Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session F48: 2D Heterostructures II: Twisted BilayersLive
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Sponsoring Units: DCMP Chair: GuangXin Ni, Florida State University |
Tuesday, March 16, 2021 11:30AM - 11:42AM Live |
F48.00001: Absence of back-scattering in twisted bilayer graphene with disorder Hector Sainz-Cruz, Tommaso Cea, Pierre Anthony Pantaleon, Francisco Guinea The phase diagram of twisted bilayer graphene (TBG) depends profoundly on the twist angle, which is a novel tuning parameter but also a source of disorder. Here we mimic TBG with a tight-binding model of two nanotubes with opposite chiral angles (△θ∼3.5°), the 1D analogue of TBG. We study how the densities of states and electrical conductivity are affected by twist angle disorder. A re-scaling procedure enables the simulation of the magic angle regime (△θ∼1°) [1]. For the calculation of the conductivity through a region with disorder, we develop a decimation technique in the spirit of [2], which yields the pristine electrodes self-energies. Twist disorder damps van-Hove peaks and creates charge localization. However, the conductivity of disordered systems (e.g. 1.02°@1.12°) is free of back-scattering within the flat bands, indicating that in this regime carriers might be impervious to such disorder. |
Tuesday, March 16, 2021 11:42AM - 11:54AM Live |
F48.00002: Intrinsic Bandgap in Twisted Doublebilayer graphene due to Crystal Fields Giulia Zheng, Peter Rickhaus, Elias Portoles, Folkert De Vries, Petar Tomic Electronic properties of twisted graphene multilayers can be engineered by tuning several parameters such as the number of layers, the twist angle, applied electric and magnetic biases, electronic interactions and elastic lattice relaxations. Here, we present an additional parameter: the crystal field. This field occurs due to a potential difference between chemically different atomic species. We experimentally demonstrate that twisted double bilayer graphene (tDBG), encapsulated between hBN layers, exhibits an intrinsic bandgap. By applying an electric field, the gaps in the individual bilayers can be closed, allowing to determine the crystal fields. We find that crystal fields point from the outer to the inner layers with strengths in the bottom/top bilayer Eb = 0.13V/nm ≈ −Et = 0.12V/nm. We show both by means of first principles calculations and low energy models that crystal fields open a band gap in the groundstate. Our results put forward a physical scenario in which a crystal field effect in carbon substantially impacts the low energy properties of tDBG, suggesting that such contributions must be taken into account in other regimes to faithfully predict the electronic properties of twisted graphene multilayers. |
Tuesday, March 16, 2021 11:54AM - 12:06PM Live |
F48.00003: Gate-tunable topological flat bands in twisted monolayer-bilayer graphene Youngju Park, Bheema Lingam Chittari, Jeil Jung We investigate the band structure of twisted monolayer-bilayer graphene (tMBG) trilayers, or twisted graphene on bilayer graphene, as a function of twist angles and perpendicular electric fields in search of optimal conditions for achieving isolated nearly flat bands. Narrow bandwidths comparable to or smaller than the effective Coulomb energies satisfying Ueff/W≥1 are expected for twist angles in the range of 0.3o- 1.5o, more specifically in islands around θ ~ 0.5o, 0.85o, 1.3o for appropriate perpendicular electric field magnitudes and directions. The valley Chern numbers of the electron-hole asymmetric bands depend intrinsically on the details of the hopping terms in the bilayer graphene, and extrinsically on factors like electric fields or average staggered potentials in the graphene layer aligned with the contacting hexagonal boron nitride substrate. This tunability of the band isolation, bandwidth, and valley Chern numbers makes tMBG trilayers a more versatile system than twisted bilayer graphene for finding nearly flat bands prone to strong correlations. |
Tuesday, March 16, 2021 12:06PM - 12:18PM Live |
F48.00004: Moiré metrology of energy landscapes in van der Waals heterostructures Dorri Halbertal, Nathan Finney, Sai Sunku, Alexander Kerelsky, Carmen Rubio Verdú, Sara Shabani, Lede Xian, Stephen Carr, Shaowen Chen, Charles Zhang, Lei Wang, Derick Gonzalez-Acevedo, Alexander McLeod, Daniel Rhodes, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Cory Dean, James Hone, Abhay Narayan, Dante Kennes, Angel Rubio, Dmitri Basov The emerging field of twistronics has revolutionized quantum materials research. At the small twist limit, and particularly under strain, as atomic relaxation prevails, the emergent moiré superlattice encodes elusive insights into the local interlayer interaction. In this work we introduce moiré metrology as a combined experiment-theory framework to probe the stacking energy landscape of bilayer structures at the 0.1 meV/atom scale, outperforming the gold-standard of quantum chemistry. Through studying the shapes of moiré domains with numerous nano-imaging techniques, and correlating with multi-scale modelling, we assess and refine first-principle models for the interlayer interaction. We document the prowess of moiré metrology for three representative twisted systems: bilayer graphene, double bilayer graphene and H-stacked MoSe2/WSe2. Moiré metrology establishes sought after experimental benchmarks for interlayer interaction, thus enabling accurate modelling of twisted multilayers. |
Tuesday, March 16, 2021 12:18PM - 12:30PM Live |
F48.00005: A moiré superlattice on the surface of a topological insulator Justin Wilson, Jennifer Cano, Shiang Fang, Jed Pixley Twisting two materials produces moiré patterns and can induce correlated many-body states, as seen in twisted bilayer graphene, for example. We investigate the surface state of a topological insulator subject to a moiré superlattice potential. With diagrammatic perturbation theory, lattice model simulations, and ab initio calculations, we uncover the unique aspects of twisting a single Dirac cone with an induced moiré superlattice and the role of bulk topology on the reconstructed bands. The Dirac cone velocity renormalizes, but no gap opens up; instead, a whole ladder of satellite Dirac cones appears, some of which can be made relatively flat with a large nearby density of states. We discuss the implications of our findings to correlated physics and future experiments. |
Tuesday, March 16, 2021 12:30PM - 12:42PM Live |
F48.00006: Nanoscale Lattice Dynamics in Hexagonal Boron Nitride Moiré Superlattices Samuel Moore, Dorri Halbertal, Christopher Ciccarino, Leo J McGilly, Nathan Finney, Kaiyuan Yao, GuangXin Ni, Yinming Shao, Aaron Sternbach, Evan Telford, Brian S Kim, Abhay Narayan, Cory Dean, James Hone, Michael Fogler, Prineha Narang, P James Schuck, Dmitri Basov Moiré superlattices enable a new means to tune the properties of quantum materials. Past research has found extensive control of electronic properties with moiré effects. There, scanning probe measurements have supported those measurements with microscopic origins for the observations. However, less is known about moiré vibrational properties, with measurements limited to far-field Raman spectroscopy. Here, we present a deterministic procedure for obtaining large-area moiré hexagonal boron nitride (hBN) domains. We use near-field infrared imaging to study the strong in-plane hBN phonon resonance in a moiré superlattice with 250-nm periodicity. We explain the observations with mechanical relaxation simulations and vibrational ab-initio calculations. By revealing a strong interplay between vibrational, mechanical, and atomic registry so far unidentified in far-field experiments, we take a larger step towards moiré tunability of quantum material vibrational properties. |
Tuesday, March 16, 2021 12:42PM - 12:54PM Live |
F48.00007: Quantum Anomalous Hall Effect of Twisted Bilayer Graphene/Hexagonal Boron Nitride Double-Moiré Structures Jingtian Shi, Jihang Zhu, Allan MacDonald Twisted bilayer graphene (tBG) nearly aligned with a hexagonal boron nitride (hBN) substrate hosts two moiré patterns and sometimes shows quantum anomalous Hall effects (QAHE). The influence of hBN on low-energy states in tBG has normally been approximated by a spatially uniform sublattice-asymmetric potential, in spite of the fact that moiré periodic terms in the Hamiltonian are estimated to be equally large. I will present a study of the QAHE of tBG/hBN trilayers using a double-moiré model. I will first identify a series of twist-angle pairs at which the two moiré patterns are commensurate, allowing moiré band theory to be applied, and then illustrate the significant effect of rigid in-plane hBN translation on the moiré band structures and Chern numbers. Using this information, I will discuss the anomalous Hall properties of general tBG/hBN trilayers in two regimes: i) when the twist-angle pair is close to a commensurate point, so that a supermoiré pattern arises between the two patterns; ii) when the twist-angle pair is far from a commensurate point, creating a disorder-like effect. Finally, I will suggest a number of possible experiments that could verify the proposed critical role of the G/hBN moiré pattern. |
Tuesday, March 16, 2021 12:54PM - 1:06PM Live |
F48.00008: Optical Susceptibility Tuning in Twisted Bilayer Insulators Andrew Smith, Chong Wang, Di Xiao In hexagonal bilayer systems with different sublattices, twists can tune a crystal structure between two high symmetry stacked structures, one potentially having inversion symmetry. Inversion symmetry requires that the second-order optical susceptibility is zero, suppressing the nonlinear optical response. In cases like hexagonal Boron Nitride, since no such constraint is present in one of the two high symmetry stackings, twisting has the potential to present a natural platform for tuning non-linear response. Our work examines how the optical susceptibilities vary as the twist angle is tuned over the parameter space of shifting and twisting the layers with respect to one another. Removing the signal of the uncoupled bilayer, we discuss the change in response due to the twist induced chiral interlayer coupling. Our computations suggest additional ultraviolet resonances in the SHG signal and we discuss connections to the changes in the peak structure of the density of states due to the Moire potential and the underlying geometry. |
Tuesday, March 16, 2021 1:06PM - 1:18PM Live |
F48.00009: Reconstruction of moiré lattices in twisted transition metal dichalcogenide bilayers Indrajit Maity, Manish Jain, Prabal K Maiti, Hullikal R Krishnamurthy An important step in understanding the exotic electronic, vibrational, and optical properties of the |
Tuesday, March 16, 2021 1:18PM - 1:30PM Live |
F48.00010: Unconventional ferroelectricity in moiré heterostructures Zhiren Zheng, Qiong Ma, Zhen Bi, Sergio de la Barrera, Ming-hao Liu, Nannan Mao, Yang Zhang, Natasha Kiper, Kenji Watanabe, Takashi Taniguchi, Jing Kong, William A. Tisdale, Raymond Ashoori, Nuh Gedik, Liang Fu, Suyang Xu, Pablo Jarillo-Herrero The freedom of stacking different layers of 2D van der Waals (vdW) materials with an arbitrary twist angle opens up tremendous opportunities in studying emergent properties that are not accessible in the natural crystal form. In particular, the recently discovered magic-angle twisted bilayer graphene shows that the twist angle and moiré potential can substantially enhance electron correlations for spontaneously broken symmetry states and turn semi-metallic graphene into insulator, superconductor, and quantum anomalous Hall insulator at will. With the right system and careful design, interaction-driven broken symmetry states can have many other manifestations that are not only fundamentally intriguing but also invaluable to new device applications. Here, we report on our observation of switchable ferroelectricity in graphene-based moiré vdW heterostructures. Our systematic transport measurements reveal rich and striking response as a function of the displacement field and electron filling, not previously observed in any 2D systems, and beyond the framework of conventional ferroelectrics. Our study may pave the way for a new class of 2D multiferroics, which has a potential implementation in many important applications, such as memristive devices for neuromorphic computing. |
Tuesday, March 16, 2021 1:30PM - 1:42PM On Demand |
F48.00011: Electron-hole asymmetry and band gaps of commensurate double moiré patterns in twisted bilayer graphene on hexagonal boron nitride Jiseon Shin, Youngju Park, Bheema Lingam Chittari, Jeil Jung Spontaneous orbital magnetism observed in twisted bilayer graphene (tBG) on nearly aligned hexagonal boron nitride (BN) substrate builds on top of the electronic structure resulting from combined G/G and G/BN double moiré interfaces. Here we show that tBG/BN commensurate double moiré patterns can be classified into two types, each favoring the narrowing of either the conduction or valence bands on average, and obtain the evolution of the bands as a function of the interlayer sliding vectors and electric fields. Finite valley Chern numbers ±1 are found in a wide range of parameter space when the moiré bands are isolated through gaps. We illustrate the impact of the BN substrate for a particularly pronounced electron-hole asymmetric band structure by calculating the optical conductivities of twisted bilayer graphene near the magic angle as a function of carrier density. The band structures corresponding to other N-multiple commensurate moire period ratios indicate it is possible to achieve narrow width W ≤ 10meV isolated folded band bundles for tBG angles θ ≤ 1°. |
Tuesday, March 16, 2021 1:42PM - 1:54PM On Demand |
F48.00012: Interactions of topological and correlated states in twisted monolayer–bilayer graphene Zhang Zhou, Siyu Li, Zhengwen Wang, Yucheng Xue, Yuhang Jiang, Jinhai Mao The twisted moiré superlattice of graphene has induced various correlated many-body effects. The twisted monolayer–bilayer graphene is naturally born with lower crystal symmetry, breaking both two-fold rotation and mirror reflection symmetry, which could help to reveal the degenerated energy bands with non-zero Chern numbers. The discovery of flat electronic bands with topological character in magic-angle twisted monolayer-bilayer graphene has created a unique opportunity to search for new strongly correlated topological phases. Here we mainly focus on the novel transport properties emerging in the interactions among electrons and the topology of their band structure. |
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