11,370 research outputs found
Frictional drag between quantum wells mediated by phonon exchange
We use the Kubo formalism to evaluate the contribution of acoustic phonon
exchange to the frictional drag between nearby two-dimensional electron
systems. In the case of free phonons, we find a divergent drag rate
(). However, becomes finite when phonon
scattering from either lattice imperfections or electronic excitations is
accounted for. In the case of GaAs quantum wells, we find that for a phonon
mean free path smaller than a critical value, imperfection
scattering dominates and the drag rate varies as over many
orders of magnitude of the layer separation . When exceeds the
critical value, the drag rate is dominated by coupling through an
electron-phonon collective mode localized in the vicinity of the electron
layers. We argue that the coupled electron-phonon mode may be observable for
realistic parameters. Our theory is in good agreement with experimental results
for the temperature, density, and -dependence of the drag rate.Comment: 45 pages, LaTeX, 8 postscript file figure
Scattering of carriers by coupled plasmon-phonon modes in bulk polar semiconductors and polar semiconductor heterostructures
We present a general treatment of carrier scattering by coupled phonon-plasmon collective modes in polar semiconductors, taking anharmonic phonon decay into account and self-consistently calculating carrier momentum relaxation rates and carrier mobility in a parabolic band model. We iteratively solve the weak-field Boltzmann equations for carriers and collective modes and obtain their nonequilibrium distribution functions. Both the scattering rates and the anharmonic decay of the coupled modes are expressed through the total dielectric function of the semiconductor, consisting of a damped lattice dielectric function, and a temperature dependent random phase approximation dielectric function for the carrier plasma. We show that the decay of the coupled modes has a significant effect on the contribution to the mobility limited by carrier-coupled mode scattering. We also propose a scalar quantity, the phonon dissipation weight factor, with which this effect can be estimated from an analytic expression. We apply this treatment to dynamically screened electron-longitudinal optical phonon scattering in bulk polar semiconductors, and to dynamically screened remote phonon scattering in polar heterostructures where monolayers of MoS2 are sandwiched between various polar dielectrics. We find that a dynamic treatment of the remote phonon scattering yields mobilities up to 75% higher than a static screening approximation does for structures which consist of a monolayer of MoS2 between hafnia and silica. Moreover, we show that accounting for the nonzero thickness of the MoS2 interface layer has an important effect on the calculated mobility in the same structure
Diffusive Transport in Quasi-2D and Quasi-1D Electron Systems
Quantum-confined semiconductor structures are the cornerstone of modern-day
electronics. Spatial confinement in these structures leads to formation of
discrete low-dimensional subbands. At room temperature, carriers transfer among
different states due to efficient scattering with phonons, charged impurities,
surface roughness and other electrons, so transport is scattering-limited
(diffusive) and well described by the Boltzmann transport equation. In this
review, we present the theoretical framework used for the description and
simulation of diffusive electron transport in quasi-two-dimensional and
quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs
and nanowires is presented in detail.Comment: Review article, to appear in Journal of Computational and Theoretical
Nanoscienc
Modeling techniques for quantum cascade lasers
Quantum cascade lasers are unipolar semiconductor lasers covering a wide
range of the infrared and terahertz spectrum. Lasing action is achieved by
using optical intersubband transitions between quantized states in specifically
designed multiple-quantum-well heterostructures. A systematic improvement of
quantum cascade lasers with respect to operating temperature, efficiency and
spectral range requires detailed modeling of the underlying physical processes
in these structures. Moreover, the quantum cascade laser constitutes a
versatile model device for the development and improvement of simulation
techniques in nano- and optoelectronics. This review provides a comprehensive
survey and discussion of the modeling techniques used for the simulation of
quantum cascade lasers. The main focus is on the modeling of carrier transport
in the nanostructured gain medium, while the simulation of the optical cavity
is covered at a more basic level. Specifically, the transfer matrix and finite
difference methods for solving the one-dimensional Schr\"odinger equation and
Schr\"odinger-Poisson system are discussed, providing the quantized states in
the multiple-quantum-well active region. The modeling of the optical cavity is
covered with a focus on basic waveguide resonator structures. Furthermore,
various carrier transport simulation methods are discussed, ranging from basic
empirical approaches to advanced self-consistent techniques. The methods
include empirical rate equation and related Maxwell-Bloch equation approaches,
self-consistent rate equation and ensemble Monte Carlo methods, as well as
quantum transport approaches, in particular the density matrix and
non-equilibrium Green's function (NEGF) formalism. The derived scattering rates
and self-energies are generally valid for n-type devices based on
one-dimensional quantum confinement, such as quantum well structures
Heat transport in harmonic lattices
We work out the non-equilibrium steady state properties of a harmonic lattice
which is connected to heat reservoirs at different temperatures. The heat
reservoirs are themselves modeled as harmonic systems. Our approach is to write
quantum Langevin equations for the system and solve these to obtain steady
state properties such as currents and other second moments involving the
position and momentum operators. The resulting expressions will be seen to be
similar in form to results obtained for electronic transport using the
non-equilibrium Green's function formalism. As an application of the formalism
we discuss heat conduction in a harmonic chain connected to self-consistent
reservoirs. We obtain a temperature dependent thermal conductivity which, in
the high-temperature classical limit, reproduces the exact result on this model
obtained recently by Bonetto, Lebowitz and Lukkarinen.Comment: One misprint and one error have been corrected; 22 pages, 2 figure
Theory of phonon-drag thermopower of extrinsic semiconducting single-wall carbon nanotubes and comparison with previous experimental data
A theoretical model for the calculation of the phonon-drag thermopower,
, in degenerately doped semiconducting single-wall carbon nanotubes
(SWCNTs) is proposed. Detailed calculations of are performed as a
function of temperature, tube radius and position of the Fermi level. We derive
a simple analytical expression for that can be utilized to determine
the free carrier density in doped nanotubes. At low temperatures shows
an activated behavior characteristic of the one-dimensional (1D) character of
carriers. Screening effects are taken into account and it is found that they
dramatically reduce the magnitude of . Our results are compared with
previous published experimental data in bulk p-doped SWCNT materials. Excellent
agreement is obtained in the temperature range 10-200 K for a consistent set of
parameters. This is a striking result in view of the complexity of these
systems.Comment: 21 pages, 6 figures. This version has been accepted for publication
in Phys. Rev.
Relativistic magnetotransport in graphene
We study the thermal and electric transport of a fluid of interacting Dirac
fermions as they arise in single-layer graphene. We include Coulomb
interactions, a dilute density of charged impurities and the presence of a
magnetic field to describe both the static and the low frequency response as a
function of temperature T and chemical potential mu. In the critical regime mu
<< T where both bands above and below the Dirac point contribute to transport
we find pronounced deviations from Fermi liquid behavior, universal,
collision-dominated values for transport coefficients and a cyclotron resonance
of collective nature. In the collision-dominated high temperature regime the
linear thermoelectric transport coefficients are shown to obey the constraints
of relativistic magnetohydrodynamics which we derive microscopically from
Boltzmann theory. The latter also allows us to describe the crossover to
disorder-dominated Fermi liquid behavior at large doping and low temperatures,
as well as the crossover to the ballistic regime at high fields.Comment: Proceedings of the Landau Memorial Conference 200
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