Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M8: Focus Session: Graphene - Twisted Layers, Stacking |
Hide Abstracts |
Sponsoring Units: DMP Chair: Taisuke Ohta, Sandia National Laboratories Room: 307 |
Wednesday, March 20, 2013 8:00AM - 8:36AM |
M8.00001: Single-layer behavior and its breakdown in twisted graphene layers Invited Speaker: Adina Luican-Mayer Stacking order plays a major role in the electronic properties of graphene layers because hopping between carbon atoms in neighboring layers is a key ingredient in their band structure. Twisting the layers away from the equilibrium Bernal stacking, which produces the superstructures known as Moir\'{e} patterns in scanning tunneling microscopy, decreases the overlap between atoms in adjacent layers and therefore significantly alters their electronic properties. Using scanning tunneling microscopy and spectroscopy, we obtained direct evidence for the electronic structure of twisted graphene layers.\footnote{G. Li, A. Luican, J.M. B. Lopes dos Santos, A. H. Castro Neto, A. Reina, J. Kong and E.Y. Andrei, Nature Physics 6, 109 ( 2010).} The samples were membranes of CVD grown graphene and graphite crystals which contain areas with various twist angles. In topographic images the regions where layers are twisted away from Bernal stacking exhibit Moir\'{e} patterns with periods which depend on the twist angle. We find that the density of states on the twisted layers develops two Van Hove singularities that symmetrically flank the Dirac point at an energy that depends on the twist angle. High magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers reveal that for twist angles exceeding $\sim $3 degrees the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20 degrees the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent.\footnote{Luican, G. Li, A. Reina, J. Kong, R. R. Nair, K. S. Novoselov, A. K. Geim, E.Y. Andrei, Phys. Rev. Lett. 106, 126802 (2011).} These results are compared with theoretical predictions. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M8.00002: Interacting Dirac Fermions and Neutrino-Like Oscillation in Twisted Bilayer Graphene Lede Xian, Zhengfei Wang, M.Y. Chou The low-energy quasiparticles in graphene can be described by a Dirac Hamiltonian for massless fermions, hence graphene has been proposed to be an effective medium to study exotic phenomena originally predicted for particle physics, such as Klein tunneling and Zitterbewegung. In this work, we show that another important particle-physics phenomenon -- the neutrino oscillation can be studied and observed in a particular graphene system, namely, twisted bilayer graphene. It has been found that graphene layers grown epitaxially on SiC or by the chemical vapor deposition (CVD) method on metal substrates display a stacking pattern with adjacent layers rotated by an angle with respect to each other. The quasiparticle states in two distinct graphene layers act as neutrinos with two flavors, and the interlayer interaction between them induces an appreciable coupling between these two ``flavors'' of massless fermions, leading to neutrino-like oscillations. In addition, anisotropic transport properties manifest in this specific energy window, which is accessible in experiment for twisted bilayer graphene. We demonstrate that combining two graphene layers enables us to probe the rich physics involving multiple interacting Dirac fermions. [Preview Abstract] |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M8.00003: Landau level splitting in rotationally faulted multilayer graphene Hridis Pal, Markus Kindermann In this work we explore theoretically whether the interlayer motion of electrons in rotationally faulted multilayer graphene can break the valley degeneracy. We show that in~the presence of a magnetic field and interlayer commensurations~this is indeed possible. It leads to the splitting of Landau levels linear in the field. Our theoretical work is motivated by a recent experiment [1] on epitaxially grown multilayer graphene where a splitting of Landau levels was observed. This Landau level splitting was found to be linear in the field at moderate fields. We consider both bilayer and trilayer configurations and find that in both cases a linear splitting can occur. The predicted lack of valley degeneracy is due to a simultaneous breaking of time-reversal symmetry and inversion symmetry by~the magnetic field and interlayer commensurations, respectively. [1] Y. J. Song, et al., Nature 467, 185 (2010).~ [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M8.00004: Simultaneous investigation of magnetoresistance (MR) and twisted angle of twisted bilayer graphene Sung Ju Hong, Julio Manzo, Kyung Ho Kim, Min Park, Seung Jae Baek, Dmitry Kholin, Min Woo Lee, Eun Sang Choi, Dae Hong Jeong, August Yurgens, Maria Drndic, Alan Johnson, Yung Woo Park We have measured magnetoresistance (MR) and twisted angle of twisted bilayer graphene, simultaneously. Twisted angle was measured by transmission electron microscopy (TEM) diffraction experiment on SiN$_{x}$ substrate. We performed Raman spectroscopy experiment and observed enhanced G mode which results from double resonance scattering process near van Hove singularity (vHs). MR shows superposition of two Shubnikov de Haas (SdH) oscillations and is analyzed by Landau fan diagram. [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:48AM |
M8.00005: Probing electronic and vibrational interactions in few-layer graphene by optical spectroscopy Invited Speaker: Chun Hung Lui Graphene possesses remarkable physical properties and great potential for novel applications.~ As more than one graphene layers are stacked on one another, the properties of the few-layer system can be strongly modified by the interactions between electrons and lattice vibrations in different graphene layers. We have investigated, by means of infrared and Raman spectroscopy, the electronic and vibrational properties of few-layer graphene with different layer thickness and stacking sequence. Our results reveal the critical roles of these degrees of freedom in defining the properties of few-layer graphene. We show how optical spectroscopy offers important routes to characterizing the thickness and stacking order of the graphene samples as well as probing the material's response to external perturbations. In particular, we will describe the use of Raman spectroscopy to identify the interlayer breathing modes in few-layer graphene of up to 20 layers in thickness, and the use of Infrared spectroscopy to probe the modulation of electronic structure and electron-phonon interactions in few-layer graphene with varying thickness, stacking order and doping level. This work was performed at Columbia University in collaboration with L. Brus, E. Cappelluti, G. L. Carr, Z.Y. Chen, Z.Q. Li, K.F. Mak, L.M. Malard and R. Saito. [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M8.00006: Resonance profile of Moire-pattern Raman peaks in twisted graphene layers Marcos Pimenta, Ariete Righi, Sara Costa, Cristiano Fantini, Helio Chacham, Carl Magnuson, Rod Ruoff, Wolfgang Bacsa, Luigi Colombo, Pedro Venezuela In this work, we study the Raman spectra of graphene samples grown by CVD on a Cu foil, with different laser excitation lines. The spectra exhibit a number of extra sharp Raman peaks, classified in different families, each one associated with Moire patterns of graphene layers twisted with different angles. The presence of these extra peaks is theoretically analyzed considering the interlayer potential perturbation, that gives rise to a set of wavevectors within the interior of the Brillouin zone of graphene, activating special selective double-resonance (DR) Raman modes, in a so-called umklapp DR (u-DR) process. The resonance Raman profile of the Moire peaks obtained experimentally by changing the laser energy is compared with the calculations of the u-DR process, showing that Raman spectroscopy is useful to characterize Moire patterns in graphene systems. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M8.00007: Interaction Induced Symmetry Breaking in ABA Trilayer Graphene Rohit Hegde, Allan H. MacDonald We present a mean-field phase diagram of dual-gated ABA trilayer graphene which is obtained by numerically solving the self-consistent Hartree-Fock equations. A metal-insulator phase transition occurs in neutral ABA trilayers at interaction strength $\alpha=0.18$ which is not associated with broken lattice symmetries. ABA trilayers do not possess the inversion symmetry present in bilayers, but do possess a mirror-plane symmetry which remains unbroken for realistic values of alpha for the case of spinless, valley-less fermions. The manner in which SU(4) spin-valley symmetry breaks depends on doping, interlayer bias, and the surrounding dielectric medium. We compare interaction effects in ABA graphene with those in the more familiar chirally-stacked multilayers. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M8.00008: Quantized Strain Channels in Bilayer Graphene Adam Tsen, Robert Hovden, Jonathan Alden, Pinshane Huang, Lola Brown, David Muller, Paul McEuen, Jiwoong Park For bilayer graphene, Bernal stacking presents the lowest energy configuration. However, when the two layers are free to translate, there are two mirrored Bernal stacking orders with degenerate energies [1]. In large-area bilayer systems grown by chemical vapor deposition domains of both stacking configurations have been observed [2], although the precise structure of their boundaries was not understood. Here, we image such structures with atomic resolution using scanning transmission electron microscopy (STEM). We find that domain boundaries are formed by continuous strain of one layer with respect to the other, while the direction and magnitude of their displacements are quantized by the energy landscape. Finally, we extend their characterization over many microns with standard dark-field TEM imaging and discover that the strain regions form long channels that can perhaps be exploited for their electronic properties in the future. 1. Lebedeva et al., J. Chem. Phys. 134, 104505 (2011) 2. Brown et al., Nano Lett. 12, 1609 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M8.00009: Electronic structure of multilayer graphene with a mixture of Bernal and rhombohedral stacking Mikito Koshino, Edward McCann We propose a general scheme to describe the electronic band structure of multilayer graphene with an arbitrary mixture of Bernal and rhombohedral stacking. The system can be viewed as a series of finite Bernal graphite sections connected by rhombohedral-type stacking faults. We find that the low-energy eigenstates are mostly localized in each Bernal section, and the whole spectrum is well approximated by a collection of the spectra of independent sections. In the ensemble-averaged electronic structure, there are frequently-appearing linear bands and quadratic bands with particular band velocities or curvatures, corresponding to finite Bernal sections and their combinations. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M8.00010: Electronic dispersion from long-range atomic ordering and periodic potentials in two overlapping graphene sheets Taisuke Ohta, Jeremy Robinson, Peter Feibelman, Thomas Beechem, Bogdan Diaconescu, Aaron Bostwick, Eli Rotenberg, Gary Kellogg A worldwide effort is underway to learn how to build devices that take advantage of the remarkable electronic properties of graphene and other two-dimensional crystals. An outstanding question is how stacking two or a few such crystals affects their joint electronic behavior. Our talk concerns ``twisted bilayer graphene (TBG),'' that is, two graphene layers azimuthally misoriented. Applying angle-resolved photoemission spectroscopy and density functional theory, we have found van Hove singularities (vHs) and associated mini-gaps in the TBG electronic spectrum, which represent unambiguous proof that the layers interact. Of particular interest is that the measured and calculated electronic dispersion manifests the periodicity of the moir\'e superlattice formed by the twist. Thus, there are vHs not just where the Dirac cones of the two layers overlap, but also at the boundaries of the moir\'e superlattice Brillouin zone. Moir\'es, ubiquitous in hybrid solids based on two-dimensional crystals, accordingly present themselves as tools for manipulating the electronic behavior. [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