Bulletin of the American Physical Society
APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012; Boston, Massachusetts
Session P7: Focus Session: Computational Design of Materials: Graphene - Electronic Structure and Transport |
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Sponsoring Units: DMP DCOMP Chair: Yan Wang, University of Florida Room: 207 |
Wednesday, February 29, 2012 8:00AM - 8:12AM |
P7.00001: Electronic transport properties of a graphene monolayer covered by another layer with infinite or finite width Daniel Valencia, Ji Luo, Jun-Qiang Lu Intrinsic graphene, a zero-gap semiconductor, is one of the most promising materials for nanodevices. In this work, the electronic transport properties of a graphene monolayer covered by another infinite or finite layer were studied. The results of the transmission spectrum and the local density of states (LDOS) showed a weak interaction between the two layers when the top layer is infinite or semi-infinite. Thus the transport properties of the monolayer do not change much. However, when the monolayer is covered by a finite-width nanoribbon, the change in its transmission spectrum is dependent on the width of the ribbon. In order to understand the origin of this phenomenon we calculated the transmission spectrum of one individual channel, and observed that the changes are due to antiresonance in the electronic transmission, which is caused by interlayer interference between the wavefunctions. [Preview Abstract] |
Wednesday, February 29, 2012 8:12AM - 8:24AM |
P7.00002: Electronic transport proprieties of graphene few-layers Junqiang Lu, Carlos Sierra, Daniel Valencia We study electronic transmission spectrums of graphane few-layers (up to 6 layers) with AA or AB stacking by using of first-principle transport calculations. The results demonstrate flat transmission spectrum for even layers while linear transmission for odd layers, which can be understood by different band structures of graphene few-layers due to the interlayer interaction. [Preview Abstract] |
Wednesday, February 29, 2012 8:24AM - 8:36AM |
P7.00003: Quantum Transport Properties of Modified Graphene Nanoribbons with Boron Nitride Domains at the Mesoscopic Scale Alejandro Lopez-Bezanilla Graphene nanoribons are seen as promising building blocks for engineering graphene based field effect transistor (GNR-FET) . The quest for fabricating efficient GNR-FETs requires a trade-off between a sufficiently wide energy gap and a reasonably large charge mobility. The solution might be the chemical codoping of one-atom thick layers of C with B and N atoms. These hybrid systems are attracting much attention as they can provide an efficient way to create new materials with complementary properties to those of graphene and h-BN. I will present a study of charge transport in graphene nanoribbons with BN domains randomly distributed along the ribbon surface. My results describe how the conductance of the hybrid systems is altered as a function of the incident electron energy and the BN domain density which leads to transport band gap opening. We explore the transport regimes comparing different degrees of BN codoping and BN domain size for ribbons of various widths and lengths on the order of the micrometer. A comparison with other types of defects such as oxygen atoms in epoxy configuration and functional groups covalently attached to the ribbon surface will be discussed as well. [Preview Abstract] |
Wednesday, February 29, 2012 8:36AM - 8:48AM |
P7.00004: First-principles study of charge transfer doping and electronic transport in single--walled carbon nanotubes Xiangguo Li, Hai-Ping Cheng It is well known that charge transfer doping can greatly enhance the conductivity in single-walled carbon nanotube thin films. Recent experiments showed that the tube-tube contact resistance dominates the impedance in the films. To understand effects of doping on the tube-tube contact, we studied the tube-tube distance changes and electron transport properties upon doping (K and Br) using first-principles calculations. Our results suggest that the effect of doping on the tube-tube distance depends not only on the type of dopants but also on the electronic properties of the carbon nanotubes. [Preview Abstract] |
Wednesday, February 29, 2012 8:48AM - 9:00AM |
P7.00005: Computational Study of the Thermal and Electronic Transport Properties of Rigidly-Interconnected Carbon Nano Foam Sora Park, Young-Kyun Kwon, David Tom\'anek We study the thermal and electronic transport properties of rigidly-interconnected structures having $sp^2$ carbon minimal surface called schwarzites. The system consists of core parts composed of schwarzite and interconnection parts with (4,4) carbon nanotube segments [1]. Using direct molecular dynamics simulations with the Tersoff potential, we compute the thermal conductivity of various configurations to explore the dependence on the number of core parts and on the length of interconnection parts. Our calculations show that each core part plays as a scattering center, which reduces the phonon mean free path and thus the thermal conductivity. We also investigate the electronic transport properties of the system by applying the non-equilibrium Green function approach in combination with density functional theory. We explore the effects of different core connectivity and structural defects introduced near the core parts on the electrical conductance. These thermal and electonic properties may be connected to the thermoelectric properties of the schwarzite system.[1] S. Park, K. Kittimanapun, J. S. Ahn, Y.-K. Kwon and D. Tom\'anek, J. Phys.: Condens. Matter {\bf{22}}, 334220 (2010). [Preview Abstract] |
Wednesday, February 29, 2012 9:00AM - 9:12AM |
P7.00006: Klein, anti-Klein tunneling and pseudo-spintronics in graphene heterojunctions Redwan Sajjad, Avik Ghosh Electrons in monolayer graphene act like massless spin-1/2 Dirac fermions. Backscattering is suppressed due to the pseudospin orthogonality of the forward and reverse scattering modes. The resulting Klein tunneling provides unit transmission for normally incident electrons at a pn junction, regardless of barrier height. By combining voltage gating with a tunnel barrier, we can realize a gate tunable metal insulator transition that promotes subthermal switching [1] and also makes the conduction unipolar. In contrast, bilayer graphene electrons act like parabolic spin-1 systems with perfect reflection for normal incidence (anti-Klein tunneling). For n$^{+}$n or p$^{+}$p junction, the transmission maximizes for normal incidence like single layer, but unlike monolayer graphene, it's barrier-dependent. We also perform atomistic numerical calculation of graphene sheets with experimentally relevant size (hundreds of nanometer) using non-equilibrium Green's function formalism and we show that the conductance can be varied substantially with gate voltage for multiple sequenced pn junctions with smoothly varying potential. \\[4pt] [1] Sajjad and Ghosh, APL 99, 123101 (2011). [Preview Abstract] |
Wednesday, February 29, 2012 9:12AM - 9:24AM |
P7.00007: Curvature-induced spin-orbit coupling and spin relaxation in a chmically-clean single-layer graphene Jae-Seung Jeong, Jeongkyu Shin, Hyun-Woo Lee Based on the second-order perturbation theory, we show that curvature induced by corrugations or periodic ripples in single-layer graphenes generates two types of effective spin-orbit coupling. In addition to the spin-orbit coupling reported previously that couples with sublattice pseudospin and corresponds to the Rashba-type spin-orbit coupling, there is an additional spin-orbit coupling that does not couple with the pseudospin. The additional spin-orbit coupling depends on the direction of principal curvature, which is similar with the curvature-induced spin-orbit coupling of carbon nanotubes that depends on the chiral angle. However, the spin-orbit coupling of single-layer graphenes can not be obtained from the trivial extension of the spin-orbit coupling of carbon nanotubes owing to their distinct topological structure. Via the numerical calcualtion, we show that both types of the curvature-induced spin-orbit coupling make the same order of contribution to spin relaxation in chemically-clean single-layer graphene with nanoscale corrugation. The spin relaxation dependence on the corrugation roughness is also investigated. [Preview Abstract] |
Wednesday, February 29, 2012 9:24AM - 9:36AM |
P7.00008: Fractional topological phases, triplet superconductivity and spontaneous time reversal symmetry breaking in strained graphene Pouyan Ghaemi, Jerome Cayssol, Donna Sheng, Ashvin Vishwanath Despite wide interests to realize fractional time-reversal symmetric phases, an experimentally realizable system with these exotic topological orders is still lacking. Recent experiment has confirmed that strain can be used to control the electronic states of graphene and create flat pseudo-Landau levels in the absence of external magnetic field. In this talk, I show that graphene under strain is a natural playground for the observation of exotic phases such as fractional valley Hall insulator as well as flat band superconductivity. For neutral graphene, we find a competition between the Ising valley ferromagnet and the spin ferromagnet ruled by the value of short range Hubbard and long range Coulomb interactions. At fractional filling with different intervalley and intravalley interactions, a spin triplet superconductor, a valley-polarized Laughlin state or a time-reversal symmetric fractional valley-Hall state could be realized. [Preview Abstract] |
Wednesday, February 29, 2012 9:36AM - 9:48AM |
P7.00009: Satellite Structures in Spectral Functions of Silicon and Graphene from ab initio GW and Cumulant Expansion Calculations Derek Vigil Currey, Johannes Lischner, Steven Louie The GW approximation is a well-established method for obtaining accurate quasiparticle properties in a wide range of materials. Its suitability for satellite structures (e.g., those measured in photoemission spectroscopies), however, has rarely been addressed in detail for real materials and the fact that GW overestimates the position of the plasmon satellite peaks in the spectral function of silicon indicates the need for an improved method for satellites. One such method is the cumulant expansion. The cumulant expansion is a method that includes, approximately, higher-order processes beyond GW that are important for satellite properties. We present here full-frequency results for the satellite and quasiparticle properties of silicon and doped graphene using the GW and the cumulant expansion methods, and discuss the improvements in satellite properties given by the cumulant expansion. We also compare our results to earlier model calculations on doped graphene. [Preview Abstract] |
Wednesday, February 29, 2012 9:48AM - 10:00AM |
P7.00010: Static and dynamical response of graphene Tobias Stauber, Guillermo Gomez Santos We discuss the static and dynamical response of graphene. First, we show that including the full hexagonal lattice leads to anisotropic Friedel oscillations and paramagnetic orbital susceptibility around the neutrality point [1]. We then apply the dynamical current-current correlation function to discuss graphene's fluorescence quenching including also transverse decay channels and full retardation [2]. We finally discuss the optical properties of double layer graphene. \\[4pt] [1] G. G\'omez-Santos and T. Stauber, Phys. Rev. Lett. 106, 045504 (2011).\\[0pt] [2] G. G\'omez-Santos and T. Stauber, Phys. Rev. B 84, 165438 (2011). [Preview Abstract] |
Wednesday, February 29, 2012 10:00AM - 10:12AM |
P7.00011: First-principles study of graphene - carbon nanotube contacts Brandon Cook, Kalman Varga The electron transport properties of carbon nanotube -- graphene junctions are investigated with first-principles total energy and electron transport calculations. By combining the advantageous material properties of graphene and nanotubes one can create all carbon hybrid architectures with properties that are particularly well suited to applications. The p-type Schottky barrier height is calculated in model junctions with (8,0) and (10,0) nanotubes in a top-contact configuration. Results indicate a lower barrier in carbon nanotube -- graphene junctions than in other carbon nanotue -- metal systems. [Preview Abstract] |
Wednesday, February 29, 2012 10:12AM - 10:24AM |
P7.00012: Electronic Band Structures of Graphene Nanomeshes William Oswald, Zhigang Wu Owing to its many remarkable properties, graphene is very promising for electronic and opto-electronic applications for size miniaturization and improving performance; however, bulk graphene is a semi-metal with zero band gap, and many methods have been proposed to open up a sizable band gap. In this work, we carry out first-principles calculations based on the density functional theory (DFT) to investigate electronic band structures of graphene nanomeshes (GNMs), the defected graphene containing a high-density array of nanoholes, studying the bandgap opening mechanism and evaluating band gap as functions of structural parameters, including hole size, hole shape, hole-hole distance, and hole arrangement. Our results suggest that while the band gap opening is a result of quantum confinement at nanomesh necks, the size of band gap depends strongly on the detailed GNM structures. For the simplest hexagonal holes, two thirds of GNMs remain semi-metallic and the rest one third are semiconductors. [Preview Abstract] |
Wednesday, February 29, 2012 10:24AM - 10:36AM |
P7.00013: Quantum Mechanics on a Mobius Strip: Energy Levels, Symmetries, and Level Splitting in a Magnetic Field Zehao Li, Ramdas Ram-Mohan We investigate the energy levels of an electron on a M\"obius strip. Schr\"odinger's equation on this curved surface is shown to have terms that do not have invariance under parity transformation in parameter space for the strip. The double degeneracy of energy levels that exists for flat cylindrical rings is shown to be removed for the pairs of energies in the M\"obius strip due to parity symmetry breaking. The orbital angular momentum is found to have approximately not only integer but also half-integer values of $\hbar$. The splitting of the energy levels in an external magnetic field is displayed. The effects of multiple twists are investigated to further clarify that the parity symmetry breaking is the effect of the curved geometry, while the appearance of half-integer angular momentum states is a topological effect. The implications for twisted rings composed of graphene will be discussed, and carrier transport through the M\"obius strip will be considered. This work was supported by AFLR/DARPA under grant FA8650-10-1-7046. [Preview Abstract] |
Wednesday, February 29, 2012 10:36AM - 10:48AM |
P7.00014: QED Kapitza conductance of nano-carbon thermal interconnects Slava V. Rotkin, Alexey G. Petrov The theory for the near-field Kapitza conductance across the interface of a nano-carbon material and the quartz is thoroughly investigated. The near-field photon tunneling is shown to contribute to the total heat flux between the hot and cold sides of the interface on the order of or even larger than the normal thermal conductance. Quartz is chosen as the most common example of non-conductive and strongly polar substrate material with the well known polarization properties, though the theory is not restricted to quartz only. Our approach allowed us to derive a unified expression for QED Kapitza conductance of the nanocarbon thermal interconnect material, such as graphene, a nanotube, or a nanotube forest and predict thermal phenomena, such as the heat rectification, as a function of the materials properties of the interface. [Preview Abstract] |
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