Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y1: Focus Session: Graphene - Multilayer and Interfacial Effects |
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Sponsoring Units: DMP Chair: Rui He, University of Northern Iowa Room: 001A |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y1.00001: The decoupling of epitaxial graphene on SiC by hydrogen intercalation: an \textit{ab initio} study Lydia Nemec, Patrick Rinke, Volker Blum, Matthias Scheffler Large-scale ordered epitaxial graphene can be grown on various substrates, out of which silicon carbide (SiC) is one of the most promising. The exact material properties of graphene depend on the growth conditions and its interaction with the substrate. By hydrogen intercalation of epitaxial graphene on the Si-face of SiC the graphene layer decouples from the substrate forming quasi-free-standing monolayer graphene (QFMLG) [1]. We performed an density functional theory study of QFMLG on the polar 6H-SiC(0001) surface based on a van der Waals corrected semi-local exchange-correlation functional using the all-electron numeric atom-centered basis function code FHI-aims. We find an adsorption height in excellent agreement with X-ray standing wave experiments, a very low buckling of the graphene layer, and a very homogeneous electron density at the interface. All these features improve the electronic properties of QFMLG compared to epitaxial graphene. Using the insight gleaned on the Si-face, we present the structure of a hypothetical QFMLG phase on the C-face of SiC. We find that hydrogen intercalation is a promising option to control the SiC-graphene interface. [1] C. Riedl, \textit{et. al}, PRL 103, 246804 (2009). [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y1.00002: Lithium Intercalation of Few-Layer Graphenes in the 2-Layer Limit Shu Yang Frank Zhao, Giselle A. Elbaz, Dmitri K. Efetov, Jayakanth Ravichandran, Yinsheng Guo, Louis Brus, Xavier Roy, Philip Kim Few layer graphene (FLG) intercalate compounds form a new generation of graphene derivative systems where carrier densities are expected to reach 6E14 cm$^{-2}$ per graphene layer, and novel physical phenomena such as superconductivity and magnetism may emerge. Experimental realization of intercalated FLGs have been limited by harsh intercalation processes which are often incompatible with mesoscopic device fabrication techniques. We developed techniques to electrochemically intercalate FLGs down to 2-layers with lithium in-situ in a controlled manner, minimizing sample degradation from parasitic reactions in the electrolyte by passivating sample surfaces using a combination of hBN (over graphene) and photoresist (over metal contacts). By performing simultaneous Raman spectroscopy as the FLGs intercalate, we found that as FLGs reached the 2-layer limit, the Raman signatures of intercalation began to deviate from that of bulk graphite. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y1.00003: Electronic Structure of Single-Crystal Monolayer Graphene on Hydrogen-Terminated Germanium Surface Sung Joon Ahn, Jae-Hyun Lee, Joung Real Ahn, Dongmok Whang Graphene, atomically flat 2-Dimensional layered nano material, has a lot of interesting characteristics from its unusual electronic structure. Almost properties of graphene are influenced by its crystallinity, therefore the uniform growth of single crystal graphene and layer control over the wafer scale areas remains a challenge in the fields of electronic, photonic and other devices based on graphene. Here, we report the method to make wafer scale single crystal monolayer graphene on hydrogen terminated germanium(110) surface and properties and electronic band structure of the graphene by using the tool of scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, electron transport measurement, electron diffraction and angle-resolved photoemission spectroscopy. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 9:12AM |
Y1.00004: Formation and electronic properties of coherent in-plane 2D heterostructures Invited Speaker: An-Ping Li Two-dimensional (2D) interfaces between crystalline materials have been shown to generate unusual interfacial electronic states in complex oxides. Recently, a one-dimensional (1D) interface has been suggested in hexagonal boron nitride (hBN) and graphene planar heterostructures, where a polar-on-nonpolar 1D boundary is expected to possess peculiar electronic states associated with edge states of graphene and the polarity of hBN. Here, we report on the formation and electronic properties of such a 1D interface. By implementing the concept of epitaxy to 2D space, we grow monolayer hBN from fresh edges of monolayer graphene with lattice coherence, forming a 1D boundary [L. Liu et al., Science 343, 163 (2014)]. Scanning tunneling microscopy and spectroscopy measurements reveal an abrupt 1D zigzag oriented boundary, with boundary states about 0.6 eV below or above the Fermi level depending on the termination of the hBN at the boundary [J. Park et al., Nature Commun. 5, 5403 (2014)]. The boundary states are extended along the boundary, and exponentially decay into the bulk of graphene and hBN. The origin of boundary states and the effect of the polarity discontinuity at the interface will be discussed. [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y1.00005: Defect-Stabilized Graphene-Based Organometallic Sandwich Structures Pratibha Dev, Thomas Reinecke Benzene-transition metal-graphene (Bz|M|Gr) sandwich structures are of interest in a range of applications such as catalysis, spintronics and quantum computing. Although they are predicted to form in several theoretical works, it has proven harder to create these complexes experimentally. Using density functional theory, we propose a chemical route to creating stable Bz|M|Gr sandwich structures. Acceptor-type defects, such as carbon vacancies and pyridinic nitrogen substituents in graphene are used to immobilize the metal onto graphene. Placing a benzene ring atop the metal atom further stabilizes the structure against oxidation. Structural, electronic and magnetic properties of the Bz|M|Gr complexes vary for different defects. High cohesive energies and spin polarization energies make defect-stabilized Bz|M|Gr complexes of interest for use as nanomagnets in ambient conditions. [Preview Abstract] |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y1.00006: Single-Valley Engineering in Graphene Superlattices Yafei Ren, Xinzhou Deng, Changsheng Li, Jeil Jung, Changgan Zeng, Zhenyu Zhang, Qian Niu, Zhenhua Qiao The two inequivalent valleys in graphene preclude the protection against inter-valley scattering offered by an odd-number of Dirac cones characteristic of $Z_2$ topological insulator phases. Here we propose a way to engineer a chiral single-valley phase in a honeycomb lattice via folding K and K$'$ valleys onto $\Gamma$ point in tailored $\sqrt{3}N\times\sqrt{3}N$ or $3N\times 3N$ superlattices. The corresponding effective Hamiltonians for top-site adatom adsorption leads to inter-valley coupling and valley-orbit coupling mechanisms that resemble the conventional in-plane Zeeman fields and spin-orbit coupling of the electron spins that have important implications in valleytronics to control the valley polarization coherently. By folding the inequivalent K and K$'$ valleys together, the single valley phase can be formed and engineered by the the inter-valley coupling and staggered sublattice potentials from inversion symmetry breaking. A topological phase transition can take place from the quantum valley-Hall phase at large staggered sublattice potential to a chiral single-valley phase with quadratic band crossover resembling the electronic structure of one \textit{half} AB-stacked bilayer graphene for sufficiently strong inter-valley coupling. [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y1.00007: Polycrystalline Graphene with Single Crystal Electronic Structure Edward B. Lochocki, Lola Brown, Jos\'e Avila, Cheol-Joo Kim, Yui Ogawa, Robin W. Havener, Dong-Ki Kim, Eric J. Monkman, Daniel E. Shai, Haofei I. Wei, Mark P. Levendorf, Mar\'Ia Asensio, Jiwoong Park, Kyle Shen Stacking two-dimensional materials is a promising method for creating and controlling vertical heterostructures with atomic precision. The relative rotation angles between layers can sensitively tune these structures' electronic and optical properties, so constituent layers with well-defined lattice orientations are critical for any practical application. Here we report the growth of large scale graphene and hexagonal boron nitride on commercial copper foils, where the resulting films display multiple nucleations yet exhibit a uniform orientation. We characterize the copper and graphene lattices on sizes ranging from nanometers to several centimeters using a multitude of probes including dark field transmission electron microscopy and angle-resolved photoemission spectroscopy. These measurements reveal that each individual graphene grain exhibits an identical electronic structure and orientation consistent with single crystalline graphene. Finally, we create stacked bilayer graphene with a homogeneous interlayer rotation angle, demonstrating a versatile approach for scalable fabrication of layered superlattices with accurate structures. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y1.00008: Isotope dependence of the electronic structure in graphene Takashi Koretsune, Susumu Saito It has been known that the effect of electron-phonon couplings on the electronic structure of diamond is not negligible and recently, it has been confirmed that the experiments are well reproduced using first-principles calculations. In case of graphene, the renormalization of the Fermi velocity due to the electron-phonon couplings has been predicted. Thus, we theoretically study the possibility of band structure engineering in graphene using the electron-phonon couplings and the isotope effect. First, we consider the difference of pure 12C graphene and 13C graphene. On the basis of density functional theory, it is found that the depth of so-called Dirac point, that is, work function of graphene, shows isotope dependence, indicating that it is possible to shift the depth of the Dirac point locally without using a gate voltage. We also discuss the possibility of band-gap opening by a periodic patterning of the carbon isotope. [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y1.00009: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y1.00010: Velocity Renormalization of Interacting Dirac Fermions in Undoped Graphene: Functional Renormalization Group Study Anand Sharma, Carsten Bauer, Peter Kopietz, Valeri Kotov We present a functional renormalization group (fRG) study of the velocity renormalization due to electron-electron interactions in undoped graphene. The role of long-range Coulomb interaction has remained elusive in graphene. It is known that the electronic properties of graphene can be modeled by (2 + 1)-dimensional Dirac electrons, coupled by instantaneous Coulomb forces, and have a linear energy spectrum in the low-energy limit. Using the fRG with partial bosonization of the Coulomb interaction in the forward scattering channel, we obtain the quasi-particle Green's function of the model. We derive the fRG flow equations for the fermionic and bosonic self-energy, as well as for the triangular vertex and calculate the renormalized velocity of the interacting Dirac fermions. We also determine the anomalous dimension of the Dirac field and evaluate the critical interaction strength for chiral symmetry breaking. [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 10:36AM |
Y1.00011: Dirac Cone Metric and the Origin of the Spin Connections in Monolayer Graphene Bo Yang There have been extensive efforts in modeling the strain and ripples of the monolayer graphene sheet in the form of the effective gauge fields, both from a microscopic point of view and from the quantum field theoretical (QFT) approach used in treating Dirac spinors moving in a curved space (M.A.H. Vozmediano et.al, Phys. Rep. 496, 109, F. Guinea et.al, Nat. Whys. 6, 30). With the QFT approach, it is argued that the metric from either the two-dimensional manifold of graphene sheet or from the in-plane strain field introduces a spin connection that couples to the sublattice pseudospin. Yet the microscopic origins of such an analogy, and the nature of the ``spin connection'' that couples to the sublattice pseudospin, was not clear. We solve this issue by showing that the modulation of the hopping amplitudes in the honeycomb lattice of the monolayer graphene uniquely defines a metric which corresponds to the geometry of the Dirac cone. This effective metric is different from the real space metric of the crystal lattice, and is entirely the property of the fermi surface. We show how the exact spin connection of this momentum space effective metric field can be calculated from the microscopic tight-binding Hamiltonian, and discuss its experimental implications. [Preview Abstract] |
Friday, March 6, 2015 10:36AM - 10:48AM |
Y1.00012: ABSTRACT WITHDRAWN |
Friday, March 6, 2015 10:48AM - 11:00AM |
Y1.00013: Electronic Properties of Graphene in Strong Static Electric Field Vadym Apalkov, Hamed Koochaki Kelardeh, Mark Stockman We study the dynamics of electrons in an ultra-strong static electric field (a few V/{\AA}) and obtain an analytical solution for the Wannier-Stark (WS) states and corresponding energy spectrum of graphene within the two-band tight binding model. Electron states in graphene have a WS ladder structure with energy levels separated by the Bloch frequency, which is proportional to both the electric field and the lattice period of graphene in the direction of electric field. The strength of the band mixing is determined by the magnitude of the interband dipole matrix element, which for graphene has distinct wave vector dependence seen neither in metals nor in insulators. Namely, at the Dirac points, the dipole matrix elements have sharp peaks due to strong interband coupling leading to redistribution of carrier density and very strong mixture of conduction and valence bands whereas, away from the Dirac point, it shows a broad maximum. As a result of such mixing, the energy spectrum of graphene shows anticrossing points, which are characterized by the corresponding anticrossing gaps. It is shown that the anticrossing gaps are proportional to electric field at the corresponding anticrossing points with the calculated values 2.54/l (eV), where l$=$1,2,\textellipsis is the order of the anticrossing point. The largest anticrossing gap $\approx $ 2.54 eV corresponds to the anticrossing point l $=$ 1 at the electric field $\approx $ 3.59 V/{\AA}. The achieved results will promisingly draw further attentions toward graphene-based Field Effect Transistors (g-FET). [Preview Abstract] |
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