Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D29: Focus Session: Carbon Nanotubes and Related Materials IV: Graphene |
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Sponsoring Units: DMP Chair: Igor Herbut, Simon Fraser University Room: Morial Convention Center 221 |
Monday, March 10, 2008 2:30PM - 3:06PM |
D29.00001: Electronic properties of Dirac fermions in epitaxial graphene Invited Speaker: Graphene, atomically thin layers of graphite, has attracted a lot of research interest because of its intriguing physics as well as its technological potential for next generation electronic devices. I will first present a detailed characterization of the growth of atomically thin films of epitaxial graphene on SiC, by using low energy electron microscopy (LEEM). The electronic properties of the films are hence studied by angle resolved photoemission spectroscopy (ARPES). Data as a function of doping, temperature and sample thickness are presented and the role of disorder and many body interactions will be discussed. Finally, the presence of a bandgap in the spectra of Dirac fermions will be presented and its potential for bandgap engineering will be discussed. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D29.00002: Electronic Confinement in Epitaxial Graphene As Seen by ARPES David Siegel, Shuyun Zhou, Alexei Fedorov, Andreas Schmid, Farid El Gabaly, Alessandra Lanzara The epitaxial growth of graphene on 6-H SiC and its electronic structure have been studied with low energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES) respectively. Some of the critical growth parameters that determine sample homogeneity and domain properties have been identified. The resulting electronic structure presents features that generally agree with the conical dispersion of Dirac quasiparticles, however deviations are observed near the Dirac point energy. The dependence of these deviations on real-space electronic confinement is discussed. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D29.00003: Tuning the phonon self-energy of a graphene bilayer Jun Yan, Erik Henriksen, Philip Kim, Aron Pinczuk We use low temperature Raman spectroscopy and the electric field effect to investigate the coupling of long wavelength optical phonons (the G-band) with charge carriers in bilayer graphene. The charge tunable phonon spectra exhibit a remarkable symmetry which reflects the underlying particle-hole symmery of the electron band structrue. The change of phonon line-width is interpreted as a Landau damping of the phonon into resonant electron-hole pair transitions. The phonon energy exhibits an intriguing non-monotonic evolution with charge density. We found that the electron-hole pair excitation stiffens (softens) the lattice vibration when its energy is smaller (larger) than the phonon energy, in agreement with theoretical predictions for deformation electron-phonon coupling. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D29.00004: Graphene on a graphite surface: effect of interlayer coupling. Adina Luican, Guohong Li, Eva Y. Andrei We present low temperature high magnetic field scanning tunneling microscopy and spectroscopy on a sheet of graphene suspended above a graphite substrate. The sheet consists of two regions that couple to the substrate with different strengths resulting in two distinct sequences of Landau Levels (LL). One region exhibits a sequence that is typical of single layer graphene (square root dependence on field and level index) with a reduced Fermi velocity that is renormalized by electron-phonon interactions. The sequence in the other region is anomalous and, according to recent theoretical work [1], it can be attributed to a bilayer with interlayer coupling that is $\sim $ 10 times weaker than that of normal bilayers. We find that the difference between the two regions is also reflected in the values of the Fermi velocity, suggesting that electron-phonon renormalization is suppressed by interlayer coupling. \newline [1] \textit{M. Pereira, F.M. Peeters and P. Vasilopoulos, Phys. Rev. B 76, 115419 (2007)} [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 3:54PM |
D29.00005: Scanning tunneling microscopy/spectroscopy studies of local electronic structure in Epitaxial Graphene Nikhil Sharma, Michael Sprinkle, Claire Berger, Walter DeHeer, Phillip First Epitaxial growth of graphene on hexagonal SiC by thermal desorption of Si has produced high quality films, providing a potential route to wafer-scale graphene electronics. However, many aspects of this new electronic material system remain to be understood. Using scanning tunneling microscopy and spectroscopy (STM/STS), we investigate the layer-dependent effect of atomic defects and deposited metal islands on the local electronic structure of epitaxial graphene. Metal islands locally dope the graphene due to the work function difference between materials, and atomic defects can have a similar effect. The lateral gradient in carrier density and the possible transition from hole- to electron- doping (PN junction) is investigated experimentally for these cases. [Preview Abstract] |
Monday, March 10, 2008 3:54PM - 4:06PM |
D29.00006: Transport in multilayered epitaxial graphene Claire Berger, Xiaosong Wu, Mike Sprinkle, Xuebin Li, Fan Ming, Walt de Heer We present recent results of electronic transport in multilayered epitaxial graphene (EG) grown by thermal decomposition of SiC wafers. Because of the rotational stacking of the layers, it was recently shown theoretically that the system should retain essentially the same band structure as single layer graphene. The system consists of a charged layer at the SiC/EG interface, as revealed by the period of the Shubnikov-de Haas oscillations (a few $10^{12}/cm^2$), and quasi-neutral layers on top. We discuss possible effects of the multilayering in the transport properties, such as the large positive increase in field of the magnetoresistance $\rho_{xx}$, the weak amplitude of the Shubnikov-de Haas and the overall features of the Hall effect, in particular the anomaly of the Hall resistance $\rho_{xy}$ observed at low field. [Preview Abstract] |
Monday, March 10, 2008 4:06PM - 4:18PM |
D29.00007: Electronic compressibility of a graphene bilayer Silvia Viola Kusminskiy, Johan Nilsson, David Campbell, Antonio Castro Neto We calculate the electronic compressibility arising from electron-electron interactions for a graphene bilayer within the Hartree-Fock approximation. We show that, due to the chiral nature of the particles in this system, the compressibility is rather different from those of either the two-dimensional electron gas or ordinary semiconductors. We find that an inherent competition between the contributions coming from intra-band exchange interactions (dominant at low densities) and inter-band interactions (dominant at moderate densities) leads to a non-monotonic behavior of the compressibility as a function of carrier density. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D29.00008: Charge inhomogeneity in a single and bilayer graphene Hari Dahal, Tim Wehling, Kevin Bedell, Jian-Xin Zhu, Alexander Balatsky We study the possibility of charge ordered state in both single and bilayer graphene using a real space tight binding model. We find that the single layer graphene always remains in a liquid phase; the reason being the higher kinetic energy compared to the potential energy. The bilayer graphene on the other hand can have an inhomogeneous distribution of the charge, namely the charge density wave (CDW) state. The CDW state is commensurate with the lattice. The charge ordered state is stabilized by the Coulomb interaction of the carriers of two layers. We also predicted a kinetic energy driven (KID) inhomogeneous phase. This phase can be stabilized by the inter layer hopping energy. The KID phase and the CDW phase compete with each other below the half filling whereas they cooperate above half filling. For the physical parameter of bilayer graphene CDW phase always wins over the KID phase. Hari P. Dahal, Tim O. Wehling, Kevin S. Bedell, Jian-Xin Zhu, Alexander V. Balatsky [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D29.00009: Interplay of Coulomb interactions and rippling of monolayer graphene: RG approach Oskar Vafek, Vladimir Juricic, Igor Herbut The effects of electron-electron Coulomb interactions and rippling disorder of a mono-layer graphene are studied at half-filling using renormalization group. It is found that the system flows to an infra-red stable line of fixed points which is accessible perturbatively and along which the zero temperature minimal metallic conductivity is non-universal and enhanced relative to the clean non-interacting fixed point. An estimate of the typical random vector potential representing ripples in graphene brings the theoretical value of the minimal conductivity into the vicinity of $4e^2/h$. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D29.00010: Graphene in a magnetic field and a superlattice potential J.M.B. Lopes dos Santos, N.M.R. Peres, A.H. Castro Neto The problem of electrons in a periodic potential in the presence of a magnetic field is revisited here in the context of graphene, by considering a superlattice periodic perturbation on the Dirac-Weyl equation for massless fermions. We solve the problem of a periodic potential for massless Dirac Fermions in a magnetic field. The relevance for graphene physics arises from the possibility of a superlattice modulation, both in single-layer graphene, due to the substrate, and in few layer graphene due to rotational stacking faults, which give rise to long wavelength moire patterns [1,2]. \par\noindent [1] J. Hass. \emph{et. al} arXiv:0706.2134v1 [cond-mat.mtrl-sci]\par\noindent [2] JMB Lopes dos Santos, NMR Peres and AH Castro Neto,arXiv:0704.2128v1 [cond-mat.mtrl-sci] [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D29.00011: Quenching of quantum Hall effect and the role of undoped planes in epitaxial graphene Pierre Darancet, Nicolas Wipf, Didier Mayou We propose a mechanism for the quenching of the Shubnikov de Haas oscillations and the quantum Hall effect observed in epitaxial graphene. This involves a coupling between the uncharged rotationally stacked layers and the charged graphene layer at the interface. In a magnetic field, the extraordinary graphene $n=0$ Landau level of the uncharged layers produces a high density of states at the Fermi level. Consequently we find that the scattering time of the conduction electron in the charged plane is magnetic field dependent and reduced to the order of the cyclotron orbit period. This scenario also explains quantitatively the recent observation of a linear magnetoresistance in epitaxial graphene. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D29.00012: Anomalous Thermoelectric Effect in Graphene Tomas Lofwander, Mikael Fogelstrom We present calculations of the thermal and electric linear response in graphene, including disorder in the self-consistent t-matrix approximation [1]. For strong impurity scattering, near the unitary limit, the formation of a band of impurity states near the Fermi level leads to that Mott's relation holds at low temperature. For higher temperatures, there are strong deviations due to the linear density of states. The low-temperature thermopower is proportional to the inverse of the impurity potential and the inverse of the impurity density. Information about impurity scattering in graphene can be extracted from the thermopower, either measured directly, or extracted via Mott's relation from the electron-density dependence of the electric conductivity.\newline \newline [1] T. L\"ofwander and M. Fogelstr\"om, Phys. Rev. B {\bf 76}, 193401 (2007). [Preview Abstract] |
Monday, March 10, 2008 5:18PM - 5:30PM |
D29.00013: Charge Carrier Interaction with a Purely Electronic Collective Mode: ``Plasmarons" and the Infrared Response of Semi-metal Bismuth N. Peter Armitage, Riccardo Tediosi, Enrico Giannini, Laszlo Forro, Dirk van der Marel We present a detailed pressure and temperature- dependent optical study of single-crystal bismuth using infrared reflectivity and ellipsometry. In the ambient pressure optical conductivity, an anomalous temperature dependent mid-infrared absorption feature is observed. An extended Drude model analysis reveals that it can be connected to a sharp upturn in the scattering rate, the frequency of which exactly tracks the strongly temperature dependent plasmon frequency. We interpret this absorption and increased scattering as direct optical evidence for a charge carrier interaction with a collective mode of purely electronic origin, here electron-plasmon scattering. The observation of a ``plasmaron'' as such is made possible by the exceptional properties of semi-metal bismuth, but it is also likely relevant to the low energy transport and thermodynamic properties of other semi-metals, like graphite and graphene. As a function of pressure, we observe massive changes in bismuth's optical and infrared conductivity as the material approaches a Lifshitz-like metal/insulator transition. [Preview Abstract] |
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