Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A51: Electronic Properties of Graphene Based Twisted Heterostructures: Transport Studies |
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Sponsoring Units: DCMP Chair: Iqbal Utama, University of California, Berkeley Room: Mile High Ballroom 1D |
Monday, March 2, 2020 8:00AM - 8:12AM |
A51.00001: Dynamic rotation of monolayer van der Walls heterostructures Josh Swann, Shaowen Chen, Nathan Finney, Derick Gonzalez-Acevedo, Matthew A Yankowitz, Cory Dean Emerging at a twist angle of around 1.1 degrees, the flat band dispersion of twisted bilayer graphene (tBLG) provides an exciting means of studying the physics of strongly correlated states. These strong correlations have recently been shown in a number of studies to give rise to superconducting and correlated insulating states as well as anomalous Hall effects. However, the physics of these states is highly sensitive to slight changes in the twist angle away from the so called ‘magic angle’. We report on a new technique for fabricating twisted bilayer graphene devices using an Atomic Force Microscope to control this angle with a high degree of precision. Techniques that enable monolayers to be rotated to arbitrary angles while also preventing the twist angle from relaxing to the Bernal stacking phase will be discussed. |
Monday, March 2, 2020 8:12AM - 8:24AM |
A51.00002: Shear phonon modes in twisted bilayer 2D crystals Seoung-Hun Kang, Kisung Chae, Mikito Koshino, Young-Woo Son Based on large scale first-principles computations and molecular dynamics simulations, we present variation of low energy in-plane phonon modes in twisted bilayer 2D crystals such as graphene and transition metal dichalcogenides. It is shown that the shear phonon modes soften as the twisted angle increases to 30 degrees while layer breathing modes does not. Discussions regarding on relation between the shear phonon softening, stability of quasicrystalline ordering and interlayer friction will be present. |
Monday, March 2, 2020 8:24AM - 8:36AM |
A51.00003: Correlated insulating and superconducting states in twisted bilayer graphene below the magic angle Emilio Codecido, Qiyue Wang, Ryan Koester, Shi Che, Haidong Tian, Rui Lv, Son Tran, Kenji Watanabe, Takashi Taniguchi, Fan Zhang, Marc Bockrath, Chun Ning Lau The emergence of flat bands and correlated behaviors in “magic angle” twisted bilayer graphene (tBLG) has sparked tremendous interest, though its many aspects are under intense debate. Here we report observation of both superconductivity and the Mott-like insulating state in a tBLG device with a twist angle of ~0.93°, which is smaller than the magic angle by 15%. At an electron concentration of ±5 electrons/moiré unit cell, we observe a narrow resistance peak with an activation energy gap ~0.1 meV. This indicates additional correlated insulating state, and is consistent with theory predicting a high-energy flat band. At doping of ±12 electrons/moiré unit cell we observe resistance peaks arising from the Dirac points in the spectrum. Our results reveal that the “magic” range of tBLG is in fact larger than what is previously expected, and provide a wealth of new information to help decipher the strongly correlated phenomena observed in tBLG. |
Monday, March 2, 2020 8:36AM - 8:48AM |
A51.00004: Interaction effects in ultra-clean ABC trilayer/hexagonal boron nitride moire heterostructures Tian Xie, Haoxin Zhou, James R. Ehrets II, Eric Spanton, Takashi Taniguchi, Kenji Watanabe, Andrea Young ABC-stacked trilayer graphene aligned to hexagonal boron nitride hosts narrow, isolated low energy bands which have been shown to ferromagnetism and superconductivity. In addition, these bands can be tuned with an out-of-plane electric field. The main challenge of fabricating this type of samples is the metastability of the ABC-stacking order, which tends to relax to the lower-energy ABA-stacking during the fabrication process. Here we will describe techniques to identify and isolate ABC domains in trilayer graphene and incorporate them into dual-graphite gated heterostructures while preserving the stacking order. Preliminary transport data reveals electric field tunable transitions between resistive states at partial band filling. |
Monday, March 2, 2020 8:48AM - 9:00AM |
A51.00005: Characterization of correlated insulating states and superconductivity in twisted bilayer graphene: Part 1 Harpreet Arora, Robert Polski, Yiran Zhang, Youngjoon Choi, Hechen Ren, Kenji Watanabe, Takashi Taniguchi, Stevan Nadj-Perge Magic Angle Twisted Bilayer Graphene (MATBG) has emerged as an exciting platform to explore correlated insulating states and unconventional superconductivity due to its gate tunable nature. Even though several theoretical models have been proposed, the nature of the many body physics leading to these exotic states is not fully understood. Here we study correlated insulating and superconducting states for a range of angles around the magic angle value. Transport measurements as a function of carrier density, temperature and magnetic field reveal new insights in this correlated system. |
Monday, March 2, 2020 9:00AM - 9:12AM |
A51.00006: Characterization of correlated insulating states and superconductivity in twisted bilayer graphene: Part 2 Robert Polski, Harpreet Arora, Yiran Zhang, Youngjoon Choi, Hechen Ren, Kenji Watanabe, Takashi Taniguchi, Stevan Nadj-Perge Near magic-angle (~1.1°) twisted bilayer graphene (TBG) has emerged as a promising system for studying electron correlations due to the wide tunability of its band structure—using twist angle—and electron/hole density through gating. However, many details on the nature of correlations and their evolution with twist angle have yet to be resolved. We report sub-Kelvin magnetotransport measurements on TBG for several angles near the magic angle, including measurements of correlated insulators and superconductivity on both hole and electron sides. |
Monday, March 2, 2020 9:12AM - 9:24AM |
A51.00007: Engineering Low-Disorder Superlattice Potentials in Graphene-Based Van der Waals Heterostructures Liam Cohen, Eric Spanton, Andrea Young, Kenji Watanabe, Takashi Taniguchi Long wavelength periodic potentials have led to the observation of a variety of electronic phenomena in graphene, including the Hofstadter butterfly, fractional Chern insulators, and flat band physics at zero magnetic field. Typically, such potentials arise due to Moiré patterns generated through lattice or rotationally mismatched layers. While Moiré patterns produce high quality, low-pitch potentials, they do not allow tuning of lattice strength, symmetry, or pitch. Here we present the fabrication of sub 40nm pitch, ultra-clean, lithographically defined metal superlattices. Using bulk capacitance measurements, we show that integrating the superlattices into a multilayer heterostructures allows controllable screening of the disorder potential while preserving a finite strength superlattice potential. I will present our progress towards engineering correlated states in artificial superlattices. |
Monday, March 2, 2020 9:24AM - 9:36AM |
A51.00008: Transport study of transition metal dichalcogenide morié superlattices Tingxin Li, Lizhong Li, Yanhao Tang, Jiacheng Zhu, Song Liu, Katayun Barmak, Kenji Watanabe, Takashi Taniguchi, Jie Shan, Kin Fai Mak Morié superlattices formed in van der Waals materials have emerged as a new platform to explore strongly correlated physics and other emergent phenomena in two-dimensional electronic systems. Superconductivity, correlated insulating states, and ferromagnetism have recently been discovered in the twisted bilayer graphene and ABC trilayer graphene/boron nitride systems. In contrast to the graphene systems, in which spin, valley, and layer degeneracies are all present, the spin and layer degeneracies are lifted in semiconducting transition metal dichalcogenide (TMD) heterobilayers by strong spin-orbital interactions and layer asymmetry. The large band mass and band gap of the TMDs also allow the flat bands to be present for a large range of twist angle. In this talk, we will present a transport study of angle aligned WSe2/WS2 bilayers and discuss properties of the observed correlated insulating states at half filling (i.e. one particle per morié site). |
Monday, March 2, 2020 9:36AM - 9:48AM |
A51.00009: High sensitivity thermodynamic measurements of fractional quantum Hall states in graphene Fangyuan Yang, Alexander A Zibrov, Ruiheng Bai, Takashi Taniguchi, Kenji Watanabe, Andrea Young Thermodynamic quantities such as chemical potential and entropy provide information about many-body ground states that cannot be directly revealed by conventional transport measurements. In this talk, I will present a technique for high sensitivity measurement of the chemical potential in ultra-clean graphene heterostructures, based on a multilayer heterostructure with an embeded graphene layer that functions as an electrometer. An integrated on-chip heater further allows rapid modulation of the temperature, from which we extract the entropy per particle, △S/△N = -△μ/△T. We apply this technique to a high mobility monolayer graphene 2D electron gas at high magnetic fields, measuring the chemical potential and entropy of fractional quantum Hall states in N = 0 and N = 1 Landau levels (LLs) and electron solid states in the N = 2 and N=3 Landau levels. We compare our experimental sensitivity with that required to detect the ground state entropy associated with non-Abelian anyons, as are thought to appear at several filling factors in bilayer graphene. |
Monday, March 2, 2020 9:48AM - 10:00AM |
A51.00010: Van der Waals Force Driven Ultrahigh-Density Piezoelectric Nano-Generators Arrays Yuhang Jiang, Jinhai Mao, Colin Robert Woods, Slavisa Milovanovic, Francois M Peeters, Konstantin S Novoselov, Eva Andrei The elegance of Van der Waals heterostructure emerges as the possibilities to design its function on-demand when choosing different functional materials. When two layers materials are taken to form moire lattice, the Van der Waals force between the two layers could stimulate even more fascinating features forming spontaneously. Here we show, when graphene and single layer BN (SLBN) are minimally twisted superposed, the interlayer interactions propel the two lattices rearrangement and induces a spatial modulated strain configuration. The moire pattern reconciled strain superlattice in single layer BN, companied by its inversion symmetry broken, induces an oscillating charge polarization as revealed by the scanning tunneling microscope. This moire pattern driven charge polarization in SLBN allows us to get a highly ordered nano-generators array with a giant density. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A51.00011: Tunable extra Dirac points in one-dimensional graphene superlattice induced by periodic ferroelectric domains Tianlin Li, Hanying Chen, Kun Wang, Yifei Hao, Le Zhang, Xia Hong In this work, we investigate the transport signature of one-dimensional superlattice (SL) in monolayer graphene induced by pre-patterned periodic domains in an interfacial ferroelectric bottom layer. We work with 50 nm single crystaline ferroelectric Pb(Zr,Ti)O3 (PZT) films deposited on (La,Sr)MnO3 buffered SrTiO3 substrates, and create periodic polarization up (Pup) and down (Pdown) stripe domains on PZT using conductive atomic force microscopy. The domain periodicity varies from 200 nm to 300 nm, and the number of periods changes from 60 to 100. We then transfer hBN-graphene stacks onto the pre-patterned domains and fabricate them into top-gated field-effect devices. The difference in carrier density between the two polarization regions reaches around 3×1013 cm-2 at 2 K due to the pyroelectric effect. We observe extra Dirac points in R(Vg) by applying voltage to the hBN top gate, which is attributed to the SL modification of the band structure. We discuss the effects of the SL period and the width ratio between the Pup and Pdown domains on the position of the extra Dirac points, and the magnetotransport properties of these SLs. |
Monday, March 2, 2020 10:12AM - 10:24AM |
A51.00012: Tunable High Workfunction Contacts: Doped Graphene Merid Belayneh Merid Belayneh1, Sergey Rashkeev1, Feras Al-Dirini2 Fahhad Alharbi2, 3 |
Monday, March 2, 2020 10:24AM - 10:36AM |
A51.00013: Self-Heated Hotspots in Superconducting Nanowires Cooled by Phonon Black-Body Radiation Andrew Dane, Di Zhu, Murat Onen, Jason Allmaras, Reza Baghdadi, Jean-Luc Tambasco, Ignacio Estay Forno, Yukimi Morimoto, Qingyuan Zhao, Marco Colangelo, Ilya Charaev, Mikhail Skvortsov, Alexander Kozorezov, Karl Berggren Heat transfer in nanostructures is of fundamental and practical interest. Sufficient cooling is critical for the operation of superconducting nanowire single photon-detectors (SNSPDs), which can support large current densities (~5 MA/cm2) just prior to being switched into a high resistivity (~250 μΩ*cm) normal state by an incident photon, leading to substantial local heating. The thermal boundary conductance (TBC) between the nanowire and the substrate determines the cooling rate, and hence the reset time, and potentially impacts the quantum efficiency of SNSPSDs. Despite the TBC’s importance, open questions remain about its correct description in few-nm thick films, and little experimental data exists on patterned nanowire devices. |
Monday, March 2, 2020 10:36AM - 10:48AM |
A51.00014: Conductance quantization in cleaved edge overgrowth GaAs quantum wires Henok Weldeyesus, Taras Patlatiuk, Christian Scheller, Gilad Barak, Amir Yacoby, Loren Pfeiffer, Kenneth West, Dominik Zumbuhl
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Monday, March 2, 2020 10:48AM - 11:00AM |
A51.00015: Emergence of dynamical energy gap in an oscillating graphene membrane Nancy Sandler, Dawei Zhai One intriguing aspect of graphene is the effect of strain on its electronic properties, which manifests as a pseudo-magnetic field producing peculiar local charge distributions and Landau level-like flat bands. It is challenging however, to generate global energy gaps for semiconductor applications- the strain needed is either too large or requires precise engineering. Irradiation with polarized light has been proposed as a mechanism for gap generation, but the high frequency and intensity needed may induce sample damage. Oscillating mechanical deformations appear like an alternative that is particularly well suited for suspended samples. Besides, spatial strain variation allows more flexibility in gap engineering. We study the effect of a Gaussian-shaped deformation in graphene with a time-periodic amplitude. Using an effective Floquet Hamiltonian, we predict the appearance of anisotropic dynamical gaps with Green’s function methods and degenerate perturbation theory. The profile of the gap closely follows the modulations in the local density of states caused by strain. We find that the optimal regime for gap generation occurs when a deformation extends over the whole sample, with a magnitude determined by its geometrical parameters. |
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