CiteULike: kas's graphene

Resonance energy transfer from a dye molecule to graphene

RS Swathi — 2008-08-19T19:54:41-00:00

The Journal of Chemical Physics, Vol. 129, No. 5. (2008)

We study the distance dependence of the rate of resonance energy transfer from the excited state of a dye to the system of graphene. Using the tight-binding model for the system and the Dirac cone approximation, we obtain the analytic expression for the rate of energy transfer from an electronically excited dye to graphene. While in traditional fluorescence resonance energy transfer, the rate has a (distance)−6 dependence, we find that the distance dependence in this case is quite different. Our calculation of rate in the case of the two dyes, pyrene and nile blue, shows that the distance dependence is Yukawa type. We have also studied the effect of doping on energy transfer to graphene. Doping does not modify the rate for electronic excitation energy transfer significantly. However, in the case of vibrational transfer, the rate is found to be increased by an order of magnitude due to doping. This can be attributed to the nonzero density of states at the Fermi level that results from doping. ©2008 American Institute of Physics

Periodized discrete elasticity models for defects in graphene

A Carpio — 2008-08-19T19:54:10-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 78, No. 8. (2008)

The cores of edge dislocations, edge dislocation dipoles, and edge dislocation loops in planar graphene have been studied by means of periodized discrete elasticity models. To build these models, we have found a way to discretize linear elasticity on a planar hexagonal lattice using combinations of difference operators that do not symmetrically involve all the neighbors of an atom. At zero temperature, dynamically stable cores of edge dislocations may be heptagon-pentagon pairs (glide dislocations) or octagons (shuffle dislocations) depending on the choice of initial configuration. Possible cores of edge dislocation dipoles are vacancies, pentagon-octagon-pentagon divacancies, Stone-Wales defects, and 7–5-5–7 defects. While symmetric vacancies, divacancies, and 7–5-5–7 defects are dynamically stable, asymmetric vacancies and 5–7-7–5 Stone-Wales defects seem to be unstable. ©2008 The American Physical Society

Zr-metal adhesion on graphenic nanostructures

Sanchez Paisal — 2008-08-19T19:53:12-00:00

Applied Physics Letters, Vol. 93, No. 5. (2008)

Our high resolution transmission electronic microscopy studies of multiwall carbon nanotubes show, after the growth of zirconia nanoparticles by a hydrothermal route, the presence of surface Zr, forming an atomically thin layer. Using first-principles calculations we investigate the nature of the Zr–C interaction, which is neither ionic nor covalent, and the optimal coverage for the Zr metal in a graphene flake. This preferred coverage is in agreement with that deduced from electron energy loss spectra experiments. We show also that the amount of charge transferred to the C layer saturates as the Zr coverage increases and the Zr–C bond becomes weaker. ©2008 American Institute of Physics

Chirality-Induced Dynamic Kohn Anomalies in Graphene

Wang Tse — 2008-08-19T19:51:50-00:00

Physical Review Letters, Vol. 101, No. 6. (2008)

We develop a theory for the renormalization of the phonon energy dispersion in graphene due to the combined effects of both Coulomb and electron-phonon (e-ph) interactions. We obtain the renormalized phonon energy spectrum by an exact analytic derivation of the phonon self-energy, finding three distinct Kohn anomalies (KAs) at the phonon wave vector q=/v, 2kF±/v for LO phonons and one at q=/v for TO phonons. The presence of these new KAs in graphene, in contrast to the usual KA q=2kF in ordinary metals, originates from the dynamical screening of e-ph interaction (with a concomitant breakdown of the Born-Oppenheimer approximation) and the peculiar chirality of the graphene e-ph coupling. ©2008 The American Physical Society

Electron screening and excitonic condensation in double-layer graphene systems

Maxim — 2008-08-18T02:02:43-00:00

(15 Aug 2008)

We theoretically investigate the possibility of excitonic condensation in a system of two graphene monolayers separated by an insulator, in which electrons and holes in the layers are induced by external gates. In contrast to the recent studies of this system, we take into account the screening of the interlayer Coulomb interaction by the carriers in the layers, and this drastically changes the result. Due to a large number of electron species in the system (two projections of spin, two valleys, and two layers) and to the suppression of backscattering in graphene, the maximum possible strength of the screened Coulomb interaction appears to be quite small making the weak-coupling treatment applicable. We calculate the mean-field transition temperature for a clean system and demonstrate that its highest possible value $T_c^\textmax∼ 10^-7ε_F≤sssim 1 \textmK$ is extremely small ($ε_F$ is the Fermi energy). In addition, any sufficiently short-range disorder with the scattering time $τ ≤sssim \hbar /T_c^\textmax$ would suppress the condensate completely. Our findings renders experimental observation of excitonic condensation in the above setup improbable even at very low temperatures.

Graphene Josephson Qubit

Colin Benjamin — 2008-08-15T06:06:00-00:00

(14 Aug 2008)

We propose to combine the advantages of graphene, such as easy tunability and long coherence times, with Josephson physics to manufacture qubits. These qubits are built around a 0 and $π$ junction and are controlled by external flux. We show that ferromagnets are not required for realizing $π$ junction in graphene, thus considerably simplifying its physical implementation. We demonstrate that one qubit gates, such as arbitrary phase rotations and the exchange gate, $σ_x$, can be implemented in much shorter times than the decoherence time of the system. This novel proposal for a graphene qubit obviates the control deficiencies of normal Josephson qubits, while adding the versatility of graphene.

Observation of half-integer quantum Hall effect in single-layer graphene using a pulsed magnet

Satoru Masubuchi — 2008-08-14T12:29:48-00:00

(13 Aug 2008)

We report magnetotransport measurements on a single-layer graphene in pulsed magnetic fields up to $B$ = 53 T. With either electron- or hole-type charge carriers, the Hall resistance $R_H$ is quantized into $R_H$ = $(h/e^2)ν ^-1$ with $ν$ = $±$2, $±$6, and $±$10, which demonstrates the observation of half-integer quantum Hall effect (QHE). At $B$ = 50 T, the half-integer QHE is even observed at room temperature in spite of a conventional carrier mobility $μ$ = 4000 cm$^2$/Vs.

Electrical Control of Exchange Bias Mediated by Graphene

YG Semenov — 2008-08-13T08:45:11-00:00

(12 Aug 2008)

The role of graphene in mediating the exchange interaction is theoretically investigated when it is placed between two ferromagnetic dielectric materials. The calculation based on a tight-binding model illustrates that the magnetic interactions at the interfaces affect not only the graphene band structure but also the thermodynamic potential of the system. This induces an indirect exchange interaction between the magnetic layers that can be considered in term of an effective exchange bias field. The analysis clearly indicates a strong dependence of the effective exchange bias on the properties of the mediating layer, revealing an effective mechanism of electrical control even at room temperature. This dependence also results in qualitatively different characteristics for the cases involving mono- and bilayer graphene.

Vacuum polarization in graphene with a topological defect

Yu — 2008-08-13T08:44:31-00:00

(12 Aug 2008)

The influence of a topological defect in graphene on the ground state of electronic quasiparticle excitations is studied in the framework of the long-wavelength continuum model originating in the tight-binding approximation for the nearest neighbour interaction in the graphitic lattice. A topological defect that rolls up a graphitic sheet into a nanocone is represented by a pointlike pseudomagnetic vortex with a flux which is related to the deficit angle of the cone. The method of self-adjoint extensions is employed to define the set of physically acceptable boundary conditions at the apex of the nanocone. The electronic system on a graphitic nanocone is found to acquire the ground state condensate and current of special type, and we determine the dependence of these quantities on the deficit angle of the nanocone, continuous parameter of the boundary condition at the apex, and the distance from the apex.

Modeling vacancies and hydrogen impurities in graphene: A molecular point of view

G Forte — 2008-08-12T13:57:31-00:00

(5 Aug 2008)

We have followed a "molecular" approach to study impurity effects in graphene. This is thought as the limiting case of an infinitely large cluster of benzene rings. Therefore, we study several carbon clusters, with increasing size, from phenalene, including three benzene rings, up to coronene 61, with 61 benzene rings. The impurities considered were a chemisorbed H atom, a vacancy, and a substitutional proton. We performed HF and UHF calculations using the STO-3G basis set. With increasing cluster size in the absence of impurities, we find a decreasing energy gap, here defined as the HOMO-LUMO difference. In the case of H chemisorption or a vacancy, the gap does not decrease appreciably, whereas it is substantially reduced in the case of a substitutional proton. The presence of an impurity invariably induces an increase of the density of states near the HOMO level. We find a zero mode only in the case of a substitutional proton. In agreement with experiments, we find that both the chemisorbed H, the substitutional proton, and the C atom near a vacancy acquire a magnetic moment. The relevance of graphene clusters for the design of novel electronic devices is also discussed.

Highly Conducting Graphene Sheets and Langmuir-Blodgett Films

Xiaolin Li — 2008-08-12T09:51:09-00:00

(4 Aug 2008)

Graphene is an intriguing material with properties that are distinct from those of other graphitic systems. The first samples of pristine graphene were obtained by peeling off and epitaxial growth. Recently, the chemical reduction of graphite oxide was used to produce covalently functionalized single-layer graphene oxide. However, chemical approaches for the large-scale production of highly conducting graphene sheets remain elusive. Here, we report that the exfoliation-reintercalation-expansion of graphite can produce high-quality single-layer graphene sheets stably suspended in organic solvents. The graphene sheets exhibit high electrical conductance at room and cryogenic temperatures. Large amounts of graphene sheets in organic solvents are made into large transparent conducting films by Langmuir-Blodgett assembly in a layer-by-layer manner. The chemically derived high quality graphene sheets could lead to future scalable graphene devices.

Atmospheric pressure graphitization of SiC(0001)- A route towards wafer-size graphene layers

Konstantin Emtsev — 2008-08-12T09:50:11-00:00

(8 Aug 2008)

We have investigated epitaxial graphene films grown on SiC(0001) by annealing in an atmosphere of Ar instead of vacuum. Using AFM and LEEM we observe a significantly improved surface morphology and graphene domain size. Hall measurements on monolayer graphene films show a carrier mobility of around 1000 cm^2/Vs at room temperature and 2000 cm^2/Vs at 27K. The growth process introduced here establishes the synthesis of graphene films on a technologically viable basis.

Polarization Charge Distribution in Gapped Graphene

Valeri Kotov — 2008-06-10T00:33:58-00:00

(7 Jun 2008)

We study the distribution of vacuum polarization charge, induced by a Coulomb impurity in massive graphene. By analytically computing the polarization function, we show that the charge density is distributed in space in a non-trivial fashion, and on a characteristic length-scale set by the effective Compton wavelength. The density crosses over from a logarithmic behavior below this scale, to a power law variation above it. Our results in the continuum limit are confirmed by explicit diagonalization of the corresponding tight-binding model on a finite-size lattice.

Divergent resistance at the Dirac point in graphene: evidence for a transition in high magnetic field

Joseph Checkelsky — 2008-08-07T15:42:36-00:00

(6 Aug 2008)

We have greatly extended measurements of the Dirac-point resistance $R_0$ in graphene, using an ultralow-dissipation ($<$3 fW) technique that avoids self-heating problems. At 0.3 K, $R_0(H)$ is observed to increase a 1000-fold to 40 M$Ω$ in a relatively narrow span of field $B$. The abruptness of the increase implies that a transition occurs to an insulating, ordered state at large $B$. Remarkably, $R_0$ accurately fits a Kosterlitz-Thouless-type correlation length over 3 decades. The results imply a field-induced phase transition to a high-field ordered state that is insulating. The effect of tuning the chemical potential away from the Dirac point is also investigated.

f-sum rule and unconventional spectral weight transfer in graphene

J Sabio — 2008-08-12T07:31:38-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 78, No. 7. (2008)

We derive and analyze the f-sum rule for a two-dimensional system of interacting electrons whose behavior is described by the Dirac equation. We apply the sum rule to analyze the spectral weight transfer in graphene within different approximations discussed in the literature. We find that the sum rule is generically dominated by interband transitions while other excitations produce subleading behavior. The f-sum rule provides strong constraints for theories of interacting electrons in graphene.

The interface structure of epitaxial graphene grown on 4H-SiC(0001)

J Hass — 2008-08-12T07:31:01-00:00

(10 Aug 2008)

We present a structural analysis of the graphene-4HSiC(0001) interface using surface x-ray reflectivity. We find that the interface is composed of an extended reconstruction of two SiC bilayers. The interface directly below the first graphene sheet is an extended layer that is more than twice the thickness of a bulk SiC bilayer (~1.7A compared to 0.63A). The distance from this interface layer to the first graphene sheet is much smaller than the graphite interlayer spacing but larger than the same distance measured for graphene grown on the (000-1) surface, as predicted previously by ab intio calculations.

Persistence of zero modes in a gauged Dirac model for bilayer graphene

R Jackiw — 2008-08-12T07:29:34-00:00

(11 Aug 2008)

A recently constructed model for low lying excitations in bilayer graphene exhibits mid-gap, zero energy modes in its Dirac-like spectrum, when a scalar order parameter takes a vortex profile. We show that these modes persist when the dynamics is extended by a gauge field interaction, which also renders finite the vortex energy. The effect of the gauge field on the zero energy wave function is to shift the phase of the (damped) oscillatory component of the wave function in the absence of the gauge field.

Tight--binding description of the quasiparticle dispersion of graphite and few--layer graphene

A Gruneis — 2008-08-12T07:29:01-00:00

(11 Aug 2008)

A universal set of third--nearest neighbour tight--binding (TB) parameters is presented for calculation of the quasiparticle (QP) dispersion of $N$ stacked $sp^2$ graphene layers ($N=1... ∞$) with $AB$ stacking sequence. The QP bands are strongly renormalized by electron--electron interactions which results in a 20% increase of the nearest neighbour in--plane and out--of--plane TB parameters when compared to band structure from density functional theory. With the new set of TB parameters we determine the Fermi surface and evaluate exciton energies, charge carrier plasmon frequencies and the conductivities which are relevant for recent angle--resolved photoemission, optical, electron energy loss and transport measurements. A comparision of these quantitities to experiments yields an excellent agreement. Furthermore we discuss the transition from few layer graphene to graphite and a semimetal to metal transition in a TB framework.

Monitoring of band gap and magnetic state of graphene nanoribbons through vacancies

M Topsakal — 2008-08-12T07:28:12-00:00

(11 Aug 2008)

Using first-principles plane wave calculations we predict that electronic and magnetic properties of graphene nanoribbons can be affected by defect-induced itinerant states. The band gaps of armchair nanoribbons can be modified by hydrogen saturated holes. Defects due to periodically repeating vacancy or divacancies induce metallization, as well as magnetization in non-magnetic semiconducting nanoribbons due to the spin-polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to edges. It is shown that these effects can, in fact, be realized without really creating defects.

Non-linear graphene optics for terahertz applications

SA Mikhailov — 2008-08-12T07:27:27-00:00

(11 Aug 2008)

The linear electrodynamic properties of graphene -- the frequency-dependent conductivity, the transmission spectra and collective excitations -- are briefly outlined. The non-linear frequency multiplication effects in graphene are studied, taking into account the influence of the self-consistent-field effects and of the magnetic field. The predicted phenomena can be used for creation of new devices for microwave and terahertz optics and electronics.

Understanding adsorption of hydrogen atoms on graphene

Simone Casolo — 2008-08-12T07:26:53-00:00

(8 Aug 2008)

Adsorption of hydrogen atoms on a single graphite sheet (graphene) has been investigated by first-principles electronic structure means, employing plane-wave based, periodic density functional theory. A reasonably large 5x5 surface unit cell has been employed to study single and multiple adsorption of H atoms. Binding and barrier energies for sequential sticking have been computed for a number of configurations involving adsorption on top of carbon atoms. We find that binding energies per atom range from ~0.8 eV to ~1.9 eV, with barriers to sticking in the range 0.0-0.2 eV. In addition, depending on the number and location of adsorbed hydrogen atoms, we find that magnetic structures may form in which spin density localizes on a $\sqrt3x\sqrt3R30^\circ$ sublattice, and that binding (barrier) energies for sequential adsorption increase (decrease) linearly with the site-integrated magnetization. These results can be rationalized with the help of the valence-bond resonance theory of planar $π$ conjugated systems, and suggest that preferential sticking due to barrierless adsorption is limited to formation of hydrogen pairs.

Influence of Disorder on Electron-Hole Pair Condensation in Graphene Bilayers

R Bistritzer — 2008-08-12T07:26:13-00:00

(8 Aug 2008)

Graphene bilayers can condense into a state with spontaneous interlayer phase coherence that supports dissipationless counterflow supercurrents. Here we address the influence of disorder on the graphene bilayer mean-field and Kosterlitz-Thouless critical temperatures and report on a simple criteria for the survival of pair condensation.

Strong terahertz conductance of graphene nanoribbons under a magnetic field

Junfeng Liu — 2008-08-09T14:15:47-00:00

Applied Physics Letters, Vol. 93, No. 4. (2008)

We demonstrate that the optical response of graphene nanoribbons in the terahertz to far-infrared regime can be significantly enhanced and tuned by an applied magnetic field. The dependence of the threshold frequency on the magnetic field is studied. The ribbons with the strongest terahertz conductance under a magnetic field are those with one-dimensional massless Dirac Fermion energy dispersion. For a given ribbon, there exists an optimal field under which the conductance resonance can occur at the lowest frequency. ©2008 American Institute of Physics

High-capacity hydrogen storage by metallized graphene

C Ataca — 2008-08-09T14:14:53-00:00

Applied Physics Letters, Vol. 93, No. 4. (2008)

First-principles plane wave calculations predict that Li can be adsorbed on graphene forming a uniform and stable coverage on both sides. A significant part of the electronic charge of the Li 2s orbital is donated to graphene and is accommodated by its distorted *-bands. As a result, semimetallic graphene and semiconducting graphene ribbons change into good metals. It is even more remarkable that Li covered graphene can serve as a high-capacity hydrogen storage medium with each adsorbed Li absorbing up to four H2 molecules amounting to a gravimetric density of 12.8 wt %. ©2008 American Institute of Physics

Symmetry Classes in Graphene Quantum Dots: Universal Spectral Statistics, Weak Localization, and Conductance Fluctuations

J Wurm — 2008-08-08T03:39:17-00:00

(7 Aug 2008)

We study the symmetry classes of graphene quantum dots, both open and closed, through the conductance and energy level statistics. For abrupt termination of the lattice, these properties are well described by the standard orthogonal and unitary ensembles. However, for smooth mass confinement, special time-reversal symmetries associated with the sublattice and valley degrees of freedom are critical: they lead to block diagonal Hamiltonians and scattering matrices with blocks belonging to the unitary symmetry class even at zero magnetic field. While the effect of this structure is clearly seen in the conductance of open dots, it is suppressed in the spectral statistics of closed dots, because the intervalley scattering time is shorter than the time required to resolve a level spacing in the closed systems but longer than the escape time of the open systems.

Suppressed conductance in a metallic graphene nano-junction

Haidong Li — 2008-08-09T14:11:14-00:00

(7 Aug 2008)

The linear conductance spectrum of a metallic graphene junction formed by interconnecting two gapless graphene nanoribbons is calculated. A strong conductance suppression appears in the vicinity of the Dirac point. We found that such a conductance suppression arises from the antiresonance effect associated with the edge state localized at the zigzag-edged shoulder of the junction. The conductance valley due to the antiresonance is rather robust in the presence of the impurity and vacancy scattering. And the center of the conductance valley can be readily tuned by an electric field exerted on the wider nanoribbon.

Dynamical polarization, screening, and plasmons in gapped graphene

PK Pyatkovskiy — 2008-08-09T14:10:50-00:00

(6 Aug 2008)

The one-loop polarization function of graphene has been calculated at zero temperature for arbitrary wave vector, frequency, chemical potential (doping) and band gap. The result is expressed in terms of elementary functions and is used to find the dispersion of the plasmon mode and the static screening within the random phase approximation. At long wavelengths the usual for two-dimensional systems square root behavior of plasmon spectrum is obtained. The presence of a small (compared to a chemical potential) gap leads to appearance of a new undamped plasmon mode. At greater values of the gap this mode merges with the long-wavelength one, and vanishes when the Fermi level enters the gap. The screening of charged impurity at large distances differs from that in gapless graphene by slower decay of Friedel oscillations ($1/r^2$ instead of $1/r^3$), similarly to conventional 2D systems.

Optimization of metal dispersion in doped graphitic materials for hydrogen storage

Gyubong Kim — 2008-08-09T14:10:20-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 78, No. 8. (2008)

The noncovalent hydrogen binding on transition-metal atoms dispersed on carbon clusters and graphene is studied with the use of the pseudopotential density-functional method. It is found that the presence of acceptorlike states in the absorbents is essential for enhancing the metal adsorption strength and for increasing the number of hydrogen molecules attached to the metal atoms. Particular configurations of boron substitutional doping are found to be very efficient for providing such states and thus enhancing storage capacity. Optimal doping conditions are suggested based on our calculations for the binding energy and ratio between metal and hydrogen molecules.

Klein Backscattering and Fabri-Perot Interference in Graphene Heterojunctions

AV Shytov — 2008-08-05T21:33:19-00:00

(4 Aug 2008)

We present a theory of quantum-coherent transport through a lateral p-n-p structure in graphene, which fully accounts for the interference of forward and backward scattering on the p-n interfaces. The backreflection amplitude changes sign at zero incidence angle because of the Klein phenomenon, adding a phase $π$ to the interference fringes. The contributions of the two p-n interfaces to the phase of the interference cancel with each other at zero magnetic field, but become imbalanced at a finite field. The resulting half a period shift in the Fabri-Perot fringe pattern, induced by a relatively weak magnetic field, can provide a clear signature of Klein scattering in graphene. This effect is shown to be robust in the presence of spatially inhomogeneous potential of moderate strength.

Controlling graphene corrugation on lattice-mismatched substrates

AB Preobrajenski — 2008-08-07T14:21:27-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 78, No. 7. (2008)

By means of synchrotron-radiation-based core-level spectroscopies we demonstrate that the degree of corrugation in graphene nanomesh on lattice-mismatched transition-metal substrates critically depends on the strength of chemical bonding at the interface. The degree of interfacial orbital hybridization between graphene and metal states is rising in the series Pt(111)-Ir(111)-Rh(111)-Ru(0001). This growing strength of hybridization is accompanied by a gradual change in graphene morphology from nearly flat to strongly corrugated. We provide a comparison of the pore size and period for the cases of graphene and h-BN nanomesh on Rh(111).

Supercritical Coulomb impurities in gapped graphene

Vitor Pereira — 2008-08-07T14:20:47-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 78, No. 8. (2008)

We study the problem of Coulomb field-induced charging of the ground state in a system of two-dimensional (2D) massive Dirac particles—gapped graphene. As in its 3D QED counterpart, the critical Coulomb coupling is renormalized to higher values compared to the massless case. We find that in gapped graphene, a different supercritical regime is possible, where the screening charge is comparable to the impurity charge; thus, leading to suppression of the Coulomb field at nanometer scales. We corroborate this with a numerical solution of the tight-binding problem on the honeycomb lattice.

Magnetoplasmons in layered graphene structures

Oleg Berman — 2008-08-07T14:11:52-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 78, No. 8. (2008)

We calculate the dispersion equations for magnetoplasmons in a single layer, a pair of parallel layers, a graphite bilayer, and a superlattice of graphene layers in a perpendicular magnetic field. We demonstrate the feasibility of a drift-induced instability of magnetoplasmons. The magnetoplasmon instability in a superlattice is enhanced compared to a single graphene layer. The energies of the unstable magnetoplasmons could be in the terahertz (THz) part of the electromagnetic spectrum. The enhanced instability makes superlattice graphene a potential source of THz radiation.

Experimental and theoretical study of the morphology of commensurate and incommensurate graphene layers on Ni single-crystal surfaces

D Usachov — 2008-08-07T14:10:58-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 78, No. 8. (2008)

The paper reports on a scanning tunneling microscopy (STM) study and computer simulation with an N-body interatomic interaction potential of a graphite monolayer (i.e., graphene) on the Ni(111), (110), (755), and (771) single-crystal surfaces. Unlike the case of graphene on Ni(111), which forms a solid single-crystal coating with (1×1) structure, graphene on the Ni(110) surface forms a complex crystal structure distorted substantially by interaction with the substrate. Calculations of the graphene/Ni(110) system have revealed that the strong chemical interaction of carbon with nickel gives rise to a noticeable curving of the graphene layer on a scale of a few angstroms. The model thus derived has permitted proper interpretation of the experimental data obtained by STM, as well as prediction of the main result of studies of graphene formed on faceted surfaces, which have revealed the ability of graphene to coat geometrically nonuniform surfaces in the form of a curved continuous film.

Vortices, zero modes and fractionalization in bilayer-graphene exciton condensate

B Seradjeh — 2008-08-07T14:10:22-00:00

(1 Aug 2008)

A real-space formulation is given for the recently discussed exciton condensate in a symmetrically biased graphene bilayer. We show that in the continuum limit an oddly-quantized vortex in this condensate binds exactly one zero mode per valley index of the bilayer. In the full lattice model the zero modes are split slightly due to intervalley mixing. We support these results by an exact numerical diagonalization of the lattice Hamiltonian. We also discuss the effect of the zero modes on the charge content of these vortices and deduce some of their interesting properties.

Quantum Interference at the Twist Boundary in Graphene

S Shallcross — 2008-08-07T14:09:04-00:00

Physical Review Letters, Vol. 101, No. 5. (2008)

We explore the consequences of a rotation between graphene layers for the electronic spectrum. We derive the commensuration condition in real space and show that the interlayer electronic coupling is governed by an equivalent commensuration in reciprocal space. The larger the commensuration cell, the weaker the interlayer coupling, with exact decoupling for incommensurate rotations and in the 0 limit. Furthermore, from first-principles calculations we determine that even for the smallest possible commensuration cell the decoupling is effectively perfect, and thus graphene layers will be seen to decouple for all rotation angles.

Pomeranchuk instability in doped graphene

Belen Valenzuela — 2008-08-07T14:07:38-00:00

(31 Jul 2008)

The density of states of graphene has Van Hove singularities that can be reached by chemical doping and have already been explored in photoemission experiments. We show that in the presence of Coulomb interactions the system at the Van Hove filling is likely to undergo a Pomeranchuk instability breaking the lattice point group symmetry. In the presence of an on--site Hubbard interaction the system is also unstable towards ferromagnetism. We explore the competition of the two instabilities and build the phase diagram. We also suggest that, for doping levels where the trigonal warping is noticeable, the Fermi liquid state in graphene can be stable up to zero temperature avoiding the Kohn--Luttinger mechanism and providing an example of two dimensional Fermi liquid at zero temperature.

Photovoltaic Hall effect in graphene

Takashi Oka — 2008-07-31T14:36:57-00:00

(30 Jul 2008)

Response of electronic systems in intense lights (AC electric fields) to DC source-drain fields is formulated with the Floquet method. We have then applied the formalism to graphene, for which we show that a non-linear effect of a circularly polarized light can open a gap in the Dirac cone, which leads to a photo-induced dc Hall current. This is numerically confirmed for a graphene ribbon attached to electrodes with the Keldysh Green's function.

Dirac equation in (1+2) dimensions for quasi-particles in graphene and quantum field theory of their Coulomb interaction

Riazuddin — 2008-07-31T14:36:14-00:00

(30 Jul 2008)

There is evidence for existance of massless Dirac quasi-particles in graphene, which satisfy Dirac equation in (1+2) dimensions near the so called Dirac points which lie at the corners at the graphene's brilluoin zone. Certain subtle points which are peculiar to odd number of space-time dimensions (in this case three), in the derivation of such an equation are clarified. It is shown that parity operator in (1+2) dimensions play an intersting role and can be used for defining conserved chiral currents [there is no $γ ^5$ in (1+2) dimensions] resulting from the underlying Lagrangian for massless Dirac quasi-particles in graphene which is shown to have chiral $U_L(2)× U_R(2)$ symmetry. Further the quantum field theory (QFT) of Coulomb interaction of 2D graphene is developed and applied to vacuum polarization and electron self energy and the renormalization of the effective coupling of this interaction and Fermi velocity.

Rabi Oscillations in Landau Quantized Graphene

B Dóra — 2008-07-31T14:35:31-00:00

(30 Jul 2008)

The canonical model of quantum optics, the Jaynes-Cummings Hamiltonian describes a two level atom in a cavity interacting with electromagnetic field. Graphene, a condensed matter system, possesses low energy excitations obeying to the Dirac equation, and mimics the physics of quantum electrodynamics. These two seemingly unrelated fields turn out to be closely related to each other. We demonstrate that Rabi oscillations, corresponding to the excitations of the atom in the former case are observable in the optical response of the latter in quantizing magnetic field, providing us with a transparent picture about the structure of optical transitions in graphene. While the longitudinal conductivity reveals chaotic Rabi oscillations, the Hall component measures coherent ones. This opens up the exciting possibility of investigating a microscopic model of a few quantum objects in a macroscopic experiment of a bulk material with tunable parameters.

Metal to insulator transition in epitaxial graphene induced by molecular doping

SY Zhou — 2008-07-31T14:34:52-00:00

(30 Jul 2008)

The capability to control the type and amount of charge carriers in a material and, in the extreme case, the transition from metal to insulator is one of the key challenges of modern electronics. By employing angle resolved photoemission spectroscopy (ARPES) we find that a reversible metal to insulator transition and a fine tuning of the charge carriers from electrons to holes can be achieved in epitaxial bilayer and single layer graphene by molecular doping. The effects of electron screening and disorder are also discussed. These results demonstrate that epitaxial graphene is suitable for electronics applications, as well as provide new opportunities for studying the hole doping regime of the Dirac cone in graphene.

Depositing graphene films on solid and perforated substrates

A Banerjee — 2008-07-31T14:33:27-00:00

Nanotechnology, Vol. 19, No. 36. (2008), 365303.

Graphene--a monolayer of graphite--has attracted vast interest recently owing to its perfect two-dimensional crystallographic nature and its potential use in a new generation of microelectronic devices. Yet, a deposition method, which results in a large coverage of monolayer thick graphite, is still lacking. By using a chemical mechanical polishing (CMP) method we were able to deposit stress-free graphene on solid and perforated substrates alike, achieving area coverage of hundreds of microns squared.

Large Reversible Li Storage of Graphene Nanosheet Families for Use in Rechargeable Lithium Ion Batteries

Eunjoo Yoo — 2008-07-31T14:32:39-00:00

Nano Lett. (24 July 2008)

Abstract: The lithium storage properties of graphene nanosheet (GNS) materials as high capacity anode materials for rechargeable lithium secondary batteries (LIB) were investigated. Graphite is a practical anode material used for LIB, because of its capability for reversible lithium ion intercalation in the layered crystals, and the structural similarities of GNS to graphite may provide another type of intercalation anode compound. While the accommodation of lithium in these layered compounds is influenced by the layer spacing between the graphene nanosheets, control of the intergraphene sheet distance through interacting molecules such as carbon nanotubes (CNT) or fullerenes (C60) might be crucial for enhancement of the storage capacity. The specific capacity of GNS was found to be 540 mAh/g, which is much larger than that of graphite, and this was increased up to 730 mAh/g and 784 mAh/g, respectively, by the incorporation of macromolecules of CNT and C60 to the GNS.

Collective properties of magnetobiexcitons in quantum wells and graphene superlattices

Oleg Berman — 2008-07-31T14:31:38-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 78, No. 3. (2008)

The Bose-Einstein condensation and superfluidity of quasi-two-dimensional spatially indirect magnetobiexcitons in a slab of superlattice with alternating electron and hole layers consisting from the semiconducting quantum wells (QWs) and graphene superlattice in high magnetic field are considered. The two different Hamiltonians of a dilute gas of magnetoexcitons with a dipole-dipole repulsion in superlattices, consisting of both QWs and graphene layers (GLs) in the limit of high magnetic field, have been reduced to one effective Hamiltonian—a dilute gas of two-dimensional excitons with the renormalized effective mass of the magnetoexciton, which depends on the magnetic field. This Hamiltonian does not include the vector potential. Moreover, for N excitons we have reduced the problem of 2N×2-dimensional space onto the problem of N×2-dimensional space by integrating over the coordinates of the relative motion of an electron and a hole. The instability of the ground state of the system of interacting two-dimensional indirect magnetoexcitons in a slab of superlattice with alternating electron and hole layers in high magnetic field is found. The stable system of indirect quasi-two-dimensional magnetobiexcitons, consisting of a pair of indirect excitons with opposite dipole moments, is considered. The density of the superfluid component ns(T) and the temperature of the Kosterlitz-Thouless phase transition to the superfluid state in the system of two-dimensional indirect magnetobiexcitons, interacting as electrical quadrupoles, are obtained for both the QW and graphene realizations.

Raman spectra of epitaxial graphene on SiC and of epitaxial graphene transferred to SiO2

Dong Lee — 2008-07-30T20:21:24-00:00

(25 Jul 2008)

Raman spectra were measured for mono-, bi- and trilayer graphene grown on SiC by solid state graphitization, whereby the number of layers was pre-assigned by angle-resolved ultraviolet photoemission spectroscopy. It was found that the only unambiguous fingerprint in Raman spectroscopy to identify the number of layers for graphene on SiC(0001) is the linewidth of the 2D (or D*) peak. The Raman spectra of epitaxial graphene show significant differences as compared to micromechanically cleaved graphene obtained from highly oriented pyrolytic graphite crystals. The G peak is found to be blue-shifted. The 2D peak does not exhibit any obvious shoulder structures but it is much broader and almost resembles a single-peak even for multilayers. Flakes of epitaxial graphene were transferred from SiC onto SiO2 for further Raman studies. A comparison of the Raman data obtained for graphene on SiC with data for epitaxial graphene transferred to SiO2 reveals that the G peak blue-shift is clearly due to the SiC substrate. The broadened 2D peak however stems from the graphene structure itself and not from the substrate.

Properties of nano-graphite ribbons with zigzag edges -- Difference between odd and even legs --

Hideo Yoshioka — 2008-07-30T20:20:37-00:00

(25 Jul 2008)

Persistent currents and transport properties are investigated for the nano-graphite ribbons with zigzag shaped edges with paying attention to system length $L$ dependence. It is found that both the persistent current in the isolated ring and the conductance of the system connected to the perfect leads show the remarkable $L$ dependences. In addition, the dependences for the systems with odd legs and those with even legs are different from each other. On the persistent current, the amplitude for the cases with odd legs shows power-low behavior as $L^-N$ with $N$ being the number of legs, whereas the maximum of it decreases exponentially for the cases with even legs. The conductance per one spin normalized by $e^2/h$ behaves as follows. In the even legs cases, it decays as $L^-2$, whereas it reaches to unity for $L \to ∞$ in the odd legs cases. Thus, the material is shown to have a remarkable property that there is the qualitative difference between the systems with odd legs and those with even legs even in the absence of the electron-electron interaction.

Nanomachining of multilayer graphene using an atomic force microscope

P Barthold — 2008-07-30T20:19:52-00:00

(25 Jul 2008)

An atomic force microscope is used to structure a film of multilayer graphene. The resistance of the sample was measured in-situ during nanomachining a narrow trench. We found a reversible behavior in the electrical resistance which we attribute to the movement of dislocations. After several attempts also permanent changes are observed. Two theoretical approaches are presented to approximate the measured resistance.

High-Frequency Properties of a Graphene Nanoribbon Field-Effect Transistor

M Ryzhii — 2008-07-30T14:13:16-00:00

(28 Jul 2008)

We propose an analytical device model for a graphene nanoribbon field-effect transistor (GNR-FET). The GNR-FET under consideration is based on a heterostructure which consists of an array of nanoribbons clad between the highly conducting substrate (the back gate) and the top gate controlling the dc and ac source-drain currents. Using the model developed, we derive explicit analytical formulas for the GNR-FET transconductance as a function of the signal frequency, collision frequency of electrons, and the top gate length. The transition from the ballistic and to strongly collisional electron transport is considered.

Dynamic STM/AFM imaging of adsorbed molecules on graphite

N Berdunov — 2008-07-30T20:18:41-00:00

(25 Jul 2008)

We have investigated the adsorption of the organic molecule perylene tetracarboxylic di-imide on a highly oriented pyrolytic graphite substrate using a combined dynamic scanning tunneling and atomic force microscope (STM/AFM). We show that these weakly bound molecules may be imaged in dynamic STM mode, in which the time averaged STM current is used as a feedback signal. Molecular resolution is readily attained and we also observe a contrast reversal between the graphite substrate and adsorbed molecules. We show that this is due to a more rapid increase in tunnel current as the tip-sample separation is reduced over the graphite, as compared with the molecular overlayer and discuss possible origins of this effect.

Dirac-point engineering and topological phase transitions in honeycomb optical lattices

B Wunsch — 2008-07-30T20:18:01-00:00

(26 Jul 2008)

We study the electronic structure and the phase diagram of non-interacting fermions confined to hexagonal optical lattices. In the first part, we compare the properties of Dirac points arising in the eigenspectrum of either honeycomb or triangular lattices. Numerical results are complemented by analytical equations for weak and strong confinements. In the second part we discuss the phase diagram and the evolution of Dirac points in honeycomb lattices applying a tight-binding description with arbitrary nearest-neighbor hoppings. With increasing asymmetry between the hoppings the Dirac points approach each other. At a critical asymmetry the Dirac points merge to open an energy gap, thus changing the topology of the eigenspectrum. We analyze the trajectory of the Dirac points and study the density of states in the different phases. Manifestations of the phase transition in the temperature dependence of the specific heat and in the structure factor are discussed.

Trigonal Band Structure and Time-Reversal Invariance in Graphene

R Winkler — 2008-07-29T07:00:37-00:00

(25 Jul 2008)

We present a symmetry analysis of the trigonal band structure in graphene. While the energy spectrum near the Fermi edge equals the spectrum of massless Dirac fermions, the transformational properties of the underlying basis functions are qualitatively different. Using group theory we develop an invariant expansion of the Hamiltonian for the electron states near the K points of the graphene Brillouin zone. We find that the k-linear dispersion near the band edge arises as an unusual consequence of time-reversal invariance. We suggest to divide the electronic properties of graphene into two categories, those that depend and those that do not depend on the transformational properties of the Bloch functions at K.