Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q30: Graphene: Electron-Electron Interactions |
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Sponsoring Units: DCMP Chair: Bruno Uchoa, University of Illinois at Urbana-Champaign Room: C147/154 |
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q30.00001: The effective fine-structure constant of freestanding graphene measured in graphite Yu Gan, James Reed, Bruno Uchoa, Young-Il Joe, Diego Casa, Eduardo Fradkin, Peter Abbamonte Electrons in graphene behave like Dirac fermions, permitting phenomena from high- energy physics to be studied in a solid-state setting. A key question is whether or not these fermions are critically influenced by Coulomb correlations. We performed inelastic x-ray scattering experiments on crystals of graphite and applied reconstruction algorithms to image the dynamical screening of charge in a freestanding graphene sheet. We found that the polarizability of the Dirac fermions is amplified by excitonic effects, improving screening of interactions between quasiparticles. The strength of interactions is characterized by a scale-dependent, effective fine-structure constant, $\alpha^{\ast}_{g}(\mathbf{k},\omega)$, the value of which approaches $1/7$ at low energy and large distances. This value is substantially smaller than the nominal $\alpha_g=2.2$, suggesting that, on the whole, graphene is more weakly interacting than previously believed. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q30.00002: Excitonic Gap from Long-Range Coulomb Interaction in Graphene Jose Gonzalez We apply renormalization group methods to analyze the development of an excitonic gap in the theory of Dirac fermions in graphene with long-range Coulomb interaction. In the large-N approximation, we show that the chiral symmetry is only broken below a critical number of two-component Dirac fermions $N_c = 32/\pi^2$, that is precisely half the value found in quantum electrodynamics. Adopting otherwise a ladder approximation, we give evidence of the existence of a critical coupling at which the order parameter of the transition to the gapped phase diverges. This result supports that the opening of an excitonic gap may be driven by a sufficiently strong Coulomb interaction, despite the divergence of the Fermi velocity at low energies in the Dirac theory of graphene. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q30.00003: Manifestation of explicit and spontaneous chiral symmetry breaking in graphene Sung-Hoon Lee, Hyun-Jong Chung, Jinseong Heo, Heejun Yang, Sunae Seo Using first-principles calculations of graphene having high- symmetry distortion or defects, we investigate the chiral symmetry breaking in graphene as the source of gap opening. We identify that the gap opening by the chiral symmetry breaking in the honeycomb lattice is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in a linear lattice, the elemental 1D Dirac lattice, and find that the chiral symmetry breaking manifests itself in graphene by the formation of an internal structure of the lattice, which represents the intrinsic internal structure of massive Dirac fermions. We then show that the gap opening of many of previously reported structures of gapped graphene occurs by explicit breaking of the chiral symmetry, rather than by quantum confinement effects or others, and also show that spontaneous chiral symmetry breaking takes place via electron- phonon coupling at certain quasi-1D graphene structures and at 2D graphene under strain. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q30.00004: Theory of Kekule superconductor on graphene's honeycomb lattice Bitan Roy, Igor Herbut A spatially non-uniform superconducting state is proposed as a variational ground state on honeycomb lattice, with the chemical potential close to and right at the Dirac point, when the nearest-neighbor attraction is the dominant component of the interaction. This state spontaneously breaks the translational invariance of the underlying lattice into the Kekule pattern of superconducting bond order parameters. Otherwise it is fully gapped, spin triplet, and odd under the exchange of two sublattices. Symmetries of the ground state for a range of nearest-neighbor interaction, the topological excitations of the Kekule superconductor, and its competition with other superconducting orders proposed in literature will also be discussed. \\[4pt] B. Roy and I. F. Herbut, Phys. Rev. B 82, 035429 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q30.00005: Pairing in graphene: A Monte Carlo study Tianxing Ma, Zhongbing Huang, Feiming Hu, Hai-Qing Lin To address the issue of possibility of inducing superconductivity in graphene, we study the behavior of pairing correlation in the extended repulsive Hubbard model on a honeycomb lattice within both determinant quantum Monte Carlo and constrained path Monte Carlo method. We find that the system shows an antiferromagnetic correlation below Van Hove fillings. In the filling range of $<$$n$$>$=1.00 $\sim$ 1.20, pairing with $d+id$ symmetry is dominant over pairing with extent $s$ symmetry, especially at low temperatures. The $d+id$-wave pairing susceptibility is enhanced as the electron filling increases, while the effective pairing interaction is suppressed. The summation of pairing correlation for long-range part is enhanced as the repulsion increases, however, for various lattice sizes and interactions, we find that the long-range part of $d+id$-wave pairing correlations both vanishes. Our results suggest that there maybe no superconductivity in pure and low doped graphene. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q30.00006: Interacting fermions on the honeycomb bilayer: From weak to strong coupling Oskar Vafek Many-body instabilities of the half-filled honeycomb bilayer are studied using weak-coupling renormalization group (RG) as well as strong-coupling expansion [1,2]. For spinless fermions, there are 4 independent four-fermion contact couplings. Generally, we find runaway RG flows which we associate with ordering tendencies. The broken symmetry state is typically a gapped insulator with either broken inversion or broken time-reversal symmetry, with a quantized anomalous Hall effect. Additionally, a tight-binding model with nearest-neighbor hopping and nearest-neighbor repulsion is studied in weak and strong couplings and in each regime a gapped phase with inversion symmetry breaking is found. In the strong-coupling limit, the ground-state wave function is constructed for vanishing in-plane hopping but finite interplane hopping, which explicitly displays the broken inversion symmetry and a finite difference between the number of particles on the two layers. In the spin-1/2 case we use Fierz identities to show that there are 9 independent four-fermion contact couplings[2]. The 9 RG equations in this case reduce to the 3 found in Ref.[1] under the assumptions stated in Ref.[1]. They are further used to show that, just as in strong coupling, the most dominant weak-coupling instability of the repulsive Hubbard model (at half filling) is an antiferromagnet. [1] O. Vafek and K. Yang, PRB {\bf 81}, 041401 (2010). [2] O. Vafek, PRB {\bf 82}, 205106 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q30.00007: Compressibility Instability of Interacting Electrons in Bilayer Graphene Xin-Zhong Yan, C.S. Ting Using the self-consistent Hartree-Fock approximation, we study the compressibility instability of the interacting electrons in bilayer grapheme at finite temperature. The chemical potential and the compressibility of the electrons can be significantly altered by an energy gap (tunable by external gate voltages) between the valence and conduction bands. For zero gap case, we show that the homogeneous system is stable. When the gap is finite, the compressibility of the electron system becomes negative at low carrier doping concentrations and low temperature. We also present the phase diagram distinguishing the stable and unstable regions of a typically gapped system in terms of temperature and doping. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q30.00008: Limits to universal conductance fluctuations of massless Dirac fermions Mario Borunda, Jesse Berezovsky, Robert Westervelt, Eric Heller We study conductance fluctuations (CFs) and the sensitivity of the conductance to the motion of a single scatterer in two-dimensional massless Dirac systems. Our extensive numerical study finds limits to the predicted universal value of CFs. We find that CFs are suppressed for ballistic systems near the Dirac point and approach the universal value at sufficiently strong disorder. The conductance of massless Dirac fermions is sensitive to the motion of a single scatterer. CFs of order $e^2/h$ result from the motion of a single impurity by a distance comparable to the Fermi wavelength. This result applies to graphene systems with a broad range of impurity strength and concentration while the dependence on the Fermi wavelength can be explored via gate voltages. Our prediction can be tested by comparing graphene samples with varying amounts of disorder and can be used to understand interference effects in graphene mesoscopic devices. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q30.00009: Interplay between curvature and in-plane magnetic field in bilayer graphene Avadh Saxena, Yogesh Joglekar For a two-dimensional electron gas (2DEG) in a uniform magnetic field, the effect of the in-plane component on the orbital motion of carriers is ignored because ``it can be gauged away.'' However, the effect of such a field on a massive quantum particle confined to a curved surface has been only recently explored [1]. We obtain the single-particle spectra for such a particle on a sphere, a cylinder, and a torus in the presence of a constant magnetic field. In addition to the geometric potential $V_G$ that arises due to the confinement on a curved surface, we find that in-plane field leads to energy shifts $\Delta E\propto V_G(R/l_B)^4$ where $R$ is the radius of curvature of the surface, and $l_B$ is the magnetic length for the in-plane field. With bilayer graphene as a model for massive quantum particle on a curved surface, we estimate the energy shift for a cylindrical geometry, and show that it is significant for typical experimental parameters. \\[4pt] [1] G. Ferrari and G. Cuoghi, Phys. Rev. Lett. {\bf 100}, 230403 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q30.00010: Plasma Instability in Graphene Bilayers Antonios Balassis, Godfrey Gumbs The problem of plasma instability in a pair of coupled semiconductor layers when a dc current is passed through one of the layers has been vigorously investigated over the years. This may be carried out by solving for the real and imaginary parts of the frequency in the polarization function making the dielectric function vanish. We analyze the conditions for plasma instability in a graphene bilayer for various chemical potentials (doping) as well as layer separation. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q30.00011: Tight-binding theory of the spin-orbit coupling in graphene structures Sergej Konschuh, Martin Gmitra, Jaroslav Fabian Spin-orbit coupling changes qualitatively the electronic band structure of graphene. Most important, the coupling induces spectral gaps at the K(K') points. Earlier theories estimated the \emph{intrinsic} gap of $1$ $\mu$eV for the single layer and several meVs for bi- and tri-layer graphene, based on $\sigma$-$\pi$ coupling. Our first-principles calculations give the value of 24 $\mu$eV for all these systems, due to the presence of the orbitals of the $d$ symmetry in the Bloch states of the $\pi$ bands. A realistic multiband tight-binding model is presented to explain the effects the $d$ orbitals play in the spin-orbit coupling of graphene and derive an effective single-orbital next-nearest-neighbor hopping model that accounts for the spin-orbit effects. We also study the \emph{extrinsic} spin-orbit coupling, due to an applied transverse electric field. In a single layer the \emph{extrinsic} effect is dominated by the $\pi$-$\sigma$ hybridization. In contrast, in the multi-layer structures the \emph{extrinsic} spin-orbit band splittings come from an interplay of the $d$-orbitals, the inter-layer hopping, and the electrostatic potential from the applied field. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q30.00012: Spin-orbit coupling in bi-layer and tri-layer graphene in transverse electric field: first-principles calculations Martin Gmitra, Sergej Konschuh, Jaroslav Fabian Few-layer graphene structures may be potentially useful for optical and transport applications, due to the possibility of electrical control of the band gaps. Here we investigate the spin-orbit coupling of bilayer and tri-layer graphene around the Fermi level. We show, by performing first-principles full potential linearized augmented plane waves calculations that the spin-orbit physics in these structures derives essentially from monolayer graphene. In particular, the spin splitting of the bands is due to the spin-orbit coupling of the d-orbitals. These give a splitting of the order of 24$\,{\rm \mu eV}$ at the K point, as in graphene. Breaking the spatial inversion symmetry by a transverse electric field does not change this (intrinsic) picture, unlike what we know from graphene. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q30.00013: Dynamical Jahn-Teller Effect at a Vacancy Center in Graphene Sashi Satpathy, Mohammad Sherafati, Birabar Nanda, Zoran Popovic We study the substitutional vacancy center in graphene from density-functional LAPW calculations and show that it is magnetic and at the same time forms a dynamical Jahn-Teller center. A net magnetic moment of $2 \mu_B$ is found, which is explained in terms of the occupation of the $sp^2\sigma$ dangling bond state and the zero-mode state derived from the $\pi$ bands. The adiabatic potential surface resulting from the $ E \otimes e$ vibronic coupling was computed and subsequently the Schr\"odinger equation was solved for the nuclear motion of the carbon atoms. Our calculations show the tunneling splitting $3 \Gamma$ to be about 80 cm$^{-1}$, which is substantially larger than the typical strain fields, leading to a dynamical Jahn-Teller effect (JTE). This explains the puzzling behavior of why in the STM measurements a symmetric carbon triangle is observed around the vacancy, while at the same time we predict the splitting of the vacancy-induced electron states by the static JTE {\it in spite of} the triangular symmetry. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q30.00014: Plasma Excitations in Graphene: Their Spectral Intensity and Temperature Dependence in Magnetic Field Jhao-Ying Wu, Szu-Chao Chen, Godfrey Gumbs, Ming-Fa Lin We calculated the dielectric function, the loss function, the magnetoplasmon dispersion relation and the temperature-induced transitions for graphene in a uniform perpendicular magnetic field. The calculations were performed using the Peierls tight-binding model to obtain the energy band structure and the random-phase approximation to determine the collective plasma excitation spectrum. The single-particle and collective excitations have been precisely identified based on the resonant peaks in the loss function. The critical wave vector at which plasmon damping takes place is clearly established. This critical wave vector depends on the magnetic field strength as well as the levels between which the transition takes place. The temperature effects were also investigated. At finite temperature, there are plasma resonances induced by the Fermi distribution function. Whether such plasmons exist is mainly determined by the field strength, temperature, and momentum. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q30.00015: Field Modulation on the Electronic Structure for the Bilayer and Trilayer Graphene Bi-Ru Wu The electronic band gap plays a central role in modern device physics and a tunable band gap provides great flexibility in device design. I present the investigation of electric filed effect on the electronic structure of the bilayer and trilayer graphene. The hexagonal and Bernal type structures are studied for the bilayer and trilayer graphene, additionally, the rhombohedral type is also take into account for the trilayer one. It is found the band gaps of the Bernal type bilayer graphene and the Rhombohedral type trilayer graphene are tunable by a perpendicular electric field. The symmetry of the graphene plays a crucial role in the field modulation. The perpendicular electric field opens the band gap of the Bernal type bilayer graphene and the Rhombohedral type trilayer graphene by breaking the symmetry in z-direction. [Preview Abstract] |
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