Extrinsic spin Nernst effect in two-dimensional electron systems

The spin accumulation due to the spin current induced by the perpendicular temperature gradient (the spin Nernst effect) is studied in a two-dimensional electron system (2DES) with spin-orbit interaction by employing the Boltzmann equation. The considered 2DES is confined within a symmetric quantum well with delta doping at the center of the well. A symmetry consideration leads to the spin-orbit interaction which is diagonal in the spin component perpendicular to the 2DES. As origins of the spin current, the skew scattering and the side jump are considered at each impurity on the center plane of the well. It is shown that, for repulsive impurity potentials, the spin-Nernst coefficient changes its sign at the impurity density where contributions from the skew scattering and the side jump cancel each other out. This is in contrast to the spin Hall effect in which the sign change of the coefficient occurs for attractive impurity potentials.Comment: 8 pages, 1 figur

Weak localisation magnetoresistance and valley symmetry in graphene.

Due to the chiral nature of electrons in a monolayer of graphite (graphene) one can expect weak antilocalisation and a positive weak-field magnetoresistance in it. However, trigonal warping (which breaks p to −p symmetry of the Fermi line in each valley) suppresses antilocalisation, while inter-valley scattering due to atomically sharp scatterers in a realistic graphene sheet or by edges in a narrow wire tends to restore conventional negative magnetoresistance. We show this by evaluating the dependence of the magnetoresistance of graphene on relaxation rates associated with various possible ways of breaking a ’hidden’ valley symmetry of the system

Length-dependent resistance model for a single-wall Carbon nanotube

The non-linear length-dependent resistance, $\mathcal{R}(l)$ observed in single-wall Carbon nanotubes (SNTs) is explained through the recently proposed ionization energy ($E_I$) based Fermi-Dirac statistics ($i$FDS). The length here corresponds to the Carbon atoms number ($\mathcal{N}$) along the SNT. It is also shown that $\mathcal{R}_y(l_y)$ $<$ $\mathcal{R}_x(l_x)$ is associated with $E_I^y$ $<$ $E_I^x$, which can be attributed to different semiconducting properties in their respective $y$ and $x$ directions.Comment: Publishe

Twist instability in strongly correlated carbon nanotubes

We show that strong Luttinger correlations of the electron liquid in armchair carbon nanotubes lead to a significant enhancement of the onset temperature of the putative twist Peierls instability. The instability results in a spontaneous uniform twist deformation of the lattice at low temperatures, and a gapped ground state. Depending on values of the coupling constants the umklapp electron scattering processes can assist or compete with the twist instability. In case of the competition the umklapp processes win in wide tubes. In narrow tubes the outcome of the competition depends on the relative strength of the e-e and e-ph backscattering. Our estimates show that the twist instability may be realized in free standing (5,5) tubes.Comment: 4 pages, 1 figur

Decay of a plasmon into neutral modes in a carbon nanotube

We evaluate the rate of energy loss of a plasmon in a disorder-free carbon nanotube. The plasmon decays into neutral bosonic excitations of the electron liquid. The process is mediated either by phonon-assisted backscattering of a single electron, or Umklapp backscattering of two electrons. To lowest order in the backscattering interactions the partial decay rates are additive. At zero doping the corresponding decay rates scale as power-laws of the temperature with positive and negative exponents for the two mechanisms, respectively. The precise values of the exponents depend on the Luttinger liquid parameter. At finite doping the decay rates are described by universal crossover functions of frequency and chemical potential measured in units of temperature. In the evaluation of the plasmon decay, we concentrate on a finite-length geometry allowing excitation of plasma resonances.Comment: 10 pages, 4 figure

Instability due to long range Coulomb interaction in a liquid of polarizable particles (polarons, etc.)

The interaction Hamiltonian for a system of polarons a la Feynman in the presence of long range Coulomb interaction is derived and the dielectric function is computed in mean field. For large enough concentration a liquid of such particles becomes unstable. The onset of the instability is signaled by the softening of a collective optical mode in which all electrons oscillate in phase in their respective self-trapping potential. We associate the instability with a metallization of the system. Optical experiments in slightly doped cuprates and doped nickelates are analyzed within this theory. We discuss why doped cuprates matallize whereas nickelates do not.Comment: 5 pages,1 figur

Crossover from Positive to Negative Interlayer Magnetoresistance in Multilayer Massless Dirac Fermion System with Non-Vertical Interlayer Tunneling

We present a theoretical description of the interlayer magnetoresistance in the layered Dirac fermion system with the application to the organic conductor \alpha-(BEDT-TTF)_2I_3 under pressure. Assuming a non-vertical interlayer tunneling and including higher Landau level effects we calculate the interlayer conductivity using the Kubo formula.We propose a physical picture of the experimentally observed crossover from the negative interlayer magnetoresistance, where the Dirac fermion zero-energy Landau level plays a central role, to the positive interlayer magnetoresistance that is the consequence of the Landau level mixing effect upon non-vertical interlayer hopping. The crossover magnetic field depends on the Landau level broadening factor and can be used to determine the Dirac fermion Landau level energy spectrum.Comment: 12 pages, 6 figure

Transport of Dirac quasiparticles in graphene: Hall and optical conductivities

The analytical expressions for both diagonal and off-diagonal ac and dc conductivities of graphene placed in an external magnetic field are derived. These conductivities exhibit rather unusual behavior as functions of frequency, chemical potential and applied field which is caused by the fact that the quasiparticle excitations in graphene are Dirac-like. One of the most striking effects observed in graphene is the odd integer quantum Hall effect. We argue that it is caused by the anomalous properties of the Dirac quasiparticles from the lowest Landau level. Other quantities such as Hall angle and Nernst signal also exhibit rather unusual behavior, in particular when there is an excitonic gap in the spectrum of the Dirac quasiparticle excitations.Comment: 25 pages, RevTeX4, 8 EPS figures; final version published in PR