Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A25: Focus Session: Graphene I: Electronic Properties |
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Sponsoring Units: DMP Chair: Jules Carbotte, McMaster University Room: 327 |
Monday, March 16, 2009 8:00AM - 8:12AM |
A25.00001: Proximity induced supercurrent in multilayer graphene Akinobu Kanda, Hidenori Goto, Sho Tanaka, Yukitoshi Nagai, Youiti Ootuka, Shunsuke Odaka, Hisao Miyazaki, Kazuhito Tsukagoshi We report experimental study on gate-dependent superconducting proximity effect in multilayer graphene. In our sample, multilayer graphene (MLG), obtained by the micromechanical cleavage of Kish graphite, is placed on a SiO$_{2}$/p$^{+}$-Si substrate, and two superconducting (Ti/Al) electrodes are connected to the top of the MLG. Dependence of the critical supercurrent on MLG length and temperature will be discussed. [Preview Abstract] |
Monday, March 16, 2009 8:12AM - 8:24AM |
A25.00002: Doping Dependent Magnetic Structure of Graphene Nanostructures Somnath Bhowmick, Umesh Waghmare, R. Shankar, Vijay Shenoy Graphene nanostructures bounded by zigzag edges are predicted to have interesting magnetic structure. We investigate how doping of the nanostructures by holes affects their magnetism. By a detailed mean-field analysis of the Hubbard model, and supported by first principles calculations, we show that doping dramatically changes the magnetic structure. In the case of a zigzag terminated nanoribbon, there is a range of doping that depends on the width of the nanoribbon, where magnetizations of both the zigzag edges are parallel (``ferro'' structure) as opposed to the undoped case where the magnetization on the two zigzag edges are anti-parallel (``anti-ferro''). We explain these results by means of a continuum field theory. We also study doping dependence of magnetic structure of other zigzag terminated nanostructures such as nanodots and find this to be a generic phenomenon in these systems. [Preview Abstract] |
Monday, March 16, 2009 8:24AM - 9:00AM |
A25.00003: A subnanometer view of the chemistry and electronic structure of graphene and graphite Invited Speaker: We have used UHV, low and variable temperature Scanning Tunneling Microscopy (STM) to investigate the structure and electronic properties of single sheets of graphite (graphene) on both insulating and metallic surfaces. STM as well as Scanning Tunneling Spectroscopy reveal variations in electron tunneling images for these one atom thick samples that differ markedly from those of ordinary, multi-layer graphite. These kinds of investigations have been extended to the study of surface chemical reactions where the Scanning Tunneling Microscope/Atomic Force Microscope is used as a ``camera'' to observe defect growth in single and multiple graphene sheets due to treatment with oxygen. ``Healing'' of defects in ordinary graphite has also been observed in samples exposed to acetylene at elevated temperatures using STM. The chemistry on these model surfaces is expected to reveal many features that determine the nanoscale properties of graphene. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A25.00004: ABSTRACT WITHDRAWN |
Monday, March 16, 2009 9:12AM - 9:24AM |
A25.00005: Interactions of Aromatic Compounds with Graphene Hugo Romero, Humberto Gutierrez, Peter Eklund We have used back-gated graphene field effect transistors (FETs) on Si/SiO$_{2}$ substrates to study the interactions of graphene with six-membered ring aromatic compounds C$_{6}$H$_{2n}$ ($n$ = 3-6). Electronic detection occurs through the shift of the of the source-drain resistance maximum (``Dirac peak'') with gate voltage. The size of the \textit{positive} shift of the Dirac peak is found to be linear in to the $\pi $ electron count of the molecule, suggesting the coupling between these $\pi $ electrons and those in the graphene may be responsible for the observed effects. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A25.00006: Slow imbalance relaxation and thermoelectric transport in graphene Igor Aleiner, Matthew Foster We compute the electronic component $(\kappa)$ of the thermal conductivity and the thermoelectric power $(\alpha)$ of monolayer graphene, within the hydrodynamic regime, taking into account the slow rate of carrier population imbalance relaxation. Interband electron-hole generation and recombination processes are inefficient due to the non-decaying nature of the relativistic energy spectrum. As a result, a population imbalance of the conduction and valence bands is generically induced upon the application of a thermal gradient. The thermoelectric response of a graphene monolayer depends upon the ratio of the sample length to an intrinsic length scale $l_Q$, set by the imbalance relaxation rate. At the same time, the metallic contacts required for the thermopower determination (under open circuit boundary conditions) can crucially influence its measurement, since carrier exchange with the contacts also relaxes the imbalance. These effects are especially pronounced for clean graphene, where the thermoelectric transport is limited exclusively by intercarrier collisions. For specimens shorter than $l_Q$ joined to sufficiently resistive contacts, we show that the population imbalance extends throughout the sample; $\kappa$ and $\alpha$ asymptote toward their zero imbalance relaxation limits. In the opposite limit of a graphene slab longer than $l_Q$, we show that at non-zero doping $\kappa$ and $\alpha$ approach intrinsic values characteristic of the infinite imbalance relaxation limit. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A25.00007: Electron localization in graphene quantum dots Vadym Apalkov, Prabath Hewageegana We study theoretically a localized state of an electron in a graphene quantum dot with a sharp boundary. Due to Klein's tunneling, the relativistic electron in graphene cannot be localized by a confinement potential. In this case electron states in a graphene quantum dot become resonances with finite trapping time. We consider these resonances as the states with complex energy. To find the energies of these states we solve the time-independent Schrodinger equation with outgoing boundary conditions. The imaginary part of the energy determines the width of the resonances and the electron trapping time. We show that if the parameters of the confinement potential satisfy a special condition, then an electron can be strongly localized by such quantum dot, i.e., the trapping time becomes infinitely large. We show how a deviation from this condition affects the electron trapping time. We also analyze the energy spectra of an electron in a graphene quantum ring with a sharp boundary. We show that in this case the condition of strong trapping can be tuned by varying parameters of confinement potential, e.g. internal radius of the ring. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A25.00008: Dirac Quasiparticle Interference in Graphene in External Magnetic Field Rudro Biswas, Laila Mattos, Hari Manoharan, Alexander Balatsky Recent experiments [1] on graphene have been able to probe impurity scattering effects locally using scanning tunneling microscopy. The spatial Fourier transforms of the local density of states (FT-STS) display prominent features corresponding to both intervalley and intravalley scattering of massless Dirac quasiparticles. We provide low energy calculations using the effective mass model to describe these experimental observations. In addition, we are able to explain the striking change in the shape of the chiral scattering peaks when graphene is placed in a perpendicular magnetic field. [1] L.S. Mattos, et al., unpublished. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A25.00009: Effects of the electron-phonon interaction on spectroscopies of graphene E.J. Nicol, J.P. Carbotte, S.G. Sharapov We examine the effect of the electron-phonon interaction in graphene on the electronic density of states, the quasiparticle spectrum and the optical conductivity. A simplified model of coupling to an Einstein mode at 200 meV is employed and self-consistent calculations are performed. The results will be discussed in relation to tunneling, ARPES and optical conductivity experiments on graphene. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A25.00010: Thermally assisted self-trimming of graphene nanoribbon edges Teng Yang, David Tom\'anek, Savas Berber Edge morphology is known to play a key role in the conductance of graphene ribbons. We use a combination of {\em ab initio} density functional total energy and molecular dynamics calculations to investigate thermally induced reconstruction occurring at graphene edges. The calculated total energy surfaces suggest that among all nanoribbon sites, atoms at edge defect sites require least energy to be displaced. At elevated temperatures, these atoms will primarily participate in diffusion and related processes at the edge that will gradually reduce the edge roughness and thus lower the edge energy. We explore various scenarios leading to such self-trimming of edges, including concerted migration processes and unravelling of chains at the edge. Close inspection of our results suggests that the preferential mechanisms and activation barriers for trimming of rough armchair and zigzag edges may be different. In selected scenarios, Joule heating of nanoribbons may not only straighten rough edges, but also modify the preferred edge morphology. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A25.00011: Fermi surface of graphene on Ru(0001) Thomas Brugger, Hugo Dil, J\"{u}rg Osterwalder, Thomas Greber, Bin Wang, Marie-Laure Bocquet, Sebastian G\"{u}nther, Joost Wintterlin The structure of a single layer graphene on Ru(0001) is compared with that of a single layer hexagonal boron nitride nanomesh on Ru(0001). Both are corrugated sp$^2$ hybridized networks and display a $\pi$-band gap at the $\overline{\rm{K}}$ point of their $1\times1$ Brillouin zone. In contrast to $h$-BN/Ru(0001), g/Ru(0001) has a distinct Fermi surface which indicates that 0.1 electrons per $1\times1$ unit cell are transferred from the Ru substrate to the graphene. Photoemission from adsorbed xenon on g/Ru(0001) identifies two distinct Xe 5p$_{1/2}$ lines, separated by 240 meV, which reveals a corrugated electrostatic potential energy surface like on $h$-BN/Rh(111) [1]. These two Xe species are related to the topography of the template and have different desorption energies.\\[4pt] [1] H. Dil, J. Lobo-Checa, R. Laskowski, P. Blaha, S. Berner, J. Osterwalder, and T.Greber, Science \textbf{319}, 1824 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A25.00012: Complex refractive index of graphene measured by picometrology Xuefeng Wang, David Nolte The complex refractive index $\tilde {n}_g $ of graphene remains unresolved because the traditional technique, ellipsometry, fails when applied to graphene with its sub-nanometer thickness, dielectric anisotropy, and small transverse sample size. Here we apply interferometric picometrology to measure $\tilde {n}_g $ at 488 nm, 532 nm and 633 nm. A strong dispersion of $\tilde {n}_g $ was found in the visible region. $\tilde {n}_g $ varies from 2.4-1.0i at 532 nm to 3.0-1.4i at 633 nm at room temperature. The dispersion is five times stronger than bulk graphite (2.67-1.34i to 2.73-1.42i from 532 nm to 633 nm). In experiments, Graphene is deposited on a substrate with complex reflection coefficient $\tilde {r}$ tuned near an antinode condition. As a dielectric film, graphene modifies $\tilde {r}$ of the substrate into $\tilde {r}'$. Picometrology measures both the amplitude and the phase change of $\tilde {r}$, and$_{ }$therefore acquires the full information needed to calculate $\tilde {n}_g $. This is accomplished by scanning a normal-incidence focused Gaussian beam (1.5 $\mu $m width) over the graphene and monitoring the asymmetric diffraction of the reflected beam. Picometrology measures the complex change of $\tilde {r}$ with a quadrant detector that simultaneously monitors both intensity and axis shift of the reflected beam and calculates $\tilde {n}_g $. The strong dispersion of graphene is reported here for the first time, and it is likely caused by the strongly modified quantum level structure of the single atomic layer. [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A25.00013: Zero-bias conductance anomaly in point-contact junctions on graphite Wan Kyu Park, Cesar Chialvo, Rich Jones, Sam Johnson, Nadya Mason, Laura Greene The electronic properties of graphene, a two-dimensional carbon allotrope, continue to attract great interest because of the interesting underlying physics and application potential of this novel electronic material. An ideal single-layer graphene is known to show a linear behavior in the electronic density of states (DOS) around the Fermi level. The ability to engineer the DOS of single- and multi-layer graphene is considered as a fundamental requirement for the realization of electronic devices. To investigate the electronic DOS in graphene/graphite, we adopt a spectroscopic technique based on nanoscale point-contact junctions, where differential conductance spectra are taken at around the liquid helium temperature. A common feature observed in all junctions on both Kish graphite and HOPG is an anomalous conductance dip at zero bias. The conductance curves show a logarithmic bias dependence in their slopes, exhibiting a systematic evolution as a function of magnetic field and contact pressure. We discuss possible origins of these behaviors including the possibility of modification in the electronic DOS of graphite. [Preview Abstract] |
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