Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N28: Focus Session: Graphene II |
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Sponsoring Units: DMP Chair: Zhihong Chen, IBM - Watson Room: Colorado Convention Center 302 |
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N28.00001: Superconducting junctions in graphene Invited Speaker: Graphene, a single sheet of graphite, is a two-dimensional material which has been long-studied theoretically, but only recently become available to experimentalists. Recent experiments have shown that the electronic properties of graphene are even more remarkable than previously thought. In my talk I will describe the fabrication and characterization of graphene devices, and introduce their basic electronic properties. I will then focus on our recent experiments where we study induced superconductivity in graphene, an observation which elucidates on the quantum coherent properties of electrons in this novel two-dimensional electron gas. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 8:48AM |
N28.00002: Mesoscopic Electron Transport in Nanostructured Graphene Barbaros Oezyilmaz, D. Efetov, K. Bolotin, M. Y. Han, P. Jarillo-Herero, P. Kim We present experimental results on low energy electric transport studies in mesoscopic graphene quantum devices. Graphene sheets have been fabricated by means of micromechanical exfoliation. Subsequently we define mesoscopic Aharonov-Bohm (AB) rings. The electron interference in such ring shaped graphene ribbons is controlled using a perpendicular magnetic field. We will discuss magnetoresistance oscillations obtained on AB rings with ring width of $\sim $ 50 nm and ring diameters ranging from 300 nm to 3000 nm as a function of both temperature and carrier density. In addition, we present our efforts on locally controlling the carrier density in graphene sheets. The latter are patterned into ribbons of $\sim $ 100nm width and contacted in a first step with source and drain electrodes. In a second step multiple lithographically-patterned electrostatic local top gates are aligned to each device. We will discuss transport measurements as a function of local gate voltages. [Preview Abstract] |
Wednesday, March 7, 2007 8:48AM - 9:00AM |
N28.00003: Observation of Proximity Effect and Multiple Andreev Reflections in Graphene/Superconductor Junctions Xu Du, Ivan Skachko, Eva Y. Andrei Graphene, a single atomic layer of graphite, has attracted much interest recently both for its unique physical properties and for its potential in electronics applications. Due to the combined effects of a linear energy-momentum dispersion and internal degrees of freedom (pseudo-spin) associated with the honeycomb lattice, the low energy excitations in graphene are expected to behave like massless relativistic fermions. This leads to many novel and unusual physical properties. We will present experimental studies on a gate controlled superconductor/graphene hybrid device. Electric field dependent superconducting proximity effect and multiple Andreev reflections will be discussed. Results obtained for junctions fabricated on graphene and on multi-layer graphite films will be compared. [Preview Abstract] |
Wednesday, March 7, 2007 9:00AM - 9:12AM |
N28.00004: Quantum Transport in Single and Bi-Layer Graphene Coupled to Superconducting Electrodes F. Miao, S. Wijeratne, U. Coskun, Y. Zhang, C. N. Lau Graphene, the two dimensional honeycomb lattice of carbon atoms, has attracted significant attention in recent years, due to its unique electrical properties. Here we present experimental studies of single and bi-layer graphenes coupled to superconducting electrodes. At low temperatures the devices display signatures of ballistic electrical transport, and the minimum conductivity varies between 6.5 and 20k$\Omega $. When the electrodes become superconducting, we observe gate-tunable low-bias conductance peaks, which are attributed to multiple Andreev reflections. Latest experimental results will be discussed in terms of various theoretical models. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:24AM |
N28.00005: Low Field Electronic Transport Properties and Scattering Mechanisms of Graphene Y.-W. Tan, Y. Zhang, M. Han, J. A. Jaszczak, P. Kim, H. L. Stormer We report low magnetic field transport properties of fourteen graphene devices with a wide spread of mobilities. The minimum conductivity at the Dirac point lies in the range $2 - 12e^ {2}/h$ with an average of $\sim 8e^2/h$. When comparing the conductivity versus density (n) curves with theoretical models, we find that high mobility samples show the features characteristic of short range scattering and low mobility samples show the features characteristic of long range scatterering. Samples exhibiting different scattering mechanisms also show different weak localization behaviors. Samples having short range disorders show total suppression of weak localization (WL), whereas samples with long range scatterering show the conventional WL peak with a reduction in amplitude. Devices in between these two limits have a sharp, narrow WL peak. Under moderate disorder scattering, we find inelastic electron-electron scattering to be the major cause of phase decoherence, and the phase coherence length has a n$^{1/4}$ dependence. [Preview Abstract] |
Wednesday, March 7, 2007 9:24AM - 9:36AM |
N28.00006: Controlling the Electronic Structure of Bilayer Graphene Taisuke Ohta, Aaron Bostwick, Jessica McChesney, Thomas Seyller, Karsten Horn, Eli Rotenberg Carbon-based materials such as carbon nanotubes, graphite intercalation compounds, fullerenes, and ultrathin graphite films exhibit many exotic phenomena such as superconductivity and an anomalous quantum Hall effect. These findings have caused renewed interest in the electronic structure of ultrathin layers of graphene: a single honeycomb carbon layer that is the building block for these materials. There is a strong motivation to incorporate graphene multilayers into atomic-scale devices, spurred on by rapid progress in their fabrication and manipulation. We have synthesized bilayer graphene thin films deposited on insulating silicon carbide and characterized their electronic band structure using angle-resolved photoemission. By selectively adjusting the carrier concentration in each layer, changes in the Coulomb potential led to control of the gap between valence and conduction bands [1]. This control over the band structure suggests the potential application of bilayer graphene to switching functions in atomic scale electronic devices. [1] T. Ohta, A. Bostwick, T. Seyller, K. Horn, E. Rotenberg, Science, 313, 951 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 9:36AM - 9:48AM |
N28.00007: Superconductivity in metal coated graphene Bruno Uchoa, Antonio Castro Neto Graphene, a single atomic layer of graphite, is a two dimensional (2D) zero gap insulator with a high electronic mobility between nearest neighbor carbon sites. The unique electronic properties of graphene, from the semi-metallic behavior to the observation of an anomalous quantum Hall effect and a zero field quantized minimum of conductivity derive from the relativistic nature of its quasiparticles. By doping graphene, it behaves in several aspects as a conventional Fermi liquid, where electrons may form Cooper pairs by coupling with a bosonic mode. In this talk, we develop a mean-field phenomenology of superconductivity in a honeycomb lattice. We predict the possibility of two distinct phases, a singlet s-wave phase and a novel p+ip wave phase in the singlet channel. At half filling, the p+ip phase is gapless and superconductivity is a hidden order. We propose a few possible sources of Cooper pairing instability in graphene coated with alkaline and transition metals, and similar low dimensional graphene based devices. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:00AM |
N28.00008: Tunneling and Josephson coupling studies of n-layer graphene Conor Puls, Neal Staley, Haohua Wang, Jeremy Forster, Kelly McCarthy, Ben Clouser, Ying Liu We investigate planar tunnel and superconductor-graphene-superconductor (SGS) junctions involving n-layer graphene. We fabricate our devices using an ultrathin quartz filament as a shadow mask over mechanically exfoliated graphene as an alternative to lithographic procedures so as to avoid possible contamination in a wet lithography process. Our tunnel junctions use Al$_2$O$_3$ as the tunnel barrier and Pb or Au as the counter-electrode. We observed a reduction of density of states in the n-layer graphene and the superconducting energy gap of Pb when Pb was an electrode. Results from work on SGS junctions and other atomically thin materials such as NbSe$_2$ will also be presented. [Preview Abstract] |
Wednesday, March 7, 2007 10:00AM - 10:12AM |
N28.00009: RPA Theory of Carrier Correlations in Graphene Yafis Barlas, Tami Pereg-Barnea, Marco Polini, Allan MacDonald We have applied RPA theory to examine some consequences of electronic correlations in doped and undoped graphene. The full wavevector and complex-frequency dependent polarization bubble was evaluated using dimensional regularization in the absence of doping and adding the appropriate carrier-scattering and Pauli-blocking corrections in doped systems. We have evaluated the RPA correlation energy as a function of charge and spin density by integrating the dynamically screened Coulomb interaction along the imaginary axis and from this have extracted the compressibility, and the spin and valley susceptibilties. We have also evaluated the frequency and wavevector dependent self-energy and used this to extract the doping dependence of velocity renormalization and the quasiparticle spectral weight at the Fermi energy. The accuracy of RPA theory applied to graphene will be critically discussed. [Preview Abstract] |
Wednesday, March 7, 2007 10:12AM - 10:24AM |
N28.00010: Localization and Polarization in Graphene Systems with Edges Jason Hill, Hongki Min, Tami Pereg-Barnea, Nikolai Sinitsyn, Allan MacDonald The properties of localized states (especially states localized at the edge of ribbons) will be presented for various graphene systems. The orientation dependence of the properties will be discussed. Methods for devising appropriate boundary conditions for Dirac ribbons will be reviewed. Localization at zero field due to finite size effects, applied magnetic fields, and spin-orbit coupling will be discussed. Tendencies toward true spin and pseudospin polarizations in graphene ribbons will also be examined. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 10:36AM |
N28.00011: Impact of substrate-graphene interaction on transport properties of graphene Jianhao Chen, Masa Ishigami, Elba-Gomar Nadal, Ellen Williams The silicon oxide substrate has nanoscale corrugations and charge traps, which influence the electronic propreties of graphene. We modify the substrate-graphene interaction by functionalizing the oxide. We are able to modify the chemical adhesive force as demonstrated by changes in the yield of graphene with different self-assembled monolayers prior to mechanical cleavage of graphite. We will discuss the impact of oxide functionalization on the transport properties of graphene. [Preview Abstract] |
Wednesday, March 7, 2007 10:36AM - 10:48AM |
N28.00012: Electrostatic Driving of Graphite Resonators Arend van der Zande, Scott Bunch, Scott Verbridge, Ian Frank, David Tanenbaum, Jeevak Parpia, Harold Craighead, Paul McEuen We fabricate nanoelectromechanical graphite resonators and drive the resonators electrostatically. Graphite sheets are suspended over trenches in SiO$_{2}$ and contacted to electrodes. Mechanical vibrations of the graphite sheets are actuated by applying a radio frequency voltage relative to a doped silicon back-gate, and the resonance frequency is tuned by varying an additional DC gate voltage. Mechanical vibrations are detected optically by laser interferometry. We detect the thermal motion of the resonators, and use the equipartition theorem to calibrate the amplitude of motion. For example, a 5nm graphite sheet at room temperature has thermal motion on resonance of 200 fm/ Hz$^{1/2}$, and shows a driven linear response in displacement up to 6 nm, comparable to the thickness of the resonator. The unusually small mass, electrically active material and reasonable dynamic range indicate that graphite resonators would make excellent force and charge sensors. [Preview Abstract] |
Wednesday, March 7, 2007 10:48AM - 11:00AM |
N28.00013: Absence of Wigner Crystallization in Graphene Hari Dahal, Yogesh Joglekar, Kevin Bedell, Alexander Balatsky We study the possibility of Wigner crystal phase as a ground state in graphene. The Wigner crystal phase results when the ratio of potential to kinetic energy is much higher than one, and generally, the particle density serves as a tuning parameter. Our calculation shows that potential and kinetic energy have the same density dependence resulting in a density independent ratio of energies. Moreover, kinetic energy is higher than potential energy; which rules out the possibility of Wigner crystallization. The physical reason of this observation can be traced back to the linear dispersion of carriers of Graphene, the Dirac ferminos. cond-mat/0609440; Physical Review B (to be published) [Preview Abstract] |
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