32,740 research outputs found
Inelastic quantum transport in superlattices: success and failure of the Boltzmann equation
Electrical transport in semiconductor superlattices is studied within a fully
self-consistent quantum transport model based on nonequilibrium Green
functions, including phonon and impurity scattering. We compute both the drift
velocity-field relation and the momentum distribution function covering the
whole field range from linear response to negative differential conductivity.
The quantum results are compared with the respective results obtained from a
Monte Carlo solution of the Boltzmann equation. Our analysis thus sets the
limits of validity for the semiclassical theory in a nonlinear transport
situation in the presence of inelastic scattering.Comment: final version with minor changes, to appear in Physical Review
Letters, sceduled tentatively for July, 26 (1999
ab initio Electronic Transport Model with Explicit Solution to the Linearized Boltzmann Transport Equation
Accurate models of carrier transport are essential for describing the
electronic properties of semiconductor materials. To the best of our knowledge,
the current models following the framework of the Boltzmann transport equation
(BTE) either rely heavily on experimental data (i.e., semi-empirical), or
utilize simplifying assumptions, such as the constant relaxation time
approximation (BTE-cRTA). While these models offer valuable physical insights
and accurate calculations of transport properties in some cases, they often
lack sufficient accuracy -- particularly in capturing the correct trends with
temperature and carrier concentration. We present here a general transport
model for calculating low-field electrical drift mobility and Seebeck
coefficient of n-type semiconductors, by explicitly considering all relevant
physical phenomena (i.e. elastic and inelastic scattering mechanisms). We first
rewrite expressions for the rates of elastic scattering mechanisms, in terms of
ab initio properties, such as the band structure, density of states, and polar
optical phonon frequency. We then solve the linear BTE to obtain the
perturbation to the electron distribution -- resulting from the dominant
scattering mechanisms -- and use this to calculate the overall mobility and
Seebeck coefficient. Using our model, we accurately calculate electrical
transport properties of the compound n-type semiconductors, GaAs and InN, over
various ranges of temperature and carrier concentration. Our fully predictive
model provides high accuracy when compared to experimental measurements on both
GaAs and InN, and vastly outperforms both semi-empirical models and the
BTE-cRTA. Therefore, we assert that this approach represents a first step
towards a fully ab initio carrier transport model that is valid in all compound
semiconductors
Transport of strong-coupling polarons in optical lattices
We study the transport of ultracold impurity atoms immersed in a
Bose-Einstein condensate (BEC) and trapped in a tight optical lattice. Within
the strong-coupling regime, we derive an extended Hubbard model describing the
dynamics of the impurities in terms of polarons, i.e. impurities dressed by a
coherent state of Bogoliubov phonons. Using a generalized master equation based
on this microscopic model we show that inelastic and dissipative phonon
scattering results in (i) a crossover from coherent to incoherent transport of
impurities with increasing BEC temperature and (ii) the emergence of a net
atomic current across a tilted optical lattice. The dependence of the atomic
current on the lattice tilt changes from ohmic conductance to negative
differential conductance within an experimentally accessible parameter regime.
This transition is accurately described by an Esaki-Tsu-type relation with the
effective relaxation time of the impurities as a temperature-dependent
parameter.Comment: 25 pages, 6 figure
Microscopic nonequilibrium theory of double-barrier Josephson junctions
We study nonequilibrium charge transport in a double-barrier Josephson
junction, including nonstationary phenomena, using the time-dependent
quasiclassical Keldysh Green's function formalism. We supplement the kinetic
equations by appropriate time-dependent boundary conditions and solve the
time-dependent problem in a number of regimes. From the solutions,
current-voltage characteristics are derived. It is understood why the
quasiparticle current can show excess current as well as deficit current and
how the subgap conductance behaves as function of junction parameters. A
time-dependent nonequilibrium contribution to the distribution function is
found to cause a non-zero averaged supercurrent even in the presence of an
applied voltage. Energy relaxation due to inelastic scattering in the
interlayer has a prominent role in determining the transport properties of
double-barrier junctions. Actual inelastic scattering parameters are derived
from experiments. It is shown as an application of the microscopic model, how
the nature of the intrinsic shunt in double-barrier junctions can be explained
in terms of energy relaxation and the opening of Andreev channels.Comment: Accepted for Phys. Rev.
Comparison of inelastic and quasi-elastic scattering effects on nonlinear electron transport in quantum wires
When impurity and phonon scattering coexist, the Boltzmann equation has been
solved accurately for nonlinear electron transport in a quantum wire. Based on
the calculated non-equilibrium distribution of electrons in momentum space, the
scattering effects on both the non-differential (for a fixed dc field) and
differential (for a fixed temperature) mobilities of electrons as functions of
temperature and dc field were demonstrated. The non-differential mobility of
electrons is switched from a linearly increasing function of temperature to a
parabolic-like temperature dependence as the quantum wire is tuned from an
impurity-dominated system to a phonon-dominated one [see T. Fang, {\em et al.},
\prb {\bf 78}, 205403 (2008)]. In addition, a maximum has been obtained in the
dc-field dependence of the differential mobility of electrons. The low-field
differential mobility is dominated by the impurity scattering, whereas the
high-field differential mobility is limited by the phonon scattering [see M.
Hauser, {\em et al.}, Semicond. Sci. Technol. {\bf 9}, 951 (1994)]. Once a
quantum wire is dominated by quasi-elastic scattering, the peak of the
momentum-space distribution function becomes sharpened and both tails of the
equilibrium electron distribution centered at the Fermi edges are raised by the
dc field after a redistribution of the electrons is fulfilled in a symmetric
way in the low-field regime. If a quantum wire is dominated by inelastic
scattering, on the other hand, the peak of the momentum-space distribution
function is unchanged while both shoulders centered at the Fermi edges shift
leftward correspondingly with increasing dc field through an asymmetric
redistribution of the electrons even in low-field regime [see C. Wirner, {\em
et al.}, \prl {\bf 70}, 2609 (1993)]
Conductivity of graphene with resonant and non-resonant adsorbates
We propose a unified description of transport in graphene with adsorbates
that fully takes into account localization effects and loss of electronic
coherence due to inelastic processes. We focus in particular on the role of the
scattering properties of the adsorbates and analyze in detail cases with
resonant or non resonant scattering. For both models we identify several
regimes of conduction depending on the value of the Fermi energy. Sufficiently
far from the Dirac energy and at sufficiently small concentrations the
semi-classical theory can be a good approximation. Near the Dirac energy we
identify different quantum regimes, where the conductivity presents universal
behaviors.Comment: 6 page
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