Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties V |
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Sponsoring Units: DMP Chair: Justin Song, Harvard University Room: Mile High Ballroom 1E |
Tuesday, March 4, 2014 11:15AM - 11:27AM |
G51.00001: Revealing the atomic, electronic and optical properties of two-dimensional Van der Waals heterostructures Leonardo Basile, Juan-Carlos Idrobo In this talk, we will present a study of the optical absorption of graphene on hexagonal boron nitride (h-BN) using a combination of first-principles calculations and aberration-corrected scanning transmission electron microscopy (STEM). We will show the emergence of a novel interesting electron-optical phenomenon present on 2D heterostructures. Specifically, the absorption spectrum of a graphene layer on a h-BN layer under illumination with a dichroic signal was calculated. The results indicate that the rotation angle between graphene and h-BN layers can be used as a tuning variable to achieve valley polarization, that is, to localize electrons to specific momentum valleys. We will discuss how the emergent field of valleytronics, in 2D heterostructures, can be accessed at the atomic scale using a monochromated aberration-corrected STEM and novel vortex electron probes carrying orbital angular momentum. This research was supported by the National Secretariat of Higher Education, Science, Technology and Innovation of Ecuador (SENESCYT) (LB), and the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy (JCI). [Preview Abstract] |
Tuesday, March 4, 2014 11:27AM - 11:39AM |
G51.00002: Probing Pseudospin-mixing Potential in Graphene/Boron Nitride Moire Superlattice by Infrared Spectroscopy Zhiwen Shi, Chenhao Jin, Wei Yang, Long Ju, Jason Horng, Guangyu Zhang, Feng Wang Graphene/boron nitride (BN) Moire superlattice, where the slow superlattice period is superimposed on a fast B-N oscillation, provides an attractive approach to engineer the electron pseudospin in graphene that goes beyond an electrostatic. Here we perform micro-infrared spectroscopy on graphene/BN superlattice, and find that the BN substrate effect in the context of pseudo-spin is much richer than an electrostatic potential. We further show that the BN substrate effect can be modified through electrostatic gating. [Preview Abstract] |
Tuesday, March 4, 2014 11:39AM - 11:51AM |
G51.00003: Landau level spectroscopy of incompressible quantum hall states in BN/bilayer graphene/NbSe2 heterostructures Dmitri Efetov, Clevin Handschin, Lei Wang, Cory Dean, James Hone, Philip Kim Inducing Superconductivity (SC) via proximity effect into the topological edge states of a 2-dimensional (2D) conductor in the Quantum Hall Regime (QHE) has been a long standing proposition which has recently reinvigorated attention. Such devices would allow to study the proximity effect in the ballistic 2D limit, where predictions go as far as specular Andreev Reflections and formation of Andreev Edge States in strong magnetic fields. Here we present a new route of fabrication of such devices made entirely out of cleanly stacked layered van der Waals materials BN/Graphene/NbSe$_{\mathrm{2}}$. Electric contact between NbSe$_{\mathrm{2\thinspace }}$and high mobility BN/graphene channels allows us to perform the Andreev reflection spectroscopy in the fully developed Quantum Hall states. We find that the NbSe2/graphene superconductor-normal metal interface (SN) has a very high transparency with extremely low electrical resistances of R$\sim $100Ohm and gives rise to Andreev reflections in graphene below the critical superconducting transition temperature. The high mobility of the graphene on h-BN and the relatively high upper critical magnetic field of NbSe2 provide a wide magnetic field range where the SC and the QHE coexist. We observe a clear enhancement of the Andreev Reflection probability when Cooper Pairs are injected into the incompressible Quantum Hall states. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G51.00004: Landau levels of graphene on h-BN probed by magneto-optics Zhiguo Chen, Zhiwen Shi, Wei Yang, Guangyu Zhang, Feng Wang, Zhiqiang Li Hexagonal boron nitride (h-BN) is an ideal substrate for achieving high-mobility graphene devices due to its atomically flat and clean surface. Moreover, the coupling between h-BN and graphene at small twist angles gives rise to a long-range moire supperlattice potential, which can significantly modify the electronic properties of graphene. Here, we will present infrared transmission measurements on graphene on h-BN in high magnetic fields applied perpendicular to the samples. Several inter-Landau-level transitions of graphene on h-BN were observed in fields, which exhibit pronounced deviations from the SQRT(B) field dependence for Landau levels of bare graphene. We will discuss possible mechanisms for the modifications of Landau levels of graphene by h-BN. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G51.00005: Hofstadter's Butterfly in a Lithographically Defined Super-Lattice Carlos Forsythe, Diego Scarabelli, Patrick Maher, Kenji Watanabe, Takashi Taniguchi, Shalom Wind, Cory Dean, Philip Kim We will present magnetotransport measurements of a gated superlattice formed on hBN encapsulated graphene in which a local gate provides a lattice-like external potential. Recent improvements in lithography have allowed for the patterning of periodic lattice structures with lattice constants of 40 nm or less. While the Hofstadter fractal energy spectrum has been studied previously through the Moire interference pattern between graphene and hBN, it has not been observed in such a tunable environment. Our multi-gate structure allows us to independently set carrier density and lattice strength, allowing us to investigate the competition between multiple allowed states associated with different lattices in our 2DEG. By comparing different lattice geometries we will also present a richer understanding of how lattice symmetry alters the Hofstadter energy spectrum. [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G51.00006: Substrate Coupling, Transport, and Exchange Interactions of Graphene on Boron Nitride Ashley DaSilva, Jeil Jung, Shaffique Adam, Allan MacDonald Boron nitride has proven to be a nearly ideal substrate for high quality graphene devices. Graphene sheet electrons are nevertheless coupled to the boron nitride by Coulomb interactions with boron and nitrogen ions, and by weak distortions of the graphene sheet bonding structure. Because the lattice constants of graphene and boron nitride differ and because the hexagonal lattices of the two sheets are usually misoriented, the substrate interaction produces a long-period moire pattern. We report on a theory of how the substrate interactions influence the electronic structure and the transport properties of the graphene sheet. Our theory is based on a low-energy effective model [1] of the graphene sheet which accounts for both electrostatic interactions and bonding pattern variations. We find that the conductivity has a minimum when the number of carriers per spin per moire period is an integer, and that exchange interactions have a large influence on the gap at the Dirac point. [1] 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 12:27PM - 12:39PM |
G51.00007: Anomalous Topological Currents in Graphene Superlattices Polnop Samutpraphoot, Justin Song, Leonid Levitov Berry's phases naturally arise from the spinor structure of Dirac systems, yet observation of non-trivial Berry's phase effects in the transport characteristics of Dirac systems, such as the Valley Hal effect, has proved elusive. Recently, layered graphene heterostructures have emerged as a promising setting to observe novel electron dynamics. We will discuss how novel features in Berry's curvature arise in Graphene/h-BN superlattices to allow long range topological currents to develop. Non-intuitively, we find superlattice mini-bands that have non-trivial Valley Chern number even though the sub-lattice asymmetric potential oscillates in sign. This results in clear non-local transport signatures for the topological character of the bands formed in Graphene/h-BN heterostructures. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G51.00008: Fermi velocity renormalization in misoriented graphene on hexagonal boron nitride Mahesh Neupane, Darshana Wickramaratne, Supeng Ge, Gen Yin, Roger Lake The electronic structure, Fermi velocity, and bandgap are calculated for graphene on BN as a function of misorientation angle. The Fermi velocity of Bernal stacked graphene on BN increases to 1.6 $v_0$ where $v_0$ is the velocity of single-layer graphene. For misorientation angles ranging from 5 to 27 degrees, the Fermi velocities of the Dirac electrons in graphene are relatively insensitive to the angle with values ranging between 0.85 and 0.9 $v_0$. In addition, the bandgap at the Dirac point for rotated graphene on BN decreases by an order of magnitude compared to that of perfectly registered graphene on h-BN. This suggests a reduction in the interlayer coupling between the graphene and BN layers due to the rotation. Calculations are performed using density functional theory. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G51.00009: Moire bands in twisted double layer graphene separated by a hBN monolayer Luis Brey We consider double layer graphene separated by a hBN monolayer. The three layers have a relative twist between them. We obtain that although a single monolayer graphene is only slightly perturbed by a hBN layer, in this sandwich structure the two monolayers graphene become strongly coupled. At small twist angle, the Fermi velocity is significantly reduced with respect the isolated single layer value. We study the velocity renormalization as function of the band gap of hBN and the band offset with respect the graphene Dirac point. The coupling between the graphene monolayers is reduced when they are separated by two hBN layers. [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G51.00010: Engineering Electronic Band Structure in Graphene Superlattices on Hexagonal Boron Nitride Guorui Chen, Mengqiao Sui, Yijun Yu, Wei Yang, Kenji Watanabe, Takashi Taniguchi, Guangyu Zhang, Yuanbo Zhang When subjected to a periodic potential, the Dirac fermion spectrum in graphene undergoes dramatic transformation. This makes it possible to engineer electronic band structure in graphene through the formation of Moir\'e patterns on hexagonal Boron Nitride (hBN) substrate. By varying the angle between graphene and hBN substrate, we are able to produce graphene superlattices with different period. We further probe the electronic structure of the graphene superlattices through electronic transport measurements. Vastly different band structures are observed in graphene superlattices with different Moir\'e wavelength, which is in agreement with our theoretical model. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G51.00011: Magneto-phonon resonances in exfoliated graphene on hexagonal boron nitride Christoph Neumann, Sven Reichardt, Marc Droegeler, Kenji Watanabe, Takashi Taniguchi, Slava V. Rotkin, Bernd Beschoten, Christoph Stampfer Raman microscopy has become a powerful and widespread tool in graphene research. An interesting scenario emerges when Raman microscopy is combined with magnetic fields, as transitions between distinct Landau levels can couple to the optical phonon modes responsible for the graphene G-Line, forming magneto-phonon resonances (MPRs). Here, we investigate exfoliated graphene flakes partly deposited on SiO$_{2}$ and partly on hexagonal boron nitride (hBN). Employing a confocal Raman setup with 500 nm spot size and variable magnetic field of up to 9 T, we compare the regions with different substrates. Distinct MPRs occur only in the graphene on hBN area. From the dominant MPR at around 3.7 T we extract an increased Fermi velocity of above 1.15 x 10$^{6}$ m/s, owing to very low doping in our samples. [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G51.00012: Graphene ``butterflies'' G.L. Yu, A. Mishchenko, R. V. Gorbachev, L.A. Ponomarenko, R. Jalil, J.S. Tu, C. R. Woods, D.C. Elias, K.S. Novoselov, A.K. Geim By stacking different 2D crystals on top of each other, some astonishingly properties and new phenomena may be shown. Typically, when graphene is transferred onto atomically flat boron nitride substrate with a certain angel, a moire pattern may come into being. Within this superlattice structure, elections will rearrange themselves to make multiple clones of Dirac fermions. At higher field even more clones would be created, accordingly, the pattern of Hofstadter butterfly can turn out. Here, both of the resistive and capacitive measurements are used to research the Hofstadter spectrum experimentally. Resistive measurement shows the longitudinal conductivity has 1/B oscillations independent of carrier density, while the Hall Effect repeatedly changes its sign with B. Quantum capacitance measurement is employed to examine directly the density of states (DoS) in graphene superlattice devices and its evolution into a clear Hofstadter spectrum. In both case, self-similarity could be observed at the fractions where the magnetic sates begin to entwine, forming a Hofstadter-like pattern. While many more minigaps are observed in the capacitance measurement. [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G51.00013: Scanning Tunneling Microscopy and Landau Level Spectroscopy of twisted graphene double layers on SiO$_{2}$ and hBN substrates Chih-Pin Lu, Eva Andrei, Guohong Li, Adina Luican, T. Taniguchi, K. Watanabe Electrons in Graphene, being confined within a one-atom thick crystal, are very sensitive to environmental disturbances. This makes it possible to engineer vertical heterostructures with designer electronic properties by stacking graphene together with other thin layers. In particular superposing two graphene layers twisted away from Bernal stacking mitigates the effect of substrate-induced random potential fluctuations and provides access to the intrinsic electronic properties near the Dirac point. We studied samples consisting of two stacked Graphene layers deposited on SiO$_{2}$ or BN and configured in a device geometry which allows varying the carrier density by gating across a 300nm layer of SiO$_{2}$. Using low temperature high-field Scanning Tunneling Microscopy and Landau level spectroscopy, we demonstrated that the random potential is significantly weakened compared to the case of single layer graphene deposited on the same substrate. As a result we were able to observe high quality Landau level spectra, comparable to those seen in graphene on graphite, starting at fields as low as 1T. We will report on the effect of isolated impurities on the Landau level spectra and on the evolution of Landau levels into edge states. Supported by DOE-FG02-99ER45742 and NSF DMR 1207108. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G51.00014: Hofstadter Butterfly Formation for Modulated Graphene Godfrey Gumbs, Andrii Iurov, Danhong Huang, Paula Fekete, Liubov Zhemchuzhna A two-dimensional (2D) periodic array of scatterers has been introduced to monolayer graphene in the presence of a uniform perpendicular magnetic field. The corresponding eigenvalue equation has been solved numerically to display the mixing of Landau orbits to form minibans. Comparison of the Hofstadter butterfly in graphene is made with that in modulated 2D electron gas (EG). Additionally, we calculated the \textit{density-of-states} in the low, intermediate and high magnetic field regimes. The results reflect the effect of Landau level structure in the three regimes and specifically the fractal structure at intermediate magnetic fields. [Preview Abstract] |
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