Phonon-induced many-body renormalization of the electronic properties of graphene

We develop a theory for the electron-phonon interaction effects on the electronic properties of graphene. We analytically calculate the electron self-energy, spectral function, and the band velocity renormalization due to phonon-mediated electron-electron interaction, finding that phonon-mediated electron-electron coupling has a large effect on the graphene band structure renormalization. Our analytic theory successfully captures the essential features of the observed graphene electron spectra in the angle-resolved photoemission experiments, predicting a kink at approximately 200 meV below the Fermi level and a reduction of the band velocity by approximately 10-20% at the experimental doping level.     

Theory of the evolution of 2D band in the Raman spectra of monolayer and bilayer graphene with laser excitation energy

A double‐resonance process gives rise to the 2D band in the Raman spectra of monolayer and bilayer graphene. Based on the electronic and vibrational dispersion energies of graphene, the wavenumbers of the 2D band were calculated under different laser excitation energies (from 1.0 to 4.4 eV). Calculated results are in good agreement with experimental data and reproduce the experimental dispersion slope of the 2D band very well. The calculated wavenumbers of the 2D band do not show a linear dependence on the laser excitation energies. Moreover, it is explained that the lowest wavenumber peak of the 2D band of the bilayer graphene, which is composed of four components, has the largest slope with laser excitation energy. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of Holstein phonons on the optical conductivity of gapped graphene

We study the optical conductivity of a doped graphene when a sublattice symmetry breaking is occurred in the presence of the electron-phonon interaction. Our study is based on the Kubo formula that is established upon the retarded self-energy. We report new features of both the real and imaginary parts of the quasiparticle self-energy in the presence of a gap opening. We find an analytical expression for the renormalized Fermi velocity of massive Dirac Fermions over broad ranges of electron densities, gap values and the electron-phonon coupling constants. Finally we conclude that the inclusion of the renormalized Fermi energy and the band gap effects are indeed crucial to get reasonable feature for the optical conductivity.     

Self-energy of phonons in anharmonic one-dimensional crystals

Using the expression obtained by Green's function methods the self-energy of phonons, interacting through anharmonic terms of third and fourth order in the expansion of the potential energy, is calculated for a linear chain without further approximations. The phonon energy shift and width show strong dependence on the frequency and the wave-vector.     

Quasiparticle lifetime in graphite studied by ultrahigh-resolution ARPES

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES) to elucidate the nature of quasiparticle dynamics in graphite. We found fairly sharp quasiparticle peak of π band around K(H) point in the vicinity of the Fermi level, together with the strong mass renormalization of the band (kink). The imaginary part of electron self-energy (ImΣ) shows a sudden drop below 0.18 eV, indicative of a strong electron-phonon coupling. The linear energy dependence of ImΣ at higher binding energies provides an evidence for the deviation from the conventional Fermi liquid theory.     

Nernst-Ettingshausen effect in graphene

The Nernst-Ettingshausen effect corresponds to the regime of crossed magnetic and electric fields. In the current theoretical studies of this effect in graphene, the dependence of the Landau levels on the applied electric field is neglected. This dependence takes place in the case of the nonquadratic energy spectrum of the charge carriers. In this work, oscillations of the Nernst coefficient in graphene with a zero and nonzero band gap have been studied taking into account such dependence. The effect of the Coulomb interaction on these oscillations is considered.     

Isospin-dependent relativistic microscopic optical potential in the Dirac Brueckner-Hartree-Fock method

There is growing evidence to suggest that the binding energy of nucleon in nuclear matter comes from a cancellation between large Lorentz scalar and vector potentials[1,2]. The relativistic approach has been of a great success in describing not only the ground state properties of stable nuclei, but also those of exotic nuclei. In the relativistic frame, the spin-orbit coupling can be deduced automatically, which is usually given by hand in the non-relativistic approach. The relativistic method…     

中子物质中超流性的自能色散效应

利用扩展的 Brueckner- Hartree- Fock理论与推广的 BCS方法研究了自能的色散效应和基态关联对中子物质中超流性和能隙的影响 .研究结果表明 ,自能的色散效应使中子物质中能隙减小;考虑基态关联后 ,超流性将进一步减弱. The effects of the dispersion and ground state correlation of the single particle self-energy on neutron matter superfluidity have been investigated in the framework of the Extended Brueckner-Hartree-Fock and the generalized BCS approaches. A sizable reduction of the energy gap is found due to the energy dependence of the self-energy. And the inclusion of the ground state correlations in the self-energy suppresses further the neutron matter superfluidity.     

Rashba effect in the graphene/ni(111) system

We report on angle-resolved photoemission studies of the electronic pi states of high-quality epitaxial graphene layers on a Ni(111) surface. In this system the electron binding energy of the pi states shows a strong dependence on the magnetization reversal of the Ni film. The observed extraordinarily large energy shift up to 225 meV of the graphene-derived pi band peak position for opposite magnetization directions is attributed to a manifestation of the Rashba interaction between spin-polarized electrons in the pi band and the large effective electric field at the graphene/Ni interface. Our findings show that an electron spin in the graphene layer can be manipulated in a controlled way and have important implications for graphene-based spintronic devices.     

Temperature dependence of the energy GAP in GaAs

It is shown that electron-phonon self-energy effects are as important as the Debye-Waller screening of the pseudopotential in the temperature dependence of the band gap when the valence band is degenerate.     

Temperature dependence of the electron self-energy in a polar-crystal slab

In this paper we investigate temperature dependence of the electron self-energy in the polar-crystal slab using Green-function method. We introduce Q2D free Green's function for the first time. Numerical calculations of the electron self-energy using GaAs as an example are performed. The results show that the electron self-energy is a decreasing function of temperature. In calculation, we consider the effect of the excited states on the electron self-energy and find the ground-state energy be about 11% lower than that of bulk polaron. The results also imply that the high excited states pay a larger contribution to the electron self-energy with increasing temperature.     

Coulomb effects on the gain and absorption spectra of the electron-hole plasma in GaAs

Many-body effects due to electron-hole (e-h) attraction and self-energy corrections are investigated on gain and absorption line shapes of degenerate e-h plasma in direct-gap semiconductors. It is demonstrated for GaAs that a large enhancement in experimental gain and absorption coefficients near crossover, which is not reproduced in single-particle treatments, is accounted for by the excitonic e-h interaction. The self-energy corrections, containing the renormalization due to e-e and e-phonon interactions, reduce the direct band gap in GaAs. Their weak $\text{k}$ dependence further improves agreement with experiment.     

Interband quantum kinetics with static disorder scattering

Simultaneous action of strong light pulses and of a strong scattering on the compositional disorder in semiconductor alloys can be treated in the CPA analytically and numerically up to explicit results for monochromatic pulses, which are adiabatic, i.e., changing slowly on the time scale of the quasiparticle formation time. We show that it is possible to construct the adiabatic electron self-energy as a sequence of self-energies for constant illumination parametrically dependent on the transient illumination strength. These are obtained by mapping the problem on a well known alloy case with an effective band hybridisation. Main effects concentrate to a narrow energy region around the one-photon resonance. We study in detail the behavior of the self-energy in this region in dependence on the basic model parameters. An important question is the occurence of the optically induced gap. Both the numerical analysis and an analytical treatment demonstrate a delayed formation of the gap in comparison with the pure crystal case, with a gradual distortion of the electronic structure around the resonance. The gap width scales according to the 2/3 power law.     

Evidence for an energy scale for quasiparticle dispersion in Bi2Sr2CaCu2O8

Quasiparticle dispersion in Bi2Sr2CaCu2O8 is investigated with improved angular resolution as a function of temperature and doping. Unlike the linear dispersion predicted by the band calculation, the data show a sharp break in dispersion at 50+/-15 meV binding energy where the velocity changes by a factor of 2 or more. This change provides an energy scale in the quasiparticle self-energy. This break in dispersion is evident at and away from the d-wave node line, but the magnitude of the dispersion change decreases with temperature and with increasing doping.     

Anisotropic resistivity of the monolayer graphene in the trigonal warping and connected Fermi curve regimes

In the present study, the anisotropic resistivity of the monolayer graphene has been obtained in semiclassical regime beyond the Dirac point approximation. In particular, detailed investigations were made on the dependence of conductivity on the Fermi energy. At low energies, in the vicinity of the Dirac points, band energy of the monolayer graphene is isotropic at the Fermi level. Meanwhile, at the intermediate Fermi energies anisotropic effects such as trigonal warping is expected to be the origin of the anisotropic resistivity. However, besides the band anisotropy there also exists an other source of anisotropic resistivity which was introduced by scattering matrix. At high energies it was shown that the band anisotropy is less effective than the anisotropy generated by the scattering matrix. It was also shown that there exist two distinct regimes of anisotropic resistivity corresponding the trigonal warping and connected Fermi curve at intermediate and high energies respectively.     

Quasiparticle energies and band gaps in graphene nanoribbons

We present calculations of the quasiparticle energies and band gaps of graphene nanoribbons (GNRs) carried out using a first-principles many-electron Green's function approach within the GW approximation. Because of the quasi-one-dimensional nature of a GNR, electron-electron interaction effects due to the enhanced screened Coulomb interaction and confinement geometry greatly influence the quasiparticle band gap. Compared with previous tight-binding and density functional theory studies, our calculated quasiparticle band gaps show significant self-energy corrections for both armchair and zigzag GNRs, in the range of 0.5-3.0 eV for ribbons of width 2.4-0.4 nm. The quasiparticle band gaps found here suggest that use of GNRs for electronic device components in ambient conditions may be viable.     

Structural and electronic properties of YH 3 at high pressure – band calculation by the GW approximation

Using the GW calculation for one particle energy of electrons, we studied the pressure dependence of the band gaps in YH ₃ for controversial structures reported in experimental and theoretical studies. For some types of band structures, perturbational treatment taking only diagonal matrix elements of the self-energy into account, which is the most commonly used method in the GW calculation, fails to give band gaps. In those cases, calculations in which the non-diagonal matrix elements of the self-energy are taken into account are needed to get the band gaps. In the fcc-YH ₃, band gap disappears around 5 GPa which is much lower pressure than those reported in experimental studies for the metallization in YH ₃. The band gap by the GW calculation in the C2/m structure, which is theoretically predicted to be most probable up to around 40 GPa, survives to much higher pressures than those predicted by the generalized gradient approximation (GGA), and probably to over 60 GPa. The very recent report of metallization pressure around 70 GPa on the YH ₃ metallization by means of DC conductivity measurements suggests that some structures other than the C 2/m should appear as an intermediate structure before the fcc-YH ₃ will appear by the pressurization.     

Velocity filtration and temperature inversion in a system with long-range interactions

Graphene has vast promising applications on the nanoelectronics and spintronics because of its unique magnetic and electronic properties. Making use of an ab initio spin-polarized density functional theory, implemented by the Heyd-Scuseria-Ernzerhof 06 hybrid functional method, abbreviated as HSE06, the properties of semi-metal nitrogen-substitutional graphene are investigated. From our investigations, we conclude that introducing nitrogen doping would possibly perform the spin symmetry breaking, resulting energy degeneracy at some doping configurations. The spin symmetry breaking would cause spin-polarized effects, which induce magnetic response in graphene. This paper systematically analyzes the dependence of magnetic moments and band gaps in graphene on nitrogen-substitutional doping configurations.     

The influence of the band structure of epitaxial graphene on SiC on the transistor characteristics

We fabricated high-mobility field-effect transistors based on epitaxial graphene synthesized by vacuum graphitization of both the Si- and C-faces of SiC. Room-temperature field-effect mobilities >4000 cm²/V s for both electrons and holes were achieved, although with wide distributions. By using a high-k gate dielectric, we were able to measure the transistor characteristics in a wide carrier density range, where the mobility is seen to decrease as the carrier density increases. We formulate a simple semiclassical model of electrical transport in graphene, and explain the sublinear dependence of conductivity on carrier density from the view point of the few-layer graphene energy band structure. Our analysis reveals important differences between the few-layer graphene energy dispersions on the SiC Si- and C-faces, providing the first evidence based on electrical device characteristics for the theoretically proposed energy dispersion difference between graphene synthesized on these two faces of SiC.     

Chirality-induced dynamic kohn anomalies in graphene

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=omega/v, 2k_{F}+/-omega/v for LO phonons and one at q=omega/v for TO phonons. The presence of these new KAs in graphene, in contrast to the usual KA q=2k_{F} 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.