Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G8: Graphene: Nanoribbons, Dots, and Strain |
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Sponsoring Units: DCMP Chair: Vincent Meunier, Rensselaer Polytechnic Institute Room: 307 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G8.00001: Towards a Rigorous Proof of Magnetism on the Edges of Graphene Nanoribbons Hamed Karimi, Ian Affleck A zigzag edge of a graphene nanoribbon supports localized zero modes, ignoring interactions. Based mainly on mean field arguments and numerical approaches, it has been suggested that interactions can produce a large magnetic moment on the edges. By considering the Hubbard model in the weak coupling limit, $U\ll t$, for bearded as well as zigzag edges, we argue for such a magnetic state, based on Lieb's theorem. Projecting the Hubbard interactions onto the flat edge band, we then prove that resulting 1 dimensional model has a fully polarized ferromagnetic ground state. We also study excitons and the effects of second neighbor hopping as well as a potential energy term acting on the edge only, proposing a simple and possibly exact phase diagram with the magnetic moment varying smoothly to zero. Finally, we consider corrections of second order in $U$, arising from integrating out the gapless bulk Dirac excitations. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G8.00002: Quantum Monte Carlo study of edge-state magnetism on chiral graphene nanoribbons Michael Golor, Thomas C. Lang, Stefan Wessel We investigate the edge-state magnetism of chiral graphene nanoribbons using projective quantum Monte Carlo (QMC) simulations and a self-consistent mean-field approximation of the Hubbard model. Previous QMC simulations support edge-state ferromagnetism in sufficiently wide zigzag terminated ribbons. We extended these calculations to include the class of chiral graphene nanoribbons and investigate the influence of chirality and ribbon width on spin-spin correlations. The static magnetic correlations are found to rapidly increase with the width of the ribbons for all chiralities, such that already for ribbons of moderate widths we observe a strong trend towards mean-field-type ferromagnetic correlations along the edges. We extract dynamical edge state signatures which can be used to detect edge-state magnetism by scanning tunneling microscopy. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G8.00003: Strain induced magnetism in graphene Lucian Covaci, Francois Peeters Electron-electron interactions are believed not to be very important in graphene since the strength of the on-site Hubbard repulsion is moderate and the electron density of states is small near the Dirac points. Even so, graphene is believed to be in the proximity of a phase transition between a conducting state and an insulating one (antiferromagnetic or spin liquid). Finding a way to bring graphene across the transition is thus an important issue. We consider the effect of inhomogeneous strain from deformations induced by imperfections or steps in the substrate or from specific strain configurations that give almost constant pseudo-magnetic fields. We perform self-consistent mean field calculations for a tight-binding Hamiltonian where we consider only a repulsive on-site Hubbard term. We show that due to strain induced modifications of the kinetic energy, the staggered magnetization will become finite near regions where the strain is large. We also uncover that near deformations, spin-polarized states will appear, in a similar way the spin-polarized states appear at zig-zag edges of graphene nanoribbons. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G8.00004: Friction, Adhesion, and Elasticity of Grahene Edges D. Patrick Hunley, Tyler Flynn, Tom Dodson, Abhishek Sundararajan, Mathias Boland, Douglas Strachan Frictional, adhesive, and elastic characteristics of graphene edges are determined through lateral force microscopy. Measurements reveal a significant local frictional increase at exposed graphene edges, whereas a single overlapping layer of graphene removes this local frictional increase. Comparison of lateral force and atomic force microscopy measurements shows that local forces on the probe are successfully modeled with a vertical adhesion in the vicinity of the atomic-scale graphene steps. Lateral force microscopy performed with carefully maintained probes shows evidence of elastic straining of graphene edges which are consistent with out-of-plane bending of the edges. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G8.00005: Single point defect states in an armchair-graphene nanoribbon Chi-Hsaun Chiu, C.S. Chu We investigate in detail the electronic states induced by a single or a few defects in an armchair-graphene nanoribbon (AGNR). A semi-analytical approach is developed for the Lippmann-Schwinger formulation within the tight-binding model. The dependences of the local density of states (LDOS) in the vicinity of the defects on both the defect locations and the nanoribbon widths are explored. In particular, the LDOS characteristics in the gapped or gapless AGNR will be discussed. Our results are compared with exact diagonalization approach. The effects of these point defect states on the transport property of the AGNR will also be presented. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G8.00006: Ab initio electronic structure and transport studies of N$_2^{AA}$-doped armchair and zigzag graphene nanoribbons Jonathan Owens, Eduardo Cruz-Silva, Vincent Meunier Recent work by Lu, et al. (Nature Scientific Reports, DOI: 10.1038) on large sheets of nitrogen-doped graphene, determined that a highly predominant amount of nitrogen dopants (80 \%) form in pairs on the same sub-lattice. Graphene nanoribbons, which are essentially narrow strips of graphene, have a natural band gap and tunable electronic properties, making them a promising candidate for scalable nanoelectronics. In this work we explore various electronic structural (density of states, local density of states, and STM images) and transport properties of armchair (aGNR) and zigzag (zGNR) graphene nanoribbons under different orientations of the N$_2^{AA}$ dopants with respect to the ribbon growth direction. For all configurations of zGNRs and aGNRs, we see a substantial decrease in conductance due the dopants, as well as spatially localized states opening around the dopant sites. Most notably, however, we observe the emergence of a new stable spin configuration, wherein the spin-up spin-down polarizations of the edges in zGNRs (denoted the antiferromagnetic state) flip near the doping sites, while being in the normal zGNR AFM ground state away from the dopants. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G8.00007: Atypical structural, electronic, and thermoelectric properties of assembled graphene nanoribbons Liangbo Liang, Vincent Meunier, Eduardo Cruz-Silva, Eduardo Gir\~ao Highly ordered assembly of individual graphene nanoribbons (GNRs) into graphene nanowiggles (GNWs) has been recently demonstrated using a surface-assisted bottom-up chemical approach. GNWs are characterized by a periodic repetition of wiggle-like junctions where armchair- or zigzag-edged GNRs sectors alternate. We employed both density functional theory (DFT) and Tight-Binding+U to demonstrate their versatile electro-magnetic properties [Gir\~ao et al, Phys. Rev. Lett. 107 (2011) ]. The coexistence of parallel and oblique sectors leads GNWs to offer a broader set of geometrical parameters to fine tune the electronic band gap from 0.0 eV to 1.7 eV than GNRs [Gir\~ao et al, Phys. Rev. B 85 (2012) ]. Also, the presence of wiggle-like edges dramatically degrades thermal conductance but retains excellent electronic conduction, resulting in significant enhancement of the thermoelectric performance [Liang et al, Phys. Rev. B 86 (2012) ]. Finally, many-electron GW calculations show quasiparticle band gaps of GNWs generally more than twice of their DFT band gaps, reaching 3.7 eV. Furthermore, the gold substrate where GNWs are synthesized is found to lead to band gap reduction owing to substrate polarization effect, consistent with experiments [Liang et al, Phys. Rev. B 86 (2012)]. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G8.00008: Quasiparticle Band Gap modulation in Graphene Nanoribbons Supported on Weakly interacting Surfaces Xueping Jiang, Neerav Kharche, Paul Kohl, Timothy Boykin, Gerhard Klimeck, Mathieu Luisier, Pulickel Ajayan, Saroj Nayak Low dimensional nanostructures such as graphene nanoribbons(GNRs) and hexagonal boron nitride (hBN) have been successfully synthesized in experiments and attract a lot of attention recently. The strong electron-electron interactions due to quantum confinement could alter band gaps of nanostructures, which has been studied thoroughly for GNRs. Band gaps could also be changed by the effect of dielectric screening arising from the surrounding materials such as the substrate. However, this effect has not been thoroughly investigated for GNRs. In contrast, in almost all the experiments GNRs are deposited on different dielectric substrates leaving a gap between theoretical estimates and experimental measurements. The effect of dielectric screening cannot be captured in an effective single particle theory such as the density functional theory (DFT) and the many-body approaches such as \textit{GW} are required. We show the band gaps of the free standing GNRs are reduced as much as 1 eV in spite of weak van der Waals interactions between the GNR and the underlying substrate. This non-local effect can be explained by a semi-classical image charge model and such understanding is critical to the band gap engineering of graphene based devices. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G8.00009: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G8.00010: Snake states and Majorana's in graphene quantum dots in the presence of a p-n junction Francois Peeters, M. Zarenia, J.M. Pereira, Jr., G.A. Farias We investigate the magnetic interface states of graphene quantum dots that contain p-n junctions. Within a tight-binding approach, we consider rectangular quantum dots in the presence of a perpendicular magnetic field containing p-n, as well as p-n-p and n-p-n junctions. The results show the interplay between the edge states associated with the zigzag terminations of the sample and the snake states that arise at the p-n junction, due to the overlap between electron and hole states at the potential interface. Remarkable localized states are found at the crossing of the p-n junction with the zigzag edge having a dumb-bell shaped electron distribution. These states are localized Majorana states. The results are presented as function of the junction parameters and the applied magnetic flux. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G8.00011: Evidence for edge state photoluminescence in graphene quantum dots Kiran Lingam, Ramakrishna Podila, Haijun Qian, Steve Serkiz, Apparao M. Rao For a practical realization of graphene-based logic devices, opening of a band gap in graphene is crucial and has proved challenging. To this end, several synthesis techniques including unzipping of carbon nanotubes, chemical vapor deposition and other bottom-up fabrication techniques have been pursued for the bulk production of graphene nanoribbons (GNRs) and graphene quantum dots (GQDs). However, only a limited progress has been made towards a fundamental understanding of the electronic and optical properties of GQDs. In particular, the origin of strong photoluminescence (PL) in GQDs, which has been attributed to the presence of emissive surface traps and/or the edge states in GQD, remains inconclusive to date. Here, we experimentally show that the PL is independent of the functional groups attached to the GQDs. Following a series of annealing experiments, we further show that the PL in GQDs originates from the edge states, and an edge-passivation subsequent to synthesis quenches PL. These results are consistent with comparative studies on other carbon nanostructures such as GNRs and carbon nano-onions. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G8.00012: Spontaneous Gap Formation in an Uniaxially Strained Graphene Anand Sharma, Valeri N. Kotov, Antonio H. Castro Neto We study the condition of spontaneous gap generation due to Coulomb interaction between anisotropic Dirac fermions in an uniaxially strained graphene. The gap equation is realized as a self-consistent solution for the self- energy i.e., Dyson- Schwinger equation, within static Random Phase Approximation. The mass gap not only depends on the momentum due to long- range nature of the interaction but also on the anisotropy due to uniaxial strain. Using standard numerical analysis we solve the integral equation on a finite grid. We evaluate the mass gap as a function of dimensionless coupling constant for different values of anisotropy parameter and obtain the critical coupling at which the gap becomes non-zero. Our study indicates that with an increase in uniaxial strain in graphene, the critical coupling decreases which is in agreement with our perturbative renormalization group analysis. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G8.00013: Band Gap Opening in Periodically Modified Graphene Marc Dvorak, Zhigang Wu The gapless electronic structure of graphene must be modified to allow a meaningful on-and-off ratio for use in field-effect transistors. Many attempts to create semiconducting graphene have been made; among them, application of periodic structural modifications, such as patterned defects or nanoscale perforation creating a graphene nanomesh, is particularly promising. Extensive theoretical efforts have been spent to investigate such graphene structures, but the precise role of periodic perturbation on band gap opening remains unclear. Here, we show analytically that band gap opening in graphene under a periodic perturbation can be accurately predicted by mapping the perturbative reciprocal lattice vectors onto Dirac points. The modified graphene alternates between a semi-metal and a semiconductor with 8/9 gapless and 1/9 semiconducting. Furthermore, semiconducting modified graphene can be mapped to exactly two corresponding semimetallic carbon nanotubes or graphene nanoribbons. These predictions reveal the fundamental physics of band gap opening in periodically defected graphene and are in excellent agreement with previous and present first-principles results for graphene nanomeshes. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G8.00014: Gauge fields for rippled graphene membranes under central load Salvador Barraza-Lopez, James V. Sloan, Alejandro A. Pacheco, Cedric M. Horvath, Zheng Fei Wang Gauge fields on graphene are invariably expressed in the language of continuum elasticity. Following an approach where the atomic positions play the preponderant role, a model of strain on graphene was developed where all relevant quantities -including gauge fields- are directly expressed in terms of atomic displacements only. Suspended, rippled graphene membranes under cetral load by a sharp object were studied using this approach. The effects from both the pseudo-magnetic field and the deformation potential were included in calculations of the electron density at different spatial locations (the deformation potential acts as an on-site potential energy). The deformation potential -neglected without proper justification in many published works- appears to modify the electronic spectrum dramatically in a qualitative way. Discussion of experiments relevant to the model will also be given. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G8.00015: Theory of electromechanical coupling in dynamical graphene Mircea Trif, Pramey Upadhyaya, Yaroslav Tserkovnyak We study the coupling between mechanical motion and Dirac electrons in a dynamical sheet of graphene. We show that this coupling can be understood in terms of an effective gauge field acting on the electrons, which has two contributions: {\it quasistatic} and purely {\it dynamic} of the Berry-phase origin. As is well known, the static gauge potential is odd in the $K$ and $K'$ valley index, while we find the dynamic coupling to be even. In particular, the mechanical fluctuations can thus mediate an indirect coupling between charge and valley degrees of freedom. [Preview Abstract] |
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