Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F55: Invited Session: Electronic Properties of Twisted Van der Waals heterostructures |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Pablo Jarillo-Herrero, Massachusetts Institute of Technology Room: Four Seasons Ballroom 1 |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F55.00001: Quantum Spin Hall Effects and Interactions in Twisted Bilayer Graphene Invited Speaker: Javier D. Sanchez-Yamagishi Twisted bilayer graphene is the ultimate limit of a bilayer 2DEG, where two graphene layers are stacked directly on top of each other with an interlayer distance of only 0.34nm. This system owes its rich electronic structure to an interlayer tunnel coupling which can be continuously tuned by twisting the two layers. At large twist angles, the system behaves as two decoupled monolayer graphene sheets, where inter-layer and intra-layer Coulomb interactions compete to form new ground states. We investigate the possibility of realizing a quantum spin Hall state in twisted bilayer graphene when it is doped to form an electron-hole bilayer at moderate magnetic fields. In this regime, counter-propagating edge modes exist on different layers and the occupation of each mode can be independently controlled. We discuss the electronic properties of this twisted bilayer graphene quantum spin Hall state and the role of electron-electron interactions in its realization. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F55.00002: Stacking textures and singularities in bilayer graphene Invited Speaker: Eugene Mele Multilayer graphenes feature special functionalities that microscopically arise from the atomic registry when graphene sheets are stacked. These depend on relative lateral translations, rotations and layer symmetry breaking that can occur spontaneously or be induced. This talk will focus on bilayer graphenes (BLG) in which the stacking arrangement varies in space. We examine domain walls where the local stacking order switches from local AB to BA registry, and study the electronic modes at the boundary by analyzing their valley-projected four band continuum models augmented by numerical calculations on a lattice. We then consider the more general family of two dimensional strain-minimizing BLG stacking textures, finding that they are twisted textures of the interlayer displacement field. We study the interactions and composition rules for these elementary textures which permit a unified treatment of stacking point defects, domain walls and twisted graphenes. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F55.00003: Electronic properties of moire superlattice bands in layered two dimensional materials Invited Speaker: Jeil Jung When atomically thin two-dimensional materials are layered they often form incommensurate non-crystalline structures that exhibit long period moir\'e patterns when examined by scanning probes. In this talk, I will present a theoretical method which can be used to derive an effective Hamiltonian for these twisted van der Waals heterostructures using input from ab initio calculations performed on short-period crystalline structures. I will argue that the effective Hamiltonian can quantitatively describe the electronic properties of these layered systems for arbitrary twist angle and lattice constants [1-2]. Applying this method to the important cases of graphene on graphene and graphene on hexagonal-boron nitride, I will present a series of experimentally observable quantities that can be extracted from their electronic structure, including their density of states and local density of states as a function of twist angle, and compare with available experiments. \\[4pt] [1] Moire bands in twisted double-layer graphene, R. Bistritzer and A. H. MacDonald, PNAS 108 (30), 12233 (2011).\\[0pt] [2] Ab initio theory of moire bands in layered two-dimensional materials, J. Jung, A. Raoux, Z. H. Qiao and A. H. MacDonald, (submitted). [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F55.00004: Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene Invited Speaker: Aaron Bostwick Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions that have a bandgap tunable by a transverse electric field. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist, strain, or electronic interactions that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. We find by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as $\sim$ 0.1$^{\circ}$, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum may be widely relevant to charge transport in bilayer graphene. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 11:00AM |
F55.00005: Imaging and Spectroscopy of Graphene Heterostructures Invited Speaker: Brian LeRoy Graphene on hexagonal boron nitride (hBN) is an example of a van der Waals heterostructure where the electronic properties of the composite material can be different from either individual material. The lattice mismatch and twist angle between graphene and hBN produces a moir\'{e} pattern in STM topographic images. For all angles, we have observed that the surface roughness of the graphene is reduced by at least an order of magnitude as compared to graphene on silicon oxide devices. Near the charge neutrality point, graphene breaks up into a series of electron and hole puddles due to potential fluctuations. Using scanning tunneling spectroscopy, we have shown that at large twist angles the potential fluctuations are reduced by an order of magnitude by the presence of the hBN [1]. Using heterostructures with graphite gates underneath the hBN [2], we have observed even further reduction in the potential fluctuations. At small twist angles, the hBN substrate produces a weak periodic potential which can have a wavelength of up to 14 nm. This periodic potential creates a new set of superlattice Dirac points at the wavevector of the potential. As the relative rotation angle between the graphene and hBN changes, the energy of this superlattice Dirac point changes. These new superlattice Dirac points have a reduced and anisotropic Fermi velocity. Using gate voltage dependent scanning tunneling spectroscopy, we have observed the effect of the new Dirac points on the local density of states in graphene [3]. Our latest results on other graphene heterostructures will also be discussed.\\[4pt] [1] J. Xue et al., Nature Materials 10, 282 (2011).\newline [2] B. Hunt et al., Science 340, 1427 (2013).\newline [3] M. Yankowitz et al., Nature Physics 8, 382 (2012). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700