184 research outputs found
Effects of impurity scattering on electron-phonon resonances in semiconductor superlattice high-field transport
A non-equilibrium Green's function method is applied to model high-field
quantum transport and electron-phonon resonances in semiconductor
superlattices. The field-dependent density of states for elastic (impurity)
scattering is found non-perturbatively in an approach which can be applied to
both high and low electric fields. I-V curves, and specifically electron-phonon
resonances, are calculated by treating the inelastic (LO phonon) scattering
perturbatively. Calculations show how strong impurity scattering suppresses the
electron-phonon resonance peaks in I-V curves, and their detailed sensitivity
to the size, strength and concentration of impurities.Comment: 7 figures, 1 tabl
A Variational Approach to Nonlocal Exciton-Phonon Coupling
In this paper we apply variational energy band theory to a form of the
Holstein Hamiltonian in which the influence of lattice vibrations (optical
phonons) on both local site energies (local coupling) and transfers of
electronic excitations between neighboring sites (nonlocal coupling) is taken
into account. A flexible spanning set of orthonormal eigenfunctions of the
joint exciton-phonon crystal momentum is used to arrive at a variational
estimate (bound) of the ground state energy for every value of the joint
crystal momentum, yielding a variational estimate of the lowest polaron energy
band across the entire Brillouin zone, as well as the complete set of polaron
Bloch functions associated with this band. The variation is implemented
numerically, avoiding restrictive assumptions that have limited the scope of
previous assaults on the same and similar problems. Polaron energy bands and
the structure of the associated Bloch states are studied at general points in
the three-dimensional parameter space of the model Hamiltonian (electronic
tunneling, local coupling, nonlocal coupling), though our principal emphasis
lay in under-studied area of nonlocal coupling and its interplay with
electronic tunneling; a phase diagram summarizing the latter is presented. The
common notion of a "self-trapping transition" is addressed and generalized.Comment: 33 pages, 11 figure
Localized modes in defective multilayer structures
In this paper, the localized surface modes in a defective multilayer
structure has been investigated. It is shown that the defective multilayer
structures can support two different kind of localized modes depending on the
position and the thickness of the defect layer. One of these modes is localized
at the interface between the multilayer structure and a homogeneous medium (the
so-called surface mode) and the other one is localized at the defect layer
(defect localized mode). We reveal that the presence of defect layer pushes the
dispersion curve of surface modes to the lower or the upper edge of the
photonic bandgap depending on the homogeneous medium is a left-handed or
right-handed medium (e.g. vacuum), respectively. So, the existence region of
the surface modes restricted. Moreover, the effect of defect on the energy flow
velocity of the surface modes is discussed.Comment: 5 pages, 7 figure
On dispersive energy transport and relaxation in the hopping regime
A new method for investigating relaxation phenomena for charge carriers
hopping between localized tail states has been developed. It allows us to
consider both charge and energy {\it dispersive} transport. The method is based
on the idea of quasi-elasticity: the typical energy loss during a hop is much
less than all other characteristic energies. We have investigated two models
with different density of states energy dependencies with our method. In
general, we have found that the motion of a packet in energy space is affected
by two competing tendencies. First, there is a packet broadening, i.e. the
dispersive energy transport. Second, there is a narrowing of the packet, if the
density of states is depleting with decreasing energy. It is the interplay of
these two tendencies that determines the overall evolution. If the density of
states is constant, only broadening exists. In this case a packet in energy
space evolves into Gaussian one, moving with constant drift velocity and mean
square deviation increasing linearly in time. If the density of states depletes
exponentially with decreasing energy, the motion of the packet tremendously
slows down with time. For large times the mean square deviation of the packet
becomes constant, so that the motion of the packet is ``soliton-like''.Comment: 26 pages, RevTeX, 10 EPS figures, submitted to Phys. Rev.
Theory of electric-field-induced spin accumulation and spin current in the two-dimensional Rashba model
Based on the spin-density-matrix approach, both the electric-field-induced
spin accumulation and the spin current are systematically studied for the
two-dimensional Rashba model. Eigenmodes of spin excitations give rise to
resonances in the frequency domain. Utilizing a general and physically
well-founded definition of the spin current, we obtain results that differ
remarkably from previous findings. It is shown that there is a close
relationship between the spin accumulation and the spin current, which is due
to the prescription of a quasi-chemical potential and which does not result
from a conservation law. Physical ambiguities are removed that plagued former
approaches with respect to a spin-Hall current that is independent of the
electric field. For the clean Rashba model, the intrinsic spin-Hall
conductivity exhibits a logarithmic divergency in the low-frequency regime.Comment: 19 pages including figure
Polaron and bipolaron transport in a charge segregated state of doped strongly correlated 2D semiconductor
The 2D lattice gas model with competing short and long range interactions is
appliedused for calculation of the incoherent charge transport in the classical
strongly-correlated charge segregated polaronic state. We show, by means of
Monte-Carlo simulations, that at high temperature the transport is dominated by
hopping of the dissociated correlated polarons, where with thetheir mobility is
inversely proportional to the temperature. At the temperatures below the
clustering transition temperature the bipolaron transport becomes dominant. The
energy barrier for the bipolaron hopping is determined by the Coulomb effects
and is found to be lower than the barrier for the single-polaron hopping. This
leads to drastically different temperature dependencies of mobilities for
polarons and bipolarons at low temperatures
Universality of ac-conduction in anisotropic disordered systems: An effective medium approximation study
Anisotropic disordered system are studied in this work within the random
barrier model. In such systems the transition probabilities in different
directions have different probability density functions. The
frequency-dependent conductivity at low temperatures is obtained using an
effective medium approximation. It is shown that the isotropic universal
ac-conduction law, , is recovered if properly scaled
conductivity () and frequency () variables are used.Comment: 5 pages, no figures, final form (with corrected equations
On the structure of the energy distribution function in the hopping regime
The impact of the dispersion of the transport coefficients on the structure
of the energy distribution function for charge carriers far from equilibrium
has been investigated in effective-medium approximation for model densities of
states. The investigations show that two regimes can be observed in energy
relaxation processes. Below a characteristic temperature the structure of the
energy distribution function is determined by the dispersion of the transport
coefficients. Thermal energy diffusion is irrelevant in this regime. Above the
characteristic temperature the structure of the energy distribution function is
determined by energy diffusion. The characteristic temperature depends on the
degree of disorder and increases with increasing disorder. Explicit expressions
for the energy distribution function in both regimes are derived for a constant
and an exponential density of states.Comment: 16 page
The Essential Interactions in Oxides and Spectral Weight Transfer in Doped Manganites
We calculate the value of the Fr\"ohlich electron-phonon interaction in
manganites, cuprates, and some other charge-transfer insulators and show that
this interaction is much stronger than any relevant magnetic interaction. A
polaron shift due to the Fr\"ohlich interaction, which is about 1 eV, suggests
that carriers in those systems are small (bi)polarons at all temperatures and
doping levels, in agreement with the oxygen isotope effect and other data. An
opposite conclusion, recently suggested in the literature, is shown to be
incorrect. The frequency and temperature dependence of the optical conductivity
of ferromagnetic manganites is explained within the framework of the bipolaron
theory.Comment: 6 pages, REVTeX 3.1 with 3 eps-figures. Journal versio
Electric-field induced spin excitations in two-dimensional spin-orbit coupled systems
Rigorous coupled spin-charge drift-diffusion equations are derived from
quantum-kinetic equations for the spin-density matrix that incorporate effects
due to k-linear spin-orbit interaction, an in-plane electric field, and the
elastic scattering on nonmagnetic impurities. The explicit analytical solution
for the induced magnetization exhibits a pole structure, from which the
dispersion relations of spin excitations are identified. Applications of the
general approach refer to the excitation of long-lived field-induced spin waves
by optically generated spin and charge patterns. This approach transfers
methods known in the physics of space-charge waves to the treatment of spin
eigenmodes. In addition, the amplification of an oscillating electric field by
spin injection is demonstrated.Comment: 17 pages, 3 figure
- …