232 research outputs found
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 āp 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 āhiddenā valley symmetry of the system
Length-dependent resistance model for a single-wall Carbon nanotube
The non-linear length-dependent resistance, observed in
single-wall Carbon nanotubes (SNTs) is explained through the recently proposed
ionization energy () based Fermi-Dirac statistics (FDS). The length
here corresponds to the Carbon atoms number () along the SNT. It
is also shown that is associated
with , which can be attributed to different semiconducting
properties in their respective and 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
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