74 research outputs found
Degeneracy breaking and intervalley scattering due to short-ranged impurities in finite single-wall carbon nanotubes
We present a theoretical study of degeneracy breaking due to short-ranged
impurities in finite, single-wall, metallic carbon nanotubes. The effective
mass model is used to describe the slowly varying spatial envelope
wavefunctions of spinless electrons near the Fermi level at two inequivalent
valleys (K-points) in terms of the four component Dirac equation for massless
fermions, with the role of spin assumed by pseudospin due to the relative
amplitude of the wave function on the sublattice atoms (``A'' and ``B''). Using
boundary conditions at the ends of the tube that neither break valley
degeneracy nor mix pseudospin eigenvectors, we use degenerate perturbation
theory to show that the presence of impurities has two effects. Firstly, the
position of the impurity with respect to the spatial variation of the envelope
standing waves results in a sinusoidal oscillation of energy level shift as a
function of energy. Secondly, the position of the impurity within the hexagonal
graphite unit cell produces a particular 4 by 4 matrix structure of the
corresponding effective Hamiltonian. The symmetry of this Hamiltonian with
respect to pseudospin flip is related to degeneracy breaking and, for an
armchair tube, the symmetry with respect to mirror reflection in the nanotube
axis is related to pseudospin mixing.Comment: 20 pages, 10 eps figure
Triplet pairing due to spin-orbit-assisted electron-phonon coupling
We propose a microscopic mechanism for triplet pairing due to
spin-orbit-assisted electron interaction with optical phonons in a crystal with
a complex unit cell. Using two examples of electrons with symmetric Fermi
surfaces in crystals with either a cubic or a layered square lattice, we show
that spin-orbit-assisted electron-phonon coupling can, indeed, generate triplet
pairing and that, in each case, it predetermines the tensor structure of a
p-wave order parameter
Spectral features due to inter-Landau-level transitions in the Raman spectrum of bilayer graphene
We investigate the contribution of the low-energy electronic excitations
towards the Raman spectrum of bilayer graphene for the incoming photon energy
Omega >> 1eV. Starting with the four-band tight-binding model, we derive an
effective scattering amplitude that can be incorporated into the commonly used
two-band approximation. Due to the influence of the high-energy bands, this
effective scattering amplitude is different from the contact interaction
amplitude obtained within the two-band model alone. We then calculate the
spectral density of the inelastic light scattering accompanied by the
excitation of electron-hole pairs in bilayer graphene. In the absence of a
magnetic field, due to the parabolic dispersion of the low-energy bands in a
bilayer crystal, this contribution is constant and in doped structures has a
threshold at twice the Fermi energy. In an external magnetic field, the
dominant Raman-active modes are the n_{-} to n_{+} inter-Landau-level
transitions with crossed polarisation of in/out photons. We estimate the
quantum efficiency of a single n_{-} to n_{+} transition in the magnetic field
of 10T as I_{n_{-} to n_{+}}~10^{-12}.Comment: 7 pages, 3 figures, expanded version published in PR
z/-z Symmetry of spin-orbit coupling and weak localization in graphene
We show that the influence of spin-orbit (SO) coupling on the weak
localization effect for electrons in graphene depends on the lack or presence
of z/-z symmetry in the system. While for z/-z asymmetric SO coupling,
disordered graphene should display a weak anti-localization behavior at lowest
temperature, z/-z symmetric coupling leads to an effective saturation of
decoherence time which can be partially lifted by an in-plane magnetic field,
thus, tending to restore the weak localization effect.Comment: 5 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
Selective transmission of Dirac electrons and ballistic magnetoresistance of \textit{n-p} junctions in graphene
We show that an electrostatically created n-p junction separating the
electron and hole gas regions in a graphene monolayer transmits only those
quasiparticles that approach it almost perpendicularly to the n-p interface.
Such a selective transmission of carriers by a single n-p junction would
manifest itself in non-local magnetoresistance effect in arrays of such
junctions and determines the unusual Fano factor in the current noise universal
for the n-p junctions in graphene.Comment: 4 pages, 2 fig
Quantum and classical surface acoustic wave induced magnetoresistance oscillations in a 2D electron gas
We study theoretically the geometrical and temporal commensurability
oscillations induced in the resistivity of 2D electrons in a perpendicular
magnetic field by surface acoustic waves (SAWs). We show that there is a
positive anisotropic dynamical classical contribution and an isotropic
non-equilibrium quantum contribution to the resistivity. We describe how the
commensurability oscillations modulate the resonances in the SAW-induced
resistivity at multiples of the cyclotron frequency. We study the effects of
both short-range and long-range disorder on the resistivity corrections for
both the classical and quantum non-equilibrium cases. We predict that the
quantum correction will give rise to zero-resistance states with associated
geometrical commensurability oscillations at large SAW amplitude for
sufficiently large inelastic scattering times. These zero resistance states are
qualitatively similar to those observed under microwave illumination, and their
nature depends crucially on whether the disorder is short- or long-range.
Finally, we discuss the implications of our results for current and future
experiments on two dimensional electron gases.Comment: 16 pages, 8 figure
Orbital effect of in-plane magnetic field on quantum transport in chaotic lateral dots
We show how the in-plane magnetic field, which breaks time-reversal and
rotational symmetries of the orbital motion of electrons in a heterostructure
due to the momentum-dependent inter-subband mixing, affects weak localisation
correction to conductance of a large-area chaotic lateral quantum dot and
parameteric dependences of universal conductance fluctuations in it.Comment: 4 pages with a figur
Distribution of time-constants for tunneling through a 1D Disordered Chain
The dynamics of electronic tunneling through a disordered 1D chain of finite
length is considered. We calculate distributions of the transmission
coefficient T, Wigner delay time and, and the transport time,
. The central bodies of these distributions have a power-law
form, what can be understood in terms of the resonant tunneling through
localised states.Comment: 5 pages, 3 figures, submitted to PR
Correlation-function spectroscopy of inelastic lifetime in heavily doped GaAs heterostructures
Measurements of resonant tunneling through a localized impurity state are
used to probe fluctuations in the local density of states of heavily doped
GaAs. The measured differential conductance is analyzed in terms of correlation
functions with respect to voltage. A qualitative picture based on the scaling
theory of Thouless is developed to relate the observed fluctuations to the
statistics of single particle wavefunctions. In a quantitative theory
correlation functions are calculated. By comparing the experimental and
theoretical correlation functions the effective dimensionality of the emitter
is analyzed and the dependence of the inelastic lifetime on energy is
extracted.Comment: 41 pages, 14 figure
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