March 8, 2023 3:36PM - 3:48PM |
Q40.00002: Bumpy graphene membranes: a roadmap for flat-band engineering and topological transitions. Nancy P Sandler, Md. Tareq Mahmoud, Dawei Zhai Graphene’s high flexibility is a property that allows using strain engineering to control electronic properties. Wrinkled or rippled graphene -suspended or on substrates-develops inhomogeneous charge distributions due to underlying strain fields. STM measurements on locally deformed samples demonstrated electron confinement with broken sublattice symmetry and induced valley currents. Motivated by these ideas, we analyzed charge distributions in models of graphene with several Gaussian-shaped deformations arranged in different geometries. Our work revealed moiré-like patterns with pockets of localized charges like those reported for twisted bilayer graphene. Next, we modeled experimental settings of graphene on top of substrates with periodic arrays of deformations. Strains induced by the substrate geometry modify electron dynamics and suggest a practical method for band structure engineering. We identify geometries for the emergence of flat bands and optimal parameters for maximal gaps. Surprisingly, electronic states separate into ‘trivial’-bound and ‘percolating'-extended states that coexist in spatial regions with distinct lattice properties. For a wide range of geometries, and breaking of valley degeneracy, flat bands acquire topological properties with the corresponding emergence of valley chiral edge states. |
Wednesday, March 8, 2023 3:48PM - 4:00PM |
Q40.00003: Machine learning moiré quantum flatland Xiaowei Zhang, Di Xiao, Ting Cao Moiré systems are fertile platforms to explore emergent quantum phenomena. Recent experimental discoveries have suggested crucial roles of structural reconstruction in the understandings of the moiré quantum phenomena. However, current modeling and first-principles frameworks face significant challenges in resolving these structural features at the moiré length scale. For example, density functional theory and many-body perturbation theory calculations, while serving as standard approaches for conventional solids, are too expensive in computational cost for most moiré superlattices. In this talk, we employed a neural network method trained by first-principles results to overcome this difficulty. We studied structural reconstructions and lattice dynamics in a variety of moiré structures. Our calculations discovered salient structural features and electronic properties controlled by twist angles, layer constituents, and other tuning knobs. We then connect and compare our findings with available experimental observations. Our work offers new theoretical and computational strategies to study moiré materials. |
Wednesday, March 8, 2023 4:00PM - 4:36PM |
Q40.00004: 2D heat, light, and mass transport in engineered 2D systems Invited Speaker: Jiwoong Park 2D atomic crystals, including graphene, hBN and TMDs, and their heterostacks have provide excellent platforms for exploring both conventional and correlated electronic phenomena with broad scientific and technological impacts. It is increasingly recognized that the same 2D nature of these systems provide unprecendented opprotunites for designing and controling novel transport phenomena with other fundamental degrees of freedom: phonons, photons, and mass. In this talk, we will present my group's recent efforts to generate large-scale 2D materials and stacked films specifically engineered to realize 2D photonic, photonic and mass transport phenomena. This general picture, which is mostly demonstrated with waferscale TMD monolayers so far, will be also discussed in the context of new molecule based crystals and hybrid systems for further expansion of available 2D systems. |
Wednesday, March 8, 2023 4:36PM - 4:48PM |
Q40.00005: Chiral Charge Distribution near Domain Boundary in Twisted Bilayer Graphene Aligned with hexagonal Boron Nitride Xinyuan Lai, Guohong Li, Kenji Watanabe, Takashi Taniguchi, Eva Y Andrei Magic-angle twisted bilayer graphene (MATBG) hosts low energy flat bands leading to |
Wednesday, March 8, 2023 4:48PM - 5:00PM |
Q40.00006: Atomistic Hartree theory of twisted double bilayer graphene near the magic angle Christopher Tat Shun T Cheung, Zachary A Goodwin, Valerio Vitale, Johannes C Lischner, Arash A Mostofi The discovery of superconductivity and correlated states in magic angle twisted bilayer graphene (tBLG) has prompted the study of other twisted graphene heterostructures for novel electronic properties. In this work [1] we used atomistic Hartree theory to calculate the electronic response of twisted double bilayer graphene (tDBLG) to gate doping and the application of perpendicular external electric fields when long-range electron-electron interactions are included. In stark constrast to tBLG, we find that tDBLG does not exhibit any significant deformations of its bandstructure when doped. However, we find that an electric field changes the band gap and breaks the valley degeneracy of the bands. Finally, whilst Hartree theory predicts the difference in on-site energies between the inner and outer graphene layers in tDBLG (also called the crystal field) at a qualitative level, we show that the subtle interplay between electron-ion and electron-electron interactions necessitates first-principles calculations for quantitative predictions. |
Wednesday, March 8, 2023 5:00PM - 5:12PM |
Q40.00007: Effect of annealing on interfacial moiré relaxation in marginally twisted hexagonal boron nitride Marisa Hocking, Christina Henzinger, Steven Tran, Mihir Pendharkar, Nathan J Bittner, Kenji Watanabe, Takashi Taniguchi, David Goldhaber-Gordon, Andrew J Mannix In emerging twistronics, the precise control of interlayer twist angles is necessary to generate novel correlated states or engineer confinement within moiré potentials. Many of these van der Waals heterostructures (vdWh) are encapsulated in hexagonal boron nitride (hBN) using dry transfer techniques that may require high temperatures. There is a general concern that extended heating of vdWh may drive moiré relaxation and perturb the interfacial twist angle. Although moiré relaxation due to high temperature annealing has been studied in graphene and chalcogenide twisted bilayers, the thermal relaxation of the twisted hBN interface is poorly understood. Here, we study the relationship between thermal annealing and twist angle relaxation at twisted hBN interfaces using a combination of scanning probe microscopy and optical spectroscopy. Based on time-resolved annealing studies, we establish preliminary bounds on the thermal budget of the hBN/hBN moiré stability, and find that marginally twisted hBN shows no significant changes in twist angle after annealing for time intervals and temperatures consistent with – or even exceeding – standard dry transfers. These results suggest that some moiré interfaces exhibit greater stability against thermally-driven lattice relaxation, which will inform future efforts to control moiré interfaces. |
Wednesday, March 8, 2023 5:12PM - 5:24PM |
Q40.00008: Tunable Excitons in Rhombohedral Trilayer Graphene Zhengguang Lu, Xiaowei Zhang, Tianyi Han, Tonghang Han, Dasol Kim, Takashi Taniguchi, Kenji Watanabe, Ting Cao, Long Ju Rhombohedral stacked multilayer graphene is a unique material where the bandgap can be continuously tuned by an out-of-plane electric field. It hosts excitons with unconventional optical selection rule and provides a novel platform to study exciton physics in the context of valley pseudospin and band topology. We observed tunable exciton states in rhombohedral trilayer graphene via a photocurrent spectroscopy technique. Unlike in AB-stacked bilayer graphene, the optical spectrum of rhombohedral trilayer graphene is dominated by exciton p and d states, while the s exciton state becomes completely dark. This observation is a direct consequence of the pseudospin texture near the band extrema. I will also discuss the moire effects on the exciton physics in a trilayer graphene/hBN superlattice. |
Wednesday, March 8, 2023 5:24PM - 5:36PM |
Q40.00009: Engineering New Moire Heterostructures in Twisted Trilayer Systems Aaron P Dunbrack, Jennifer Cano We introduce a method of engineering moire heterostructures in trilayer systems without requiring near-matched lattice constants, allowing for a wider variety of material combinations. We call these structures "intrinsically trilayer moire" because they do not naturally fit into the "moire of moire" paradigm used for systems like twisted trilayer graphene. The local structures of these patterns are quasicrystalline, but they exhibit a crystalline moire pattern. This talk focuses on explaining the emergence of these patterns and briefly examining their local configuration space. |
Wednesday, March 8, 2023 5:36PM - 5:48PM |
Q40.00010: G-band enhancement in ABt Twisted Trilayer Graphene Xiaofeng Li, Ronghui Luo, Xiao Li, Zhibo Liu Twisted bilayer graphene is a new platform for investigating exotic properties such as correlated insulator and superconducting states. In particular, it is known to exhibit a G-band enhancement in Raman experiments, which is closely related to the divergent density of states near the van Hove singularities. However, the Raman signatures of twisted trilayer graphene remain unclear. In this talk I will introduce our recent experimental investigation on this question. We experimentally observed the presence of G-band enhancement in twisted trilayer graphene, which consists of one twisted layer on top of an AB-stacked bilayer (ABt twisted trilayer graphene). We identified two centers of G-band enhancements that are centered around 12.3° and 16°, respectively. They correspond to two pairs of van Hove singularities formed by the anti-crossing between the linearly dispersive bands of the twisted layer and the parabolic bands of the AB-stacked bilayer. We utilized the continuum model to map out the G-band intensity with respect to twist angles in Raman measurements, which properly captures these two enhancement centers. |
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