Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E43: Strain, Stack, Fold, and Twist: New Strategies for Engineering the Electronic Properties of 2D MaterialsInvited
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Sponsoring Units: DCMP Chair: Eva Andrei, Rutgers University, New Brunswick Room: BCEC 210B |
Tuesday, March 5, 2019 8:00AM - 8:36AM |
E43.00001: Rotationally Controlled Atomic Layer Heterostructures Invited Speaker: Emanuel Tutuc Heterostructures of atomic layers such as graphene, hexagonal boron-nitride, and transition metal dichalcogenides (TMDs) can serve as testbed for novel quantum phenomena in two-dimensions (2D). A key ingredient that can add a new dimension to the atomic layer heterostructures palette is the rotational control and alignment of different 2D layers. We review here an experimental technique that enables rotationally controlled heterostructures with accurate alignment of individual layer crystal axes [1]. We illustrate the applicability of this technique to the realization of tunable moiré patterns in twisted bilayer graphene [2], and rotationally aligned double layers of graphene [3], or TMDs [4] separated by a tunnel barrier, which display resonant, energy- and momentum-conserving tunneling in vertical transport. Rotationally aligned double bilayer graphene separated by a WSe2 tunnel barrier, designed to study equilibrium indirect exciton formation, display a strongly enhanced tunneling at low temperatures when the two graphene bilayers are populated with carriers of opposite polarity and equal density [5]. The enhanced tunneling at overall neutrality departs markedly from single-particle model calculations, and suggests the emergence of a many-body state with when electrons and holes of equal densities populate the two layers. |
Tuesday, March 5, 2019 8:36AM - 9:12AM |
E43.00002: Strain-induced Pseudo-Magnetic Fields and flat band in Graphene Invited Speaker: Yuhang Jiang The discovery of free-standing 2D atomic layers ushered in a new era of materials whose properties can be shaped controlled and engineered by unconventional means such as strain, bend or twist. An externally imposed periodic potential breaks up the electronic band structure into a series of mini-bands which, under certain circumstances, become almost dispersionless. Such flat bands can occur naturally in the presence of a vector potential generated by a strain-induced periodic pseudo-magnetic field (PMF). Within these flat bands, the strongly suppressed kinetic energy favors the formation of interaction-driven exotic phases which can give rise to novel electronic properties. In this talk, I will present experimental results demonstrating two pathways to generate strain induced PMFs and flat bands. In the first method, a graphene membrane is supported and strained by a two-dimensional (2D) periodic array (~ 1mm) of Au nano-pillars grown on a hexagonal Boron Nitride (hBN) substrate1. A direct measure of the local strain is achieved by scanning tunneling microscopy (STM) through the magnifying effect of the Moiré pattern formed against the hBN substrate. The strain-induced PMF (~10T) is obtained from the pseudo-Landau level spectra observed in scanning tunneling spectroscopy (STS). The second method utilizes the buckling transition of a compressed graphene membrane to generate a 2D periodic strain pattern. This produces spatially modulated PMFs with periods of order ~10nm and amplitudes exceeding 100T. Using STM, STS and numerical simulations we find that the pseudo-Landau levels associated with the strain-induced periodic PMF give rise to a flat-band at charge-neutrality, providing a pathway to engineering correlated phases by non-chemical means. |
Tuesday, March 5, 2019 9:12AM - 9:48AM |
E43.00003: Superconductivity in twisted graphene layers: electronic structure and interactions. Invited Speaker: Francisco Guinea A special feature of the states in the low energy bands of twisted graphene bilayers is that their charge distribution shows a significant momentum dependence. As a result, the filling of these bands leads to electrostatic potentials, which, in turn, modify the energy levels and the structure of the bands. The dependence of the band shape on the band filling can be described in terms of the emergence of new electron-electron interactions. We quantify and analyze these interactions, and discuss their relation with the observed superconducting and insulating states. |
Tuesday, March 5, 2019 9:48AM - 10:24AM |
E43.00004: Transport in Twisted Bilayer Graphene at Extreme Angles Invited Speaker: Peter Rickhaus Two misaligned layers of graphene exhibit intriguing transport properties that depend drastically on the twist angle. At very large twists, the system behaves similar to two decoupled graphene layers. Upon reduction of the angle, the graphene layers begin to couple, leading to a decrease in Fermi velocity. At a magical angle of 1.1°, superconductivity emerges [1]. Finally, at tiny twist angles, large regions of strongly coupled bilayer graphene dominate the transport. By gating, these regions can become insulating leading to a triangular network of topological channels [2]. An intriguing probe for transport phenomena in graphene consists in the formation of an electronic Fabry-Pérot interferometer [3,4]. P-n junctions serve as semi-transparent mirrors and exhibit relativistic phenomena such as Klein-tunneling and we discuss how they can be used to access the interesting physics twisted bilayer graphene at tiny [5] and at very large twist angles. |
Tuesday, March 5, 2019 10:24AM - 11:00AM |
E43.00005: Electrically tunable frustrated lattices and magnetism in twisted bilayer graphene Invited Speaker: Jose Lado Twisted graphene bilayers have become a powerful solid-state platform to realize a plethora of electronic states, stemming from the emergence of controllable moire superlattices and their electrical tunability. Here we theoretically show that twisted bilayer graphene is a versatile platform to realize tunable frustrated magnets, by realizing triangular and Kagome lattices of localized modes in two dramatically different regimes [1,2]. First, for rotation angles around 1.5 degrees, we show that the magnetic instabilities of the emergent triangular AA lattice can be electrically controlled [1]. In particular, we show that the ferromagnetic and antiferromagnetic ordering inside the AA regions can be electrically switched, giving rise to superlattice spin spirals in the emergent triangular moire superlattice. Second, we show that in the tiny angle regime of 0.2 degrees, an emergent Kagome lattice of localized modes can be electrically generated, stemming from an artificial gauge field created by the interlayer bias [2]. In particular, we show that the microscopic properties of these Kagome modes are controlled by the magnitude of the electrically generated gauge field, yielding a powerful solid-state platform to experimentally engineer a paradigmatic model of frustrated magnetism. These rich and electrically controllable frustrated lattices provide a potential route to experimentally explore frustrated magnetism and quantum spin liquid physics in twisted graphene bilayers. |
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