Minimal conductivity in graphene: interaction corrections and ultraviolet anomaly

ManuscriptConductivity of a disorder-free intrinsic graphene is studied to the first order in the long-range Coulomb interaction and is found to be ? = ?0(1+0.01g), where g is the dimensionless ("fine structure") coupling constant. The calculations are performed using three different methods: i) electron polarization function, ii) Kubo formula for the conductivity, iii) quantum transport equation. Surprisingly, these methods yield different results unless a proper ultraviolet cut-off procedure is implemented, which requires that the interaction potential in the effective Dirac Hamiltonian is cut-off at small distances (large momenta)

Shot noise in a diffusive ferromagnetic-paramagnetic-ferromagnetic spin valve

Journal ArticleFluctuations of electric current in a spin valve consisting of a diffusive conductor connected to ferromagnetic leads and operated in the giant magnetoresistance regime are studied. It is shown that fluctuations due to spin-flip scattering enhance strongly shot noise up to a point where the Fano factor approaches the full Poissonian value

Electronic Raman scattering in a magnetic field

Journal ArticleRaman scattering in a magnetic field is proposed as a possible method to solve the problem of whether the light scattering in high-Tc superconductors comes from conduction electrons or not. The electronic Raman light scattering in a magnetic field is studied theoretically. The semiclassical approach based on the system of Boltzmann and Maxwell equations is applied. Two types of resonances in the scattering cross section are found to be from electrons moving along the semiclassical trajectories in a magnetic field. The first one, the cyclotron resonance, is associated with the time dispersion and has the symmetrical logarithmic shape. It is sharp in a clean metal. We show that in a dirty metal the relaxation continuum obtained by Zawadowski and Cardona is modified drastically into the set of smaller continua which are located at the frequencies of simple and multiple cyclotron resonances. The other resonance, associated with the space dispersion, results from the electrons moving coherently with the effective external field. It has the asymmetrical shape. The effects of skin depth and Coulomb interaction are analyzed. The strong peak associated with excitation or absorption of helicons is found in the electronic Raman spectra in a high magnetic field. This peak is considered for the case of dirty metal

Electrostatics of straight and bent single-walled carbon nanotubes

Journal ArticleResponse of a single-walled carbon nanotube to external electric field, F, is calculated analytically within the classical electrostatics. Field-induced charge density distribution is approximately linear along the axis of a metallic nanotube and depends rather weakly, as ln(h/ r), on the nanotube length, h (here r is the nanotube radius). In a semiconducting nanotube with a gap, Eg, charge separation occurs as F exceeds the threshold value Fth=Eg /eh. For F>Fth, positively and negatively charged regions at the ends of nanotube are separated by a neutral strip in the middle. Properties of this neutral strip, length and induced charge distribution near the ends, are studied in detail. We also consider a bent nanotube and demonstrate that the number of neutral strips can be one or two depending on the direction of F

Breached superfluidity via p-wave coupling

Journal ArticleAnisotropic pairing between fermion species with different Fermi momenta opens two-dimensional areas of gapless excitations, thus producing a spatially homogeneous state with coexisting superfluid and normal fluids. This breached pairing state is stable and robust for arbitrarily small mismatch and weak p-wave coupling

Frequency dependence of the photonic noise spectrum in an absorbing or amplifying diffusive medium

ManuscriptA theory is presented for the frequency dependence of the power spectrum of photon current fluctuations originating from a disordered medium. Both the cases of an absorbing medium ("grey body") and of an amplifying medium ("random laser") are considered in a waveguide geometry. The semiclassical approach (based on a Boltzmann-Langevin equation) is shown to be in complete agreement with a fully quantum mechanical theory, provided that the effects of wave localization can be neglected. The width of the peak in the power spectrum around zero frequency is much smaller than the inverse coherence time, characteristic for black-body radiation. Simple expressions for the shape of this peak are obtained, in the absorbing case, for waveguide lengths large compared to the absorption length, and, in the amplifying case, close to the laser threshold

Gapless surfaces in anisotropic superfluids

Journal ArticleWe demonstrate when p-wave pairing occurs between species whose free Fermi surfaces are mismatched the gap generally vanishes over a two-dimensional surface. We present detailed calculations of condensation energy, superfluid density (Meissner mass) and specific heat for such states. We also consider stability against separation into mixed phases. According to several independent criteria that can be checked at weak coupling, the resulting "breached" state appears to be stable over a substantial range of parameters. The simple models we consider are homogeneous in position space, and break rotation symmetry spontaneously. They should be realizable in cold atom systems

Spin detection in quantum dots by electric currents

Journal ArticleWe develop a theoretical description of transport through quantum dots connected to reservoirs via spin-polarized ballistic contacts. Rate equations account for spin accumulation inside the dot, electron-electron interactions, and stochastic fluctuations. It is shown that both the ac response (admittance) and the frequency-dependent shot noise are governed by spin-flip scattering, which can be used to detect spin-polarization and spin-flip processes in the dot