Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session B30: Graphene: Transport and Correlations |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Vladimir Falko, Lancaster University, UK Room: C147/154 |
Monday, March 21, 2011 11:15AM - 11:27AM |
B30.00001: Magnetoconductance Oscillations in High-Mobility Suspended Bilayer and Trilayer Graphene Wenzhong Bao, Zeng Zhao, Hang Zhang, Gang Liu, Philip Kratz, Lei Jing, Jairo Velasc, Dmitry Smirnov, Chun Ning Lau We observed pronounced magnetoconductance oscillations on suspended bilayer and trilayer graphene devices with mobilities up to 270,000 cm$^{2}$/Vs. For bilayer devices, we observe conductance minima at all integer filling factors n between 0 and -8, as well as a small plateau at n=1/3. For trilayer devices, we observe features at n=-1, -2, -3 and -4, and at n$\sim $0.5 that persist to 4.5K at B=8T. All of these features persist for all accessible values of Vg and B, and could suggest the onset of symmetry breaking of the first few Landau (LL) levels and fractional quantum Hall states. [Preview Abstract] |
Monday, March 21, 2011 11:27AM - 11:39AM |
B30.00002: Novel Excitonic Effects in Graphene and Bilayer Graphene Li Yang Through first-principles calculations with many-body effects included, we have revealed unique excitonic effects in the high-frequency regime (10 $\sim $ 20 eV) of optical spectra of graphene and bilayer graphene (BLG). Despite their different symmetries, the parallel $\sigma $ and $\pi $* bands result in enhanced excitonic effects in such two-dimensional semimetals; one narrow resonant exciton is discovered to form an isolated peak below the prominent absorption continuum with a surprisingly large binding energy, 270 meV in graphene and 80 meV in BLG. Moreover, because of its extremely weak resonant character, this exciton exhibits a bound electron-hole wave function. [Preview Abstract] |
Monday, March 21, 2011 11:39AM - 11:51AM |
B30.00003: Metal-insulator transitions in graphene Mario Amado, Enrique Diez, Francesco Rossella, Vittorio Bellani, David Lopez-Romero, Duncan Maude We investigate the metal-insulator quantum phase transitions that appear in the quantum Hall effect, namely the plateau-insulator and plateau-plateau transitions We have performed magnetotransport experiments with the magnetic field as the driving parameter in the temperature range from 4K up to 230K and magnetic fields up to 28T. The analysis of the temperature dependence of the Hall and longitudinal resistivity reveals the non-universality of the critical exponent for the metal-insulator transition when varying the density of carriers. We also find relevant discrepancies with recent works concerning the value of the critical exponent of the plateau-plateau transition. [Preview Abstract] |
Monday, March 21, 2011 11:51AM - 12:03PM |
B30.00004: Conductivity of Coulomb interacting massless Dirac particles in graphene Vladimir Juricic, Oskar Vafek, Igor Herbut The ac conductivity of the Coulomb interacting Dirac fermions in graphene is considered in the collisionless regime using a variant of the dimensional regularization with the spatial dimension $D=2-\epsilon$ for $\epsilon>0$. We show that this regularization procedure preserves the Ward-Takahashi identity associated with the charge conservation [1], and as such it can serve as a consistent regularization of the entire interacting field theory. As a consequence of the explicitly preserved $U(1)$ gauge symmetry, the dimensional regularization yields the same result for the Coulomb correction to the conductivity when calculated using the current-current and the density-density correlators, which is, nevertheless, different than the ones previously reported in the literature. References: [1] V. Juricic, O. Vafek, and I. F. Herbut, ArXiv:1009.3269 (Phys. Rev. B, in press). [Preview Abstract] |
Monday, March 21, 2011 12:03PM - 12:15PM |
B30.00005: Plasma Excitations of Dressed Dirac Electrons in Graphene Oleksiy Roslyak, Danhong Huang, Andrii Iurov, Godfrey Gumbs The dispersion relation for the collective plasma excitations of optically dressed Dirac electrons in single and double graphene layers is calculated in the random-phase approximation. The presence of circularly polarized light gives rise to an energy gap $\epsilon_g$ between the conductance and valence bands. The value of $\epsilon_g$ may be adjusted by varying the frequency and intensity of the light which could be sizable compared to that which is generated by spin-orbit coupling or sub-lattice symmetry breaking. We present numerical results for the dispersion relation for plasma excitations for various energy gaps and separation between graphene layers. The induced $\epsilon_g$ opens up a gap in the particle-hole continuum thus allowing plasmon excitations of short wave-length. An optical and acoustic phonon-like modes are obtained in the double layer configuration. Those are very sensative to the induced energy gap and symmtry breaking between the layers. [Preview Abstract] |
Monday, March 21, 2011 12:15PM - 12:27PM |
B30.00006: Variational approach to the excitonic phase transition in graphene Fernando Sols, Javier Sabio, Francisco Guinea We analyze the Coulomb interacting problem in undoped graphene layers by using an excitonic variational ansatz. By minimizing the energy, we derive a gap equation which reproduces and extends known results. We show that a full treatment of the exchange term, which includes the renormalization of the Fermi velocity, tends to suppress the phase transition by increasing the critical coupling at which the excitonic instability takes place. [Preview Abstract] |
Monday, March 21, 2011 12:27PM - 12:39PM |
B30.00007: First-principles study of polarization in graphene Priyamvada Jadaun, YuGui Yao, Leonard F. Register, Qian Niu, Sanjay Banerjee The emergence of polarization in monolayer graphene is investigated using first-principles calculations. We try to understand electronic polarization calculated using Berry phase technique as well as ionic polarization when in-plane symmetry is broken within the graphene lattice. The effect of underlying substrate as well as stress on this symmetry breaking is also explored. [Preview Abstract] |
Monday, March 21, 2011 12:39PM - 12:51PM |
B30.00008: Graphene K and K' States at the Dirac Point Lawrence Snyder, Christopher Wells The graphene band structure states at the K and K' points and the Fermi level, the Dirac point, computed when a (1x1) unit cell is employed, fall at the gamma point when a (3x3) unit cell is employed. These states at the gamma point of the Brillouin zone for the (3x3) unit cell have a zero phase factor and are conveniently represented as molecular orbitals of pi electrons. These states are illustrated and discussed. [Preview Abstract] |
Monday, March 21, 2011 12:51PM - 1:03PM |
B30.00009: Edges states and anomalous Aharonov-Bohm-type oscillation in anti-dot lattice graphenes formed by nanoporous alumina template mask J. Haruyama, T. Shimizu, J. Nakamura, T. Matsui, H. Fukuyama Edge states of graphene with a zigzag structure theoretically have extremely high electronic density of states (EDOS), electron localization, and polarized spin transport as well [1]. However, few studies have reported on the experimental observation of edge states and related quantum phenomena. Here, we report on the nonlithographic and low-damage fabrication of honeycomb-like nanopore arrays (anti-dot lattice) on thin multilayered graphenes utilizing nanoporous alumina template masks [2]. We confirm the presence of high EDOS at the edges of the nanopores using STM observation. We find periodic magnetoresistance oscillations with two different periods over a wide magnetic field range (anomalous Aharonov-Bohm-type effect [3]) (e.g., high fields at where the diameter of cyclotron-motion electrons is smaller than diameter of the nanopore). These findings clearly suggest the presence of localized electrons and edge states at the nanopore edges of graphene. \\[4pt] [1] K. Nakada, G. Dresselhaus, M. S. Dresselhaus et al., Phys. Rev. B 54, 17954 (1996). \\[0pt] [2]T. Shimizu, J. Haruyama et al., To be published on Phys. Rev. Lett. \\[0pt] [3] D. Weiss, K.von Klitzing et al., Phys. Rev. Lett. 70, 4118 (1993). [Preview Abstract] |
Monday, March 21, 2011 1:03PM - 1:15PM |
B30.00010: Snake orbits in graphene underneath an array of Ni$_{0.80}$Fe$_{0.20}$ nano-dots Adam Neal, Jiangjiang Gu, Tony Low, Peide Ye The existence of snake orbits in 2DEG formed at AlGaAs/GaAs heterojunction is theoretically predicted and experimentally demonstrated by creating a spatially inhomogeous magnetic field [1]. Due to its ambipolar nature, graphene opens up new possibilities to investigate snake orbits and other exotic phenomena by simply creating a p-n junction in a homogenous magnetic field. We have fabricated periodic arrays of Ni$_{0.80}$Fe$_{0.20}$ nano-dots on graphene with the dot diameter of 80 nm or 150 nm and the period of 160 nm or 300 nm, respectively. A quasi-periodic magneto-resistance oscillation is observed in the low-temperature magneto-transport measurement. We ascribe it to Aharonov-Bohm oscillations induced by snake orbits of carriers underneath the nano-dots. Due to the high work-function of Ni$_{0.80}$Fe$_{0.20}$, it is possible to generate local circular n-p and p--p junctions underneath the nanodots, which form the snake orbits of carriers in an external applied magnetic field. Dependence of these oscillations on temperature and carrier density and simulation work on snake orbits will be presented.\\[4pt] [1] J.E. Muller, Phys. Rev. Lett. 68, 385 (1992); P.D. Ye et al., Phys. Rev. Lett. 74, 3013 (1995). [Preview Abstract] |
Monday, March 21, 2011 1:15PM - 1:27PM |
B30.00011: Metal Electrode Effect on Electronic Transport through Graphene Cheng Gong, Weichao Wang, Geunsik Lee, Bin Shan, Kyeongjae Cho Metal-graphene contact is one of key issues in graphene-based device applications. In this work, electronic transport through metal/graphene/metal end-contact structures with zigzag interface is investigated by first-principles non-equilibrium Green's function method. Double-dips transmission characteristics in Palladium/Graphene/Palladium are observed with a common positive dip and varied negative dips for graphene of different lengths. Transmission through the structure is suppressed by mode mismatch among different carbon localities perturbed by interface hybridization, yet intensities of the suppression at two dips are featured by distinctive channel potential profiles. Finite transmissions at Fermi level are attributed to both evanescent and propagating modes. This study benefits the understanding of the origins of contact resistance at metal/graphene interfaces. [Preview Abstract] |
Monday, March 21, 2011 1:27PM - 1:39PM |
B30.00012: Casimir Interaction Between Graphene and Planar Systems David Drosdoff, Lilia Woods Casimir forces become increasingly important as systems become miniaturized. Such fluctuation forces are studied between graphene, a potential future substitute for silicon based electronics, and other materials. Because graphene is one atomic layer thick, the Casimir force is relatively weak. Yet its singular electronic properties give rise to an attraction between graphene layers in the order of a factor of $\alpha\approx 1/137$ times smaller than the interaction between two ideal metal plates. For the case of the interaction between graphene and metamaterials, a strong reduction in the Casimir attraction or even repulsion may be found if the metamaterial is mostly magnetic in nature. Metamaterials with strong magnetic responses in the optical range may soon be possible as the rapid development of metamaterials continue. Other graphene configurations with metals and metamaterials are also studied. [Preview Abstract] |
Monday, March 21, 2011 1:39PM - 1:51PM |
B30.00013: Hot electron dynamics and Schwinger mechanism in graphene Meng-Chieh Ling, J\"org Schmalian We investigate the nonlinear dc conductivity of graphene by explicitly solving the Boltzmann equation with relaxation and particle-hole pair production contributions and obtain the non- equilibrium electronic distribution function. First, by considering isotropic elastic electron-phonon scattering, we show that, in the limit of weak external electric field one recovers Ohm's law, while above a threshold field $E=(k_B T)/(ev_F \tau)$ the dc conductivity varies as the inverse of the external electric field. In particular, we obtain an explicit form for the scaling of the conductivity with respect to $E/T$. We then investigate how this result is affected by the Schwinger mechanism, which leads to particle-hole creation and, consequently, to interband transitions. [Preview Abstract] |
Monday, March 21, 2011 1:51PM - 2:03PM |
B30.00014: Band structures of bilayer graphene superlattices Si Wu, Matthew Killi, Arun Paramekanti We have studied the electronic band structures of bilayer graphene (BLG) superlattices. In BLG, there are two distinct types of superlattice modulations - chemical potential modulations and electric field induced gap modulations. We have solved energy bands for one- and two-dimensional superlattices for both kinds of modulations. We found, in particular, for a 2D superlattice with gap modulation, that the energy gap is one order smaller than that in a uniform electric field. The problem of a single charged impurity in gated BLG is also studied. Implications of our results on transport experiments are discussed. [Preview Abstract] |
Monday, March 21, 2011 2:03PM - 2:15PM |
B30.00015: Giant inelastic tunneling in epitaxial grapheme mediated by localized states Kees Flipse, Kevin Ruit van de, Jiri Cervenka Local electronic structures of nanometer-sized patches of epitaxial grapheme and its interface layer with SiC(0001) have been studied by atomically resolved scanning tunneling microscopy and spectroscopy. Localized states belonging to the interface layer of the graphene/SiC system show to have essential influence on the electronic structure of grapheme. Giant enhancement of inelastic tunneling, reaching 50{\%} of the total tunneling current, has been observed at the localized states on a nanometer-sized graphene monolayer surrounded by defects. [Preview Abstract] |
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