Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session V25: Focus Session: Graphene XIII: Spectroscopic and Transport Properties |
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Sponsoring Units: DMP Chair: Shaffique Adam, University of Maryland Room: 327 |
Thursday, March 19, 2009 8:00AM - 8:12AM |
V25.00001: Charged Impurity Scattering in Graphene induced by adsorption of calcium Masahiro Ishigami, Jyoti Katoch We have measured the impact of charged impurity scattering induced by adsorbed calcium atoms on the transport properties of graphene sheets. We vary the density of adsorbed atoms on the surface of graphene based-devices which are otherwise devoid of any surface adsorbates in ultra high vacuum environment. We will discuss the impact of calcium atoms on the charge carrier mobility, gate-dependent conductivity and minimum conductivity in comparison with earlier measurements performed using potassium. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V25.00002: Current-Phase Measurements in Single Layer Graphene Cesar Chialvo, Ion Moraru, Daniel Bahr, Nadya Mason, Dale Van Harlingen The current-phase relationship (CPR) of a Josephson junction can provide key information about the microscopic processes that make up a supercurrent. However, CPR has not been previously measured in graphene. We have successfully fabricated a variety of Josephson junctions containing single-layer graphene as a weak link, and with different junction width to length ratios. We present results of measurements based on a phase-sensitive SQUID technique, where we determine the supercurrent amplitude and phase, as well as a possibly anomalous shape of the CPR. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 9:00AM |
V25.00003: Landau level spectroscopy of Dirac fermions in multilayer epitaxial graphene and graphite Invited Speaker: The results of magneto-absorption studies of epitaxial multilayer graphene on SiC and of graphite will be presented. The talk will be focused on inter Landau level transitions characteristic of electronic states with Dirac-like dispersion relations which are distinctive of graphene but persist in multilayer epitaxial graphene and are also present at the H-point of bulk graphite. The high energy limits of the Dirac-cone in epitaxial graphene will be discussed from experiments carried out in the near-infrared spectral range. Probing the nearest vicinity of the Dirac point with far-infrared light will be testified. An exceptional quality of essentially neutral multilayers of graphene on SiC (low temperature carrier mobilities $\sim $250 000cm\^{}2/Vs) will be discussed from cyclotron resonance absorption resonance. This resonance will be shown to persist up to room temperature with negligible changes of the width what indicates no relevant thermally activated scattering process in this material. M.L. Sadowski, et al, Phys. Rev. Lett. 97, 266405 (2006), P. Plochocka et al., ibid., 100, 087401 (2008), M. Orlita et al., ibid., 100, 136403 (2008), M. Orlita et al., ibid, accepted, (arXiv:0808.3662). [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V25.00004: Charged Impurity Scattering in Bilayer Graphene Shudong Xiao, Jianhao Chen, Ellen D. Williams, Michael S. Fuhrer Materials Research Science and Engineering Center and Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, MD, 20742, USA We have examined the impact of charged impurity scattering on the charge carrier transport in bilayer graphene in ultra-high vacuum (UHV) at low temperatures. Bilayer graphene sheets are mechanically exfoliated on Si/SiO$_{2}$ substrates, and the number of layers is verified by micro-Raman spectroscopy. Charged impurity density is varied over a wide range (up to 2x10$^{13}$ cm$^{-2})$ by deposition of potassium atoms on clean bilayer graphene in UHV. At a gate-induced charge carrier density of 4.3 x 10$^{12}$ cm$^{-2}$, the mobility is inversely proportional to the charged impurity density $\mu $ = 5x10$^{15}$ V$^{-1}$s$^{-1}$/$n_{imp}$. Surprisingly, the coefficient relating $\mu $ to 1/$n_{imp}$ has a similar magnitude to that for single-layer graphene, indicating a similar strength for charged impurity scattering at this carrier density. The magnitude of charged impurity scattering, as well as the implications for the source of disorder in undoped bilayer graphene, will be discussed in the context of Boltzmann transport theory. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V25.00005: Band structure asymmetry of bilayer graphene revealed by infrared spectroscopy Zhiqiang Li, Erik Henriksen, Zhigang Jiang, Zhao Hao, Matt Zhang, Michael Fogler, Michael Martin, Philip Kim, Horst Stormer, Dimitri Basov We report on infrared spectroscopy of bilayer graphene integrated in gated structures. The dominant feature of the optical conductivity is a resonance peak due to interband transitions between the two conduction bands or two valence bands. Both the frequency and the voltage dependence of the peak show a significant asymmetry upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer and the interlayer coupling. From the conductivity data, the energy difference of the two sublattices and the interlayer coupling energy are directly determined. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 10:00AM |
V25.00006: On the dynamical and dc conductivity of graphene Invited Speaker: It was only recently shown that a simply-observable quantity as the optical transparency of suspended graphene is defined solely by the fine structure constant [1]. In this talk, I will give the theoretical explanation to this experiment, i.e., show that even in the visible-optics regime the corrections to the Dirac cone approximation are small (a few percent) and the effect of next-nearest neighbor hopping is negligible [2]. I will also discuss the infrared conductivity of graphene on a substrate where electron-phonon and impurity scattering become important [3]. Finally, I will comment on the still unsettled question of dc conductivity in graphene and discuss - apart from Coulomb scattering - an alternative scattering mechanism based on midgap states [4]. \\[4pt] [1] R. R. Nair et al., Science 320, 1308 (2008).\\[0pt] [2] T. Stauber et al., Phys. Rev. B 78, 085432 (2008).\\[0pt] [3] T. Stauber et al., Phys. Rev. B 78, 085418 (2008).\\[0pt] [4] T. Stauber et al., Phys. Rev. B 76, 205423 (2007). [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V25.00007: Thermoelectric and Magnetothermoelectric Transport Measurements of Graphene Yuri M. Zuev, Willy Chang, Philip Kim We investigated the electronic, thermoelectric, and magnetothermoelectric transport properties of graphene as a function of temperature and carrier density. Microfabricated heater and thermometer electrodes were used to simultaneously measure conductance and thermoelectric power (TEP) of graphene in the temperature range of 4-300K. Graphene exhibits both positive and negative values of TEP, with a peak value on the order of kB/e, when the Fermi energy is below and above the charge neutrality point, respectively. A quantitative comparison of the conductance and TEP can be made using the semiclassical Mott relation. We observed an excellent quantitative agreement between the measured TEP and the Mott relation based on the mesoscopic two terminal conductance in the low temperature regime (T $<$ 30K). At higher temperatures, the Mott relation employing the local conductivity is necessary. Upon applying magnetic field, the magneto-thermopower exhibits characteristic oscillations in accordance with the Shubnikov-de Hass oscillations in conductance. In the quantum Hall regime at high B field, we observed the quantizing transverse and longitudinal thermpower components which are also in good agreement with the generalized Mott relation, except near the charge neutral Dirac point. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V25.00008: Piezoresistivity of graphene-based thin films Rahul Raveendran Nair, K.S. Navoselov, Da Jiang, Soren Neubeck, Leonid Ponomarenko, A.K. Geim Large-scale production of graphene films is of particular interest because of graphene's extraordinary electronic, mechanical and optical properties. We report the properties of graphene films produced by spraying or spinning of a graphene suspension obtained by ultrasound cleavage of graphite in organic solvents, the route that does not involve graphene oxide [1]. Wafer-scale uniform films of overlapping submicron graphene crystallites were made on transparent substrates and exhibited sheet resistivity of a few k$\Omega $ with more than 80{\%} transmission with respect to white light. Electric measurements and Raman studies suggest that the films are p-doped. In particular, we have investigated the piezoresistive effect in such films by depositing them on flexible plastic substrates. Fully reversible changes in the resistance were observed as a function of strain that could exceed 8{\%} before the films started loosing their continuity. The piezoresistive gauge factor was up to $\sim $30 for our films. [1] Peter Blake \textit{et al.}, Nano Lett. 2008, 8,1704-1708. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V25.00009: Graphite in the bi-layer regime: in-plane transport Hridis Pal, Sefaattin Tongay, Dmitri Gutman, Dmitrii Maslov, Arthur Hebard The dependence of in-plane resistivity of HOPG graphite on temperature is studied both experimentally and theoretically over a wide range of temperatures(up to $\sim$900 K). For temperatures larger than the next-to-nearest-plane coupling which gives rise to an overlap of the valence and conduction bands, but still below the nearest-plane coupling, graphite can be viewed as a stack of bilayers. In this regime, the in-plane conductivity $\sigma$ is supposed to scale as $T\tau$, where $\tau$ is the scattering time. For conventional electron-phonon scattering, $\tau\propto 1/T$ and $\sigma$ is supposed to saturate at higher $T$. However, we observe experimentally that $\sigma$ decreases monotonically without any sign of saturation up to the highest temperature measured. We propose two additional scattering mechanisms which lead to a decrease of $\sigma$: intervalley scattering by phonons and multiple intravalley scattering by phonons due to anharmonicity of a layered lattice at high temperatures. A reasonable agreement between theory and experiment is obtained by using this model. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V25.00010: Tuning the Infrared Absorption of a Bilayer Graphene Field-Effect Transistor Chun Hung Lui, Kin Fai Mak, Matthew Sfeir, James Misewich, Tony Heinz Bilayers of graphene have attracted intense interest because of the possibility of tuning of their band gap by the application of a perpendicular electric field [Taisuke Ohta et al. Science 313, 951 (2006)]. Indeed, such gate electric fields induce both 1) tuning of the chemical potential and 2) modification of the bilayer electronic structure by the development of potential difference across the two layers. These effects have significant consequences for the infrared absorption, which probes the interband transitions, of bilayer samples. We have examined these issues by measuring the evolution of the optical conductivity (for photon energies of 0.2 - 0.8 eV) of graphene bilayer field-effect transistors constructed with a transparent top gate. The infrared absorption shows a significant and reproducible variation with gate voltage. The behavior for positive and negative gate voltages reveals an electron/hole asymmetry, reflecting corresponding differences in the band structure. The role of the development of a band gap in these structures and the effect of electrostatic screening will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V25.00011: Local Transport Measurements on Graphene Using Scanning Tunneling Potentiometry Weigang Wang, Ko Munakata, Michael Rozler, Francoise Kidwingira, Malcolm Beasley Scanning tunneling potentiometry (STP) is a local transport measurement that was demonstrated some time ago, but has only recently been developed in a generally useful form. Near equilibrium, STP measures the electrochemical potential along a sample surface with near nanometer spatial resolution. With our newly developed STP system, we report preliminary results on few-layer graphene at room temperature and 4.2K. Room temperature STP data show a constant drift in the electric properties. At low temperature, however, our data show no such drift. Possible evidence for Landauer resistivity dipoles will be presented. Work supported by the AFOSR. [Preview Abstract] |
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