225 research outputs found
Slow imbalance relaxation and thermoelectric transport in graphene
We compute the electronic component of the thermal conductivity (TC) and the
thermoelectric power (TEP) of monolayer graphene, within the hydrodynamic
regime, taking into account the slow rate of carrier population imbalance
relaxation. Interband electron-hole generation and recombination processes are
inefficient due to the non-decaying nature of the relativistic energy spectrum.
As a result, a population imbalance of the conduction and valence bands is
generically induced upon the application of a thermal gradient. We show that
the thermoelectric response of a graphene monolayer depends upon the ratio of
the sample length to an intrinsic length scale l_Q, set by the imbalance
relaxation rate. At the same time, we incorporate the crucial influence of the
metallic contacts required for the thermopower measurement (under open circuit
boundary conditions), since carrier exchange with the contacts also relaxes the
imbalance. These effects are especially pronounced for clean graphene, where
the thermoelectric transport is limited exclusively by intercarrier collisions.
For specimens shorter than l_Q, the population imbalance extends throughout the
sample; the TC and TEP asymptote toward their zero imbalance relaxation limits.
In the opposite limit of a graphene slab longer than l_Q, at non-zero doping
the TC and TEP approach intrinsic values characteristic of the infinite
imbalance relaxation limit. Samples of intermediate (long) length in the doped
(undoped) case are predicted to exhibit an inhomogeneous temperature profile,
whilst the TC and TEP grow linearly with the system size. In all cases except
for the shortest devices, we develop a picture of bulk electron and hole number
currents that flow between thermally conductive leads, where steady-state
recombination and generation processes relax the accumulating imbalance.Comment: 14 pages, 4 figure
Magnetotransport in disordered delta-doped heterostructures
We discuss theoretically how electrons confined to two dimensions in a
delta-doped heterostructure can arrange themselves in a droplet-like spatial
distribution due to disorder and screening effects when their density is low.
We apply this droplet picture to magnetotransport and derive the expected
dependence on electron density of several quantities relevant to this
transport, in the regimes of weak and moderate magnetic fields. We find good
qualitative and quantitative agreement between our calculations and recent
experiments on delta-doped heterostructures.Comment: 10 pages RevTeX, 2 figures, uses psfrag; published versio
Conduction band spin splitting and negative magnetoresistance in heterostructures
The quantum interference corrections to the conductivity are calculated for
an electron gas in asymmetric quantum wells in a magnetic field. The theory
takes into account two different types of the spin splitting of the conduction
band: the Dresselhaus terms, both linear and cubic in the wave vector, and the
Rashba term, linear in wave vector. It is shown that the contributions of these
terms into magnetoconductivity are not additive, as it was traditionally
assumed. While the contributions of all terms of the conduction band splitting
into the D'yakonov--Perel' spin relaxation rate are additive, in the
conductivity the two linear terms cancel each other, and, when they are equal,
in the absence of the cubic terms the conduction band spin splitting does not
show up in the magnetoconductivity at all. The theory agrees very well with
experimental results and enables one to determine experimentally parameters of
the spin-orbit splitting of the conduction band.Comment: 8 pages, RevTeX, 4 Postscript figure
Phonon-induced decay of the electron spin in quantum dots
We study spin relaxation and decoherence in a
GaAs quantum dot due to spin-orbit interaction. We derive an effective
Hamiltonian which couples the electron spin to phonons or any other fluctuation
of the dot potential. We show that the spin decoherence time is as large
as the spin relaxation time , under realistic conditions. For the
Dresselhaus and Rashba spin-orbit couplings, we find that, in leading order,
the effective magnetic field can have only fluctuations transverse to the
applied magnetic field. As a result, for arbitrarily large Zeeman
splittings, in contrast to the naively expected case
. We show that the spin decay is drastically suppressed for
certain magnetic field directions and values of the
Rashba coupling constant. Finally, for the spin coupling to acoustic phonons,
we show that
for all spin-orbit mechanisms in leading order in the
electron-phonon interaction.Comment: 5 pages, 1 figur
D'yakonov-Perel' spin relaxation for degenerate electrons in the electron-hole liquid
We present an analytical study of the D'yakonov-Perel' spin relaxation time
for degenerate electrons in a photo-excited electron-hole liquid in intrinsic
semiconductors exhibiting a spin-split band structure. The D'yakonov-Perel'
spin relaxation of electrons in these materials is controlled by electron-hole
scattering, with small corrections from electron-electron scattering and
virtually none from electron-impurity scattering. We derive simple expressions
(one-dimensional and two-dimensional integrals respectively) for the effective
electron-hole and electron-electron scattering rates which enter the spin
relaxation time calculation. The electron-hole scattering rate is found to be
comparable to the scattering rates from impurities in the electron liquid - a
common model for n-type doped semiconductors. As the density of electron-hole
pairs decreases (within the degenerate regime), a strong enhancement of the
scattering rates and a corresponding slowing down of spin relaxation is
predicted due to exchange and correlation effects in the electron-hole liquid.
In the opposite limit of high density, the original D'yakonov-Perel' model
fails due to decreasing scattering rates and is eventually superseded by free
precession of individual quasiparticle spins.Comment: 16 pages, 5 figure
Statistics of the Charging Spectrum of a Two-Dimensional Coulomb Glass Island
The fluctuations of capacitance of a two-dimensional island are studied in
the regime of low electron concentration and strong disorder, when electrons
can be considered classical particles. The universal capacitance distribution
is found, with the dispersion being of the order of the average. This
distribution is shown to be closely related to the shape of the Coulomb gap in
the one-electron density of states of the island. Behavior of the the
capacitance fluctuations near the metal - insulator transition is discussed.Comment: 4 pages, LaTex, 4 Postscript figures are included Discussion of the
situation with screening by metallic gate is adde
- …