Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session H30: Electronic Properties of Graphene and Related Structures II |
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Sponsoring Units: DCMP Chair: Giovanni Cantele, Istituto Nazionale per la Fisica della Materia Room: Morial Convention Center 222 |
Tuesday, March 11, 2008 8:00AM - 8:12AM |
H30.00001: Electronic structure of graphene in the presence of disorder Alexander Kemper, Manoj Srivastava, Hai Ping Cheng Graphene, a single layer of the carbon structure graphite, has a number of interesting electronic properties. To aid in the understanding of these properties we have performed first-principles calculations of single graphene layers in the presence of disorder of various forms, including single and double vacancies, Stone-Wales defects, and metallic dopants. We report the effects of defects and dopants on the charge density and electronic density of states. Furthermore, we discuss energetics of these systems and defect-induced spin-states. [Preview Abstract] |
Tuesday, March 11, 2008 8:12AM - 8:24AM |
H30.00002: Density of states in graphene with charged impurities Ben Yu-Kuang Hu, E. H. Hwang, S. Das Sarma We discuss the density of states of graphene in the presence of charged screened impurity scattering. The density of states is obtained from the imaginary part of the single-particle Green's function, which is evaluated in the Born and the self-consistent Born approximations, and the screened Coulomb impurity potentials are evaluated within the random phase approximation. The density of states in the presence of impurities is typically larger than that of clean graphene at any given energy. In particular, the density of states at the Dirac point, which is zero in a clean sample, becomes non-zero, with a magnitude that is given by an expression akin to that for the BCS superconducting gap. [Preview Abstract] |
Tuesday, March 11, 2008 8:24AM - 8:36AM |
H30.00003: Topological Frustration in an Alkali-Graphene-Halogen System Youjian Tang, Vincent Crespi We theoretically studied a system with alkali and halogen adsorbed to opposing sides of a graphene sheet, their mutual interactions then being modulated and constrained by the interposing presence of the sheet. Charge transfer from alkali to halogen generates a substantial dipole moment and large local electric field. Trends with respect to electron affinity, ionization energy, areal density, and the character of the bounding layer (i.e. BN versus graphene) will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 8:36AM - 8:48AM |
H30.00004: Energy gaps and Stark effects in boron nitride nanoribbons Cheol-Hwan Park, Steven G. Louie Graphene nanoribbons, which have been recently synthesized, are regarded as promising candidate materials for nanoscale electronics. It is expected that boron nitride nanoribbons may be produced in a similar way. Notwithstanding their structural similarity, the electronic properties of boron nitride nanoribbons are qualitatively different from those of graphene nanoribbons. Here, we present first-principles calculations of the electronic properties of boron nitride nanoribbons with widths up to 10 nm both without any external potential or under a transverse electric field. The results show a rich set of behaviors and promise for possible applications of boron nitride nanoribbons in nanoscale electronics. [Preview Abstract] |
Tuesday, March 11, 2008 8:48AM - 9:00AM |
H30.00005: Band Engineering in C/BN Super-stripes Jeffrey Mullen, Marco Buongiorno-Nardelli Using electronic structure calculations from first principles, we have studied the electronic characteristics of graphene/BN sheets in a planar ``super-striped'' geometry. Similarly to Hydrogen-terminated graphene nanoribbons, also C/BN super-stripes show a variation of band gaps associated with the stripe size. Moreover, the bonding with BN introduces confinement effects that can be potentially exploited to enhance the electronic transport properties of these systems. We have characterized these effects by evaluating the band offsets and the electrostatic potential profile across the super-stripe structures. [Preview Abstract] |
Tuesday, March 11, 2008 9:00AM - 9:12AM |
H30.00006: Selective nesting and sensing of molecules on optimally modified material surfaces John Russell, Boyang Wang, Petr Kral We develop a methodology of optimal modifications of material surfaces allowing us to design selective nesting sites for inorganic, organic and biological molecules [1]. The idea is to modify material surfaces by atomic dopants and charged ligands in such a way that the created local electric fields form selective Coulombic traps for the adsorbed molecules. We demonstrate this methodology by molecular dynamics simulations of short peptides docked in nesting sites designed on graphene sheets substitutionally doped with B and N atoms. We show that the same approach can be used to selectively dock proteins in water solvent on graphene layers modified by short charged ligands. As a practical application of this methodology, we design and model chemical sensors that could detect the selectively nested molecules. The detection is realized by evaluating the change of the electrical conductivity of the modified graphene sheets upon the docking of the molecules. [1] B. Wang and P. Kral, Small, 3, 580, (2007). [2] J. Russell, B. Wang and P. Kral, in preparation. [Preview Abstract] |
Tuesday, March 11, 2008 9:12AM - 9:24AM |
H30.00007: Anisotropic Electron-Phonon Coupling on Graphene-Derived Fermi Surface in CaC$_{6}$ Tonica Valla, J. Camacho, Z-H. Pan, A.V. Fedorov, A.C. Walters, C.A. Howard, M. Ellerby Superconductivity in graphite intercalated compounds had been studied for more than 40 years and it is still not fully understood, despite the recent progress and the discovery of relatively high Tc superconductivity in CaC$_{6}$ and YbC$_{6}$. Initially, even unconventional mechanisms, such as excitonic and plasmonic pairing were considered, but recent studies now suggest that superconductivity in graphite intercalated compounds is more conventional and that the electron-phonon coupling is responsible for pairing. However, it is still not clear whether the graphene-derived electronic states and vibrations or the intercalant-derived ones play more important role, or if some particular combination of graphene-intercalant states and vibrations dominates the coupling. Here, we present angle-resolved photoemission studies of electronic structure in CaC$_{6}$. We find that the electron-phonon coupling on the graphene-derived Fermi surface is very strong and anisotropic, reflecting the interaction of graphene-derived states with high-frequency graphene-derived vibrations. [Preview Abstract] |
Tuesday, March 11, 2008 9:24AM - 9:36AM |
H30.00008: Fractional statistics of topological defects in graphene and related structures Babak Seradjeh, Marcel Franz We show that fractional charges bound to topological defects in the recently proposed time-reversal-invariant models of spinless fermions on the honeycomb lattice with Kekule distortion and on the $\pi$-flux square lattice with Peierls distortion obey fractional statistics. The continuum effective low-energy description is given in terms of a `doubled' level-2 Chern-Simons field theory, which is parity and time-reversal invariant and implies two species of semions (particles with statistical angle $\pm\pi/2$) labeled by a new emergent quantum number that we identify as the fermion axial charge. [Preview Abstract] |
Tuesday, March 11, 2008 9:36AM - 9:48AM |
H30.00009: Irrational vs. rational charge and statistics in graphene-like system Chang-Yu Hou, Claudio Chamon, Roman Jackiw, Christopher Mudry, So-Young Pi, Andreas P. Schnyder Electron fractionalization is intimately related to topology. In one-dimensional systems, fractionally charged states exist at domain walls between degenerate vacua. In two-dimensional systems, fractionalization exists in quantum Hall fluids, where time-reversal symmetry is broken by a large external magnetic field. Recently, there has been a tremendous effort in the search for examples of fractionalization in two-dimensional systems with time-reversal symmetry. We will show that quasiparticle excitations with irrational charge and irrational exchange statistics exist in tight-biding systems, such as graphene-like structure, described, in the continuum approximation, by the Dirac equation in (2+1)-dimensional space and time. These excitations can be deconfined at zero temperature, but when they are, the charge re-rationalizes to the value 1/2 and the exchange statistics to that of ``quartons'' (half-semions). [Preview Abstract] |
Tuesday, March 11, 2008 9:48AM - 10:00AM |
H30.00010: Internal mobility edge in doped graphene: frustration in a renormalized lattice Gerardo Naumis We show that an internal localization mobility edge can appear around the Fermi energy in graphene by introducing impurities in the split-band regimen, or by producing vacancies in the lattice. The edge appears at the center of the spectrum and not at the band edges, in contrast with the usual picture of localization. Such result is explained by showing that the bipartite nature of lattice allows to renormalize the Hamiltonian, and the internal edge appears because of frustration effects in the renormalized lattice [1]. The size in energy of the spectral region with localized states is similar in value to that observed in narrow gap semiconductors. [1] G.G. Naumis, Phys. Rev. B 76, 153403 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 10:00AM - 10:12AM |
H30.00011: ABSTRACT WITHDRAWN |
Tuesday, March 11, 2008 10:12AM - 10:24AM |
H30.00012: Possible bandgap opening in graphene due to deposited Au nanoparticles: First-principles calculations Ronaldo J.C. Batista, Sabrina S. Carara, Helio Chacham We perform first-principles calculations to investigate electronic and structural properties of graphene with a layer of deposited Au nanoparticles. We consider Au$_{38}$ nanoparticles that can be either covered with methylthiol molecules, or not. We also consider that the nanoparticles are arranged in a hexagonal lattice, and we focus on the effect of net charge, applied electric fields, and molecular coverage on the electronic structure of the graphene+nanoparticles system. We find that covered nanoparticles interact weakly with graphene, and that the main effect of the nanoparticles on the electronic structure of graphene is a doping effect that can be modified with the application of an electric field perpendicular to the graphene plane. The system is always metallic, without the opening of a bandgap, even if the nanoparticles are charged: neither the Coulomb potential nor the weak nanoparticle-graphene interaction is able to break the graphene sublattice symmetry. In contrast, in the case of deposited bare (non-covered) Au nanoparticles, there is a relatively strong interaction between low-coordinated Au atoms and graphene carbon atoms beneath. This leads to a symmetry breaking of the graphene sublattices and to the opening of a small bandgap of a few tens of meV. [Preview Abstract] |
Tuesday, March 11, 2008 10:24AM - 10:36AM |
H30.00013: Nonequilibrium-induced metal-superconductor quantum phase transition in graphene So Takei, Yong Baek Kim We study the effects of dissipation and time-independent nonequilibrium drive on an open superconducting graphite monolayer, or graphene. In particular, we investigate how dissipation and nonequilibrium effects modify the semi-metal-BCS quantum phase transition that occurs at half-filling in equilibrium graphene with attractive interactions. Our system consists of a graphene sheet sandwiched by two semi-infinite three-dimensional Fermi liquid reservoirs, which act both as a particle pump/sink and a source of decoherence. A steady-state charge current is established in the system by equilibrating the two reservoirs at different, but constant, chemical potentials.The nonequilibrium BCS superconductivity in graphene is formulated using the Keldysh path integral formalism, and we obtain generalized gap and number density equations valid for both zero and finite voltages. The behaviour of the gap is discussed as a function of both attractive interaction strength and filling for various graphene-reservoir couplings and voltages. [Preview Abstract] |
Tuesday, March 11, 2008 10:36AM - 10:48AM |
H30.00014: Detecting single graphene layer by using fluorescence from high-speed Ar$^{7+}$ ion Yoshiyuki Miyamoto, Hong Zhang A highly-charged-ion interacting with graphite causes structural change in nano-scales [1]. While when the ion's kinetic energy reaches few MeVs, the induced is not the structural change but electronic excitation. An experiment [2] showed fluorescence from Ar$^{7+}$ ions penetrating through carbon foil with kinetic energy of 2 MeV. Motivated by this experiment, we tested interaction between an Ar$^{7+}$ ion and a graphene sheet by the time-dependent density functional approach, and found that the electronic excitation in the Ar$^ {7+}$ ion is also the case even when the incident kinetic energy is 500 KeV and the target thickness is only mono-atomic layer. This simulation suggests the possibility of detecting a suspended mono-atomic layer of graphene [3] by monitoring fluorescence from the penetrated Ar$^{7+}$ ions. We will discuss its importance for analyzing bombardment of solids by highly charged, high-speed ions and possible experiments according to the present result. References: [1] T. Meguro, et al., Appl. Phys. Lett {\bf 79}, 3866 (2001). [2] S. Bashkin, H. Oona, E. Veje, Phys, Rev. A{\bf25}, 417 (1982). [3] J. Mayer et al., Nature (London), {\bf 446}, 60 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 10:48AM - 11:00AM |
H30.00015: Charge response function and a novel plasmon mode in graphene Eugene Mishchenko, Abdel-Khalek Farid, Suhas Gangadharaiah Polarizability of non-interacting 2D Dirac electrons has a inverse square root singularity at the boundary of electron-hole excitations. The screening of this singularity by long-range electron-electron interactions is usually treated within the random phase approximation. The latter is exact only in the limit of N->infinity, where N is the ``color'' degeneracy. We find that the ladder-type vertex corrections become crucial close to the threshold. The strong singularity in the ladder series arises due to the long range interaction between electrons which move almost collinearly to the external momentum. The series is summed up analytically yielding a non- perturbative result: the density and spin response functions acquire non-zero imaginary part in an additional frequency range $ q u < \omega < q v$. The reversal of the sign of the electron polarizability in this new domain gives rise to a sharp plasmonic mode which is absent in the conventional RPA. [Preview Abstract] |
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