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 Y42: Imaging of 2D Moire SystemsLive
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Sponsoring Units: DCMP Chair: Stephanie Lough, University of Central Florida |
Friday, March 19, 2021 11:30AM - 11:42AM Live |
Y42.00001: Imaging double- moiré pattern in twisted bilayer graphene aligned on h-BN substrate Xiong Huang, Lingxiu Chen, Shujie Tang, Chengxin Jiang, chen Chen, Huishan Wang, Zhixun Shen, Haoming Wang, Yongtao Cui Moiré superlattices in van der Waals heterostructures have been a promising platform to explore emergent electronic phenomena. Here, we report the imaging experiment of a double moiré pattern in twisted bilayer-graphene that is aligned on h-BN substrate, using microwave impedance microscopy. Two graphene layers are directly grown on h-BN flakes using CVD method. The first layer grows directly on the h-BN substrate with a 0-degree alignment, while the second layer grows on top of the first one at a small twist angle. We resolve two sets of moiré patterns formed in this tri-layer heterostructure, a larger one (~15 nm) between the first graphene and h-BN and a smaller one between the two graphene layers.The smaller moiré pattern exhibits abrupt discontinuities at the boundaries of the larger moiré pattern. We analyze the parameters of the two moiré superlattices based on both a simple trilayer stacking model and FFT calculation, which agrees well with our experimental observations. |
Friday, March 19, 2021 11:42AM - 11:54AM Live |
Y42.00002: Structure of twisted transition metal dichalcogenides Madeleine Phillips, C Stephen Hellberg We present density functional theory (DFT) calculations of the reconstruction in MoSe2/WSe2 hetero-bilayers twisted a small angle away from 180-degree stacking. The bilayer reconstructs to increase the area of the domains with the lowest energy stacking. The overall twist is concentrated in the domain walls and vertices, where the energetic cost of twisting away from 180 degrees is reduced. We consider the local stacking, order parameter, and strain in the relaxed twisted bilayer. |
Friday, March 19, 2021 11:54AM - 12:06PM Live |
Y42.00003: Stripe phases in WSe2/WS2 moiré superlattices Chenhao Jin, Zui Tao, Tingxin Li, Yang Xu, Yanhao Tang, Jiacheng Zhu, Song Liu, Kenji Watanabe, Takashi Taniguchi, James Hone, Liang Fu, Jie Shan, Kin Fai Mak Two-dimensional moiré superlattices have emerged as a powerful platform to study strong electronic correlation phenomena. Stripe phases, in which the rotational symmetry of charge density is spontaneously broken, occur as a result of competing interactions and are thought to be important for high-temperature superconductivity. Here we uncover stripe phases in WSe2/WS2 moiré superlattices with continuously gate-tunable charge densities by combining optical anisotropy and electronic compressibility measurements. We find stripe crystal states (insulating) at the ½ filling of the moiré superlattice and electronic liquid crystal states (conducting) over a large doping range around the ½ filling. We further investigate melting of the stripe crystals and image the stripe domain patterns. |
Friday, March 19, 2021 12:06PM - 12:18PM Live |
Y42.00004: Observation of Electrically Tunable van Hove Singularities in Twisted Bilayer Graphene from NanoARPES Ryan Muzzio, Alfred Jones, Paulina Majchrzak, Sahar Pakdel, Davide Curcio, Klara Volckaert, Deepnarayan Biswas, jacob gobbo, Simranjeet Singh, Jeremy Robinson, Kenji Watanabe, Takashi Taniguchi, Timur Kim, Cephise Cacho, Nicola Lanata, Jill A. Miwa, Philip Hofmann, Jyoti Katoch, Søren Ulstrup The possibility of triggering correlated phenomena by placing a singularity of the density of states near the Fermi energy remains an intriguing avenue toward engineering the properties of quantum materials. Twisted bilayer graphene is a key material in this regard because the superlattice produced by the rotated graphene layers introduces a van Hove singularity and flat bands near the Fermi energy that cause the emergence of numerous correlated phases, including superconductivity. Direct demonstration of electrostatic control of the superlattice bands over a wide energy range has, so far, been critically missing. This work examines the effect of electrical doping on the electronic band structure of twisted bilayer graphene using a back-gated device architecture for angle-resolved photoemission measurements with a nano-focused light spot. A twist angle of 12.2° is selected such that the superlattice Brillouin zone is sufficiently large to enable identification of van Hove singularities and flat band segments in momentum space. The doping dependence of these features is extracted over an energy range of 0.4 eV, expanding the combinations of twist angle and doping where they can be placed at the Fermi energy and thereby induce new correlated electronic phases. |
Friday, March 19, 2021 12:18PM - 12:30PM Live |
Y42.00005: Nanoscale imaging of moiré superlattices in twisted van der Waals heterostructures Yue Luo, Rebecca Engelke, Marios Mattheakis, Michele Tamagnone, Stephen Carr, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Philip Kim, William L Wilson Direct visualization of nanometer-scale properties of moiré superlattices in van der Waals heterostructure devices is a critically needed diagnostic tool for study of the electronic and optical phenomena induced by the periodic variation of atomic structure in these complex systems. Conventional imaging methods are destructive and insensitive to the buried device geometries, preventing practical inspection. Here we report a versatile scanning probe microscopy employing infrared light for imaging moiré superlattices of twisted bilayers graphene encapsulated by hexagonal boron nitride. We map the pattern using the scattering dynamics of phonon polaritons launched in hexagonal boron nitride capping layers via its interaction with the buried moiré superlattices. We explore the origin of the double-line features imaged and show the mechanism of the underlying effective phase change of the phonon polariton reflectance at domain walls. The nano-imaging tool developed provides a non-destructive analytical approach to elucidate the complex physics of moiré engineered heterostructures. |
Friday, March 19, 2021 12:30PM - 12:42PM Live |
Y42.00006: Moiré patterns in graphene and transition metal dichalcogenide heterostructures Laurent Molino, Ryan Plumadore, Mohammed Alezzi, Shaffique Adam, Adina A Luican-Mayer Vertically stacked heterostructures of two-dimensional materials provide a platform for realizing novel electronic states due to proximity effects. In particular, moiré patterns in two-dimensional material heterostructures have been shown to create flat bands that favor the occurrence of correlated electronic states. In this work, we use scanning tunneling microscopy and spectroscopy to study moiré patterns in mechanically assembled heterostructures of graphene and transition metal dichalcogenides. By comparing to a theoretical model, in the case of graphene - ReS2 heterostructures we find the presence of stripped moiré patterns, reflecting the different crystal symmetry of the two lattices. |
Friday, March 19, 2021 12:42PM - 12:54PM Live |
Y42.00007: LEEM imaging of the moiré pattern of twisted bilayer graphene Tobias de Jong, Tjerk Benschop, Xing Chen, Dmitri K. Efetov, Felix Baumberger, Rudolf M Tromp, Michiel J.A. de Dood, Milan P. Allan, Sense Jan van der Molen The discovery that magic angle twisted bilayer graphene (MABLG) is a superconductor, yields the promise of exciting new solid state physics [1]. However, the influence of inhomogeneity of twist angle, strain and defects on charge transport properties in these exfoliated, torn and stacked flakes remains an important open question. |
Friday, March 19, 2021 12:54PM - 1:06PM Live |
Y42.00008: Thermodynamic study of magic-angle graphene using scanning SET. Uri Zondiner, Asaf Rozen, Jeong Min Park, Daniel Rodan-Legrain, Yuan Cao, Raquel Queiroz, Takashi Taniguchi, Kenji Watanabe, Ady Stern, Felix von Oppen, Yuval Oreg, Erez Berg, Pablo Jarillo-Herrero, Shahal Ilani I will present local measurements of electronic compressibility, entropy, and magnetization in magic-angle graphene, taken by a scanning single-electron transistor (SET). |
Friday, March 19, 2021 1:06PM - 1:18PM Live |
Y42.00009: Thermodynamics of spin waves in a quantum Hall ferromagnet Seung Hwan Lee, Andrew Pierce, Yonglong Xie, Patrick R Forrester, Kenji Watanabe, Takashi Taniguchi, Bertrand I Halperin, Amir Yacoby Under sufficiently large magnetic field, graphene exhibits quantum Hall ferromagnetism at partial fillings of the lowest Landau level, where strong Coulomb interaction breaks its SU(4) isospin symmetry. In particular, the ν= ±1 state hosts spin-wave excitations, whose generation and propagation have been studied using transport. In this talk, we present a local thermodynamic measurement of the ν= 1 state in the presence of magnons using a scanning single electron transistor (SET). Our electronic compressibility measurements show that pumping magnons into the bulk results in a reduction of the ν=1 gap, and allow us to extract numerous thermodynamic quantities including the magnon density, chemical potential, and average skyrmion spin. Our methods furnish an entirely novel means of probing strongly-interacting electronic phases by using electrically-addressable charge-neutral excitation, which is likely to be applicable to a variety of systems with strong interaction, in particular twisted van der Waals heterostructures with flat bands. |
Friday, March 19, 2021 1:18PM - 1:30PM Live |
Y42.00010: Isotopic Effect of Carrier Relaxation in Graphene-hBN Heterostructures Alexandra Brasington, Song Liu, James Edgar, Takashi Taniguchi, Kenji Watanabe, Arvinder S Sandhu, Brian J LeRoy In atomically thin systems, the choice of substrate plays an important role in the relaxation of photo-excited carriers. In previous work, hexagonal boron nitride (hBN) substrates have been shown to improve the thermal relaxation rates of carriers in graphene as compared to silicon oxide substrates. Naturally occurring boron contains a mixture of two isotopes with atomic masses 10 and 11 with abundances of 20% and 80% respectively. Theoretical studies have predicted a higher thermal conductivity with higher isotopic purity of hBN, due to reduced phonon scattering from isotopic defects. We utilize femtosecond pump-probe spectroscopy to observe the time dynamics of photo-excited carriers in graphene-hBN heterostructures for both natural and isotopically pure hBN and find faster relaxation in isotopically pure hbn substrates. |
Friday, March 19, 2021 1:30PM - 1:42PM Live |
Y42.00011: Imaging small angle twisted bilayer graphene aligned with hexagonal Boron Nitride Xinyuan Lai, Nikhil Tilak, Jinhai Mao, Yuhang Jiang, Kenji Watanabe, Takashi Taniguchi, Francisco Guinea, Eva Andrei Magic-angle twisted bilayer graphene (MATBG) hosts low energy eight-fold degenerate flat bands, whose quenched kinetic energy facilitates interaction-driven instabilities as the Fermi energy is swept through the band. Breaking the sublattice symmetry of the MATBG by aligning its crystallographic axis with that of a hexagonal Boron Nitride (hBN) substrate creates a staggered potential which lifts the sublattice degeneracy and opens gaps that reveal sub-bands with non-trivial topology and orbital magnetism. Using scanning tunneling microscopy and spectroscopy (STM/STS) on MATBG aligned with hBN, we observe a network of moiré solitons where the hBN and MATBG crystals are perfectly commensurate. STS maps of this system reveal a strong reconstructed local density of states within each moiré cell, which together with its magnetic field dependence sheds light on the nature of the broken symmetry phases in this system. |
Friday, March 19, 2021 1:42PM - 1:54PM Live |
Y42.00012: Direct Observation of Dirac Fermion Cloning in Graphene/SiC Heterostructure Qiangsheng Lu, Guang Bian The electronic coupling between graphene internal layers and the coupling between graphene and substrates could generate lots of interesting physics phenomenon, specially, the twisting of the different graphene layers and the lattice mismatch between graphene and the substrates could generate the cloning of the Dirac cones, and these Dirac Fermion cloning can modulate the electron properties dramatically and form Mott insulator, super-conductivity, Ferro-magnetism, Wigner Crystal, Anomalous Hall effect, etc. |
Friday, March 19, 2021 1:54PM - 2:06PM Live |
Y42.00013: Bound states in continuum in graphene interfaces Zhongwei Dai, Zhaoli Gao, Sergey Pershoguba, Nikhil Tiwale, Ashwanth Subramanian, Qicheng Zhang, Calley Eads, Samuel Tenney, Chang-Yong Nam, Jiadong Zang, charlie T johnson, Jerzy T. Sadowski We present experimental evidence of discrete bound states coexisting in the continuum (BIC) in certain graphene interface systems. The interaction of discrete states and the continuum is enabled by interface quantum confinement, and is a direct result of resonant interlayer multiple scattering. We used elastic electron back-scattering, in the very low energy range, to characterize these interlayer resonant states in graphene, using a low energy electron microscope (LEEM). We used Raman spectroscopy to further probe the inelastic light-matter interaction. We discovered a unique type of Fano-resonance around the D mode and G mode of the graphene lattice vibrations. The anomalous Fano-resonance is also a direct result of the quantum confinement and interplay between discrete phonon states and excitonic electronic continuum. |
Friday, March 19, 2021 2:06PM - 2:18PM Live |
Y42.00014: Twist angle homogeneity in twisted bilayer graphene devices studied with STM Tjerk Benschop, Tobias de Jong, Petr Stepanov, Xiaobo Lu, Vincent Stalman, Sense Jan van der Molen, Dmitri K. Efetov, Milan Allan Theoretic models that describe magic-angle twisted bilayer graphene strongly depend on how homogeneous the twist angle is. Furthermore, (in)homogeneity is suggested to be the cause for differences between similar devices fabricated by different research groups. It is thus of high importance to characterize the (in)homogeneity experimentally, on the local scale. We use scanning tunneling microscopy to explore non-encapsulated twisted bilayer graphene devices with different nominal twist angles. In this talk, I will introduce a new method to quantitatively describe the local (in)homogeneity present in these devices, potentially on different length scales. |
Friday, March 19, 2021 2:18PM - 2:30PM Live |
Y42.00015: One-dimensional moiré superlattices and flat bands in collapsed chiral carbon nanotubes Olga Arroyo Gascón, Ricardo Fernández-Perea, Eric Suarez Morell, Carlos Cabrillo, Leonor Chico We demonstrate that one-dimensional moiré patterns, analogous to those found in twisted bilayer graphene [1, 2], can arise in collapsed chiral carbon nanotubes. Resorting to a combination of approaches, namely, molecular dynamics to obtain the relaxed geometries and tight-binding calculations validated against ab initio modeling, we find that magic angle physics occur in collapsed carbon nanotubes [3]. Velocity reduction, flat bands and localization in AA regions with diminishing moiré angle are revealed, showing a magic angle close to 1°. From the spatial extension of the AA regions and the width of the flat bands, we estimate that many-body interactions in these systems are stronger than in twisted bilayer graphene. Chiral collapsed carbon nanotubes stand out as promising candidates to explore many-body effects and superconductivity in low dimensions, emerging as the one-dimensional analogues of twisted bilayer graphene. |
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