Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P30: Electronic Properties of Graphene and Related Structures III |
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Sponsoring Units: DCMP Chair: Antonio Castro Neto, Boston University Room: Morial Convention Center 222 |
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P30.00001: Electric Field and Frequency Dependence of the Dielectric Storage, Loss, and Conductivity of Multi-Wall Carbon Nanotubes Rajratan Basu , Germano Iannacchione The dielectric storage ($\epsilon'$) and loss ($\epsilon''$) of the complex dielectric constant ($\epsilon*$) are reported for multi-wall carbon nanotubes (MWCNT) up to $10^5$~Hz as a function of ac-electric field amplitude $E_{rot}$ (in-phase and same frequency as the measurement) and $E_{ac}$ (fixed phase and frequency with respect to the measurement). A slow relaxation process (mode-1) is observed that increases in peak frequency with increasing $E_{rot}$ but is independent of $E_{ac}$. A fast relaxation process (mode-2) is also observed that is independent of $E_{rot}$ and shifts to higher frequency with increasing $E_{ac}$ (opposite to that seen for mode-1). A conductivity analysis of MWCNT reveals possible mechanisms for how $E_{rot}$ and $E_{ac}$ can effect the dielectric dissipation differently. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P30.00002: The genus g=2 problem- A solution of the Persistent current for the genus g=2 -An application to the Edge currents in Graphene David Schmeltzer We report the first solution of Persistent currents for genus $g=2$ Aharonov-Bohm coupled rings which form a character ``8'' structure. For two large coupled rings with equal fluxes, we found that the persistent current in the two coupled rings is equal to that in a single ring. For opposite fluxes the energy has a chaotic structure. This results are obtained within an extension of Dirac's second class constraints. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P30.00003: Potential barriers in graphene Nimrod Stander , David Goldhaber-Gordon , Benjamin Huard , Joey Sulpizio , Kathryn Todd , Bo Yang Graphene is a single sheet of graphite. Some of its remarkable electronic properties have been predicted over the past 6 decades, but only recently, the Geim group at Manchaster succeeded in fabricating graphene and measuring the Quantum Hall effect. These measurements agreed with earlier predictions by observing plateaus at half-integer values and triggered an immense theoretical and experimental effort. It was also predicted that the tunneling through a potential step in graphene is highly anisotropic, and occurs with probability 1 at normal incidence, due to the chiral nature of its quasiparticles. This behavior can be investigated in different potential configurations, such as pn junctions or npn barriers in graphene. In this talk, I will present our experimental work on electronic transport through a tunable potential barrier in top gated graphene devices. I will show that the experiments we have done, depend on the disorder and on the profile of the potential rise across the graphene sheet. Therefore, they can also be seen as as a tool to investigate scattering and screening properties in graphene. [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P30.00004: Effects on the electron conductivity emerging from Majorana zero modes Miklos Gulacsi , Pasquale Sodano The link between single particle random Hamiltonians and non-linear sigma models (NLSM) comes about when setting up a generating function for the mean value taken by the product of Green's functions. Using the standard machinery of the replica trick we derived disorder averaged product of the retarded and advanced Green's functions from an effective NLSM describing low energy physics of graphene films with both disorder and defects. We then compute the electron transport properties and we find remarkable effects arising when the defect topology is such that the Majorana fields acquire zero modes. [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P30.00005: BEC and Superfluidity of Magnetoexcitons in Graphene Oleg Berman , Godfrey Gumbs We propose the experimental observation of Bose-Einstein condensation (BEC) and superfluidity of quasi-magnetoexcitons in bilayer graphene. Electrons are in one layer and holes in another which are controlled by an applied gate voltage. We describe the dilute gas of magnetoexcitons with dipole- dipole repulsion in a strong magnetic field $B$ by a $4\times4$ matrix Hamiltonian. This Hamiltonian is mapped on to a scalar effective mass Hamiltonian for a dilute gas of dipolar excitons without an applied magnetic field. However, the magnetic field enters through a $B$-dependent effective mass for magnetoexcitons. Moreover, for $N$ excitons, we reduced the problem in a space with $2N\times 2$ dimensions into one with $N\times 2$ dimensions. This is accomplished by integrating over the coordinates of the relative motion of electron and hole. We will present the energy spectrum of collective excitations, the sound spectrum as well as the effective magnetic mass of magnetoexcitons in the strong magnetic field limit. The superfluid density $n_S$ and the temperature of the Kosterlitz-Thouless phase transition $T_c$ are shown to be increasing functions of the excitonic density $n$ but decreasing functions of $B$ and the interlayer separation $D$. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P30.00006: Fractionalization in a square-lattice model with time-reversal symmetry Marcel Franz , Conan Weeks , Babak Seradjeh We propose a two-dimensional time-reversal invariant system of essentially non-interacting electrons on a square lattice that exhibits configurations with fractional charges $\pm e/2$. These are vortex-like topological defects in the dimerization order parameter describing spatial modulation in the electron hopping amplitudes. Charge fractionalization occurs via a mechanism similar to that in graphene with the ``Kekule'' distortion and is established by a simple electron counting argument, analytical calculation within the effective low-energy theory, and by an exact numerical diagonalization of the lattice Hamiltonian. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P30.00007: Dirac Fermions in a Magnetic Forest Andres Concha Recent experimental progress have made it possible to study of a single layer of carbon atoms, i.e. $\it {graphene}$. This newly synthetized material seems to be a promising candidate for building up nano-devices as well as a useful experimental tool to study exotic properties of its low energy excitations. In this work we consider the effect of a vortex lattice in the adjacent superconductor on the transport properties of a graphene sheet. We show that the transport properties of graphene sheets of various geometries can be substantially altered and manipulated by changing the vortex lattice structure and the amount of magnetic flux. A relatively straightforward experimental test of our results is suggested. [Preview Abstract] |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P30.00008: Phonon-induced many-body renormalization of graphene electronic properties Wang-Kong Tse , Sankar Das Sarma In this talk, we present a many-body theory for the electron-phonon interaction effects on the electronic properties of graphene. We provide analytical results for the electron self-energy, spectral function, and band velocity renormalization due to phonon-mediated electron-electron interaction, showing that phonon-mediated electron-electron coupling has a large effect on the graphene band structure renormalization. Our analytic theory successfully captures the essential features of the observed graphene electron spectra in the angle-resolved photoemission spectroscopy (ARPES) experiments, predicting a kink at $\sim 200\mathrm{meV}$ below the Fermi level and a reduction of the band velocity by $\sim 10-20\%$ at the experimental doping level. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P30.00009: The effect of electronic correlations on Josephson current and proximity effect in SNS graphene junctions Annica Black-Schaffer , Sebastian Doniach Using the self-consistent tight-binding Bogoliubov-de Gennes (BdG) formalism, we investigate the proximity effect and current-phase relationship in SNS graphene Josephson junctions. Both short and long junctions are considered, as well as different doping levels of the graphene. For short junctions at zero doping in the uncorrelated regime our results agree with those found using the non self-consistent Dirac-BdG formalism [1]. We introduce electronic correlations in the Hamiltonian by including the intrinsic nearest-neighbor spin-singlet coupling present in $p \pi$-bonded planar organic molecules. We study the possibility of coupling this intrinsic $s$- or $d$-wave superconducting pairing [2] to the extrinsic $s$-wave order parameter induced by the metal electrodes. The intrinsic $d$-wave solution, favored in doped graphene, appears for longer doped junctions. For short junctions, the $s$-wave solution can occur, although the result is sensitive to the type of interface. We also report on the two different intrinsic superconducting states' influence on the supercurrent. \newline [1] M. Titov {\it et al.} PRB {\bf 74} 041401 (2006) \newline [2] A. Black-Schaffer {\it et al.} PRB {\bf 75} 134512 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P30.00010: Magnetoplasmons in Graphene Multilayers Godfrey Gumbs , Oleg Berman With the use of a massless Dirac-fermion band structure, we calculate the dielectric response function for magnetoplasmons in multi- layered graphene. The ambient quantizing magnetic field is perpendicular to the plane of the graphene layers which are embedded in a background dielectric medium. We carry out numerical calculations when only the highest valence band is populated and completely full at $T=0$ K. Transitions between the highest valence band and the lowest three conduction bands yield the magnetoplasmon dispersion relation between the plasmon excitation energy and the in-plane wave number. We analyze the instability of these modes by solving the dispersion equation in the complex frequency plane as well as examining the rate of transfer of energy between the layered structure and a charged particle current parallel to the graphene layers. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P30.00011: Strong coupling polarons in graphene Lucian Covaci , Mona Berciu Continued interest in electronic properties of graphene has prompted for the description of polaron formation in 2D honeycomb lattices. We employ the recently developed Momentum Average (MA) approximation to describe coupling to optical phonons in graphene. The extension of the original method to unit cells that have multiple sites makes possible a fast calculation of the electron self-energy in graphene. This method has been proved very successful with Holstein polarons in square lattices and thus allows us to obtain accurate non-perturbative results for the ground state energy, the effective mass and the spectral function in any electron-phonon coupling regimes. Both types of electron-phonon scattering (inter and intra band) can be easily solved within the MA approximation. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P30.00012: First-Principles Theoretical Analysis of Carbon Allotropes and Nanostructures Tejinder Singh , Michael J. Behr , Eray S. Aydil , Dimitrios Maroudas We analyze the various crystalline phases of C observed upon exposing carbon nanotubes to H2 plasmas, which produces an amorphous carbon matrix with carbon nanocrystalls embedded in it. Structural characterization with electron diffraction and high-resolution TEM yields three distinct crystalline phases of C consistent with a fcc lattice with lattice parameter a = 4.25 {\AA}, a bcc lattice with a = 3.0 {\AA}, and a diamond lattice with a = 3.57 {\AA}. Using first-principles density functional theory (DFT) calculations, we have analyzed the structure of several allotropes of pure carbon and we discuss our results in the context of the experimental findings. In addition, we consider the possibility of H incorporation in these C phases. According to our DFT calculations, incorporation at proper concentrations of H in interstitial sites of cubic phases of C provides interpretations for the experimentally observed crystalline C phases. [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P30.00013: d+id'-wave Superconducting States in Graphene and Andreev Reflection Daoxin Yao , Yongjin Jiang , Erica W. Carlson , Han-Dong Chen , JiangPing Hu We study the d+id superconducting state in graphene through proximity effect. By introducing short range pairing (i.e. bond singlet pairing), we found that some effective, low energy pairing order parameter with symmetries will emerge. This is due to the Dirac cone structure in graphene at low energy. We focus on the d+id' pairing order between nearest neighbor sites on the honeycomb lattice of graphene. At low energy, the effective pairing has a p+ip-wave pairing component and a s-wave pairing component, which has a dramatic effect on band structure, Andreev reflection, differential conductance. For the energy gap, we found that it is dominated by p+ip-wave symmetry at low doping and saturated at high doping which is related to s-wave symmetry. The Andreev conductance spectra shows remarkable difference between d+id' wave and s-wave pairing case (including extended s-wave). We point out that an recent Andreev reflection experiment is consistent with our d+id' calculation. In the end, we discuss the possible ways to realize this d+id' superconducting state in graphene. Reference: Y. J. Jiang, D. X. Yao, E. W. Carlson, H.-D. Chen and J. P. Hu, arXiv:0710.3962 [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P30.00014: Magnetoplasmon excitations in graphene for filling factors lower than 6. Gerard Martinez , Yuri Bychkov Graphene is a monolayer of graphite with a band structure composed of two cones located at two inequivalent corners of the Brillouin zone at which conduction and valence bands merge. In contrast to conventional two dimensional electron gas, the dispersion relation obeys a Dirac law with an energy linear as a function of momentum which leads to a specific square root dependence of the Landau levels under an applied magnetic field. Because of this specific dispersion, electron-electron interactions are expected to play a significant role in the magneto-optical response of this system which should be analyzed in terms of magnetoplasmon excitations. In the frame of the Hartree-Fock approximation, the dispersion of these excitations have been calculated for all transitions corresponding to a filling factor lower than 6. It is found that each type of transitions, for wave vectors close to zero, displays a variation, as a function of the magnetic field, corresponding to a re-normalized Fermi velocity different for different types of transitions. The theoretical results will be compared to available experimental data. [Preview Abstract] |
Wednesday, March 12, 2008 10:48AM - 11:00AM |
P30.00015: Charge polarization and phonon renormalization at the K point of graphene Jose Gonzalez , Enrico Perfetto We study the renormalization of the dispersion of different phonon branches by low-energy electronic excitations at the K point of graphene. Among all the in-plane phonons, only the transverse optical modes have a nonvanishing coupling to electron-hole excitations, with a mild Kohn anomaly at the K point. We show, however, that the total charge polarization has a singular behavior and that the dispersion of the out-of-plane phonons undergoes therefore a strong renormalization, with a significant decrease of the phonon frequencies. This leads to an enhancement of the coupling between the two Dirac valleys in graphene, with the potential to open an instability in the spectrum of Dirac quasiparticles. [Preview Abstract] |
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