18 research outputs found
Fluctuation-Stimulated Variable-Range Hopping
Qualitatively new transport mechanism is suggested for hopping of carriers
according to which the variable-range hopping (VRH) arises from the resonant
tunneling between transport states brought into resonance by Coulomb potentials
produced by surrounding sites with fluctuating occupations. A semiquantitative
description of the hopping transport is given based on the assumption that
fluctuations of energies of hopping sites have spectral density 1/f
Coulomb gap in a model with finite charge transfer energy
The Coulomb gap in a donor-acceptor model with finite charge transfer energy
describing the electronic system on the dielectric side of the
metal-insulator transition is investigated by means of computer simulations on
two- and three-dimensional finite samples with a random distribution of equal
amounts of donor and acceptor sites. Rigorous relations reflecting the symmetry
of the model presented with respect to the exchange of donors and acceptors are
derived. In the immediate neighborhood of the Fermi energy the the
density of one-electron excitations is determined solely by
finite size effects and further away from is described by
an asymmetric power law with a non-universal exponent, depending on the
parameter .Comment: 10 pages, 6 figures, submitted to Phys. Rev.
Temperature-induced smearing of the coulomb gap: experiment and computer simulation
We present the first verification of the theoretically predicted effect of temperature-induced smearing of the Coulomb gap. Measurements of the variable-range-hopping conductivity (VRH) in samples of ion-implanted Si:As and computer simulation are used to study the density of states (DOS) near the Fermi level (FL) in the impurity band. The VRH is determined by the DOS integrated over some energy range that depends on temperature T and on the magnetic field B. Using the interplay between T and B we find that the DOS in the vicinity of the FL increases with increasing T
Thermodynamic and Tunneling Density of States of the Integer Quantum Hall Critical State
We examine the long wave length limit of the self-consistent Hartree-Fock
approximation irreducible static density-density response function by
evaluating the charge induced by an external charge. Our results are consistent
with the compressibility sum rule and inconsistent with earlier work that did
not account for consistency between the exchange-local-field and the disorder
potential. We conclude that the thermodynamic density of states is finite, in
spite of the vanishing tunneling density of states at the critical energy of
the integer quantum Hall transition.Comment: 5 pages, 4 figures, minor revisions, published versio
Backward correlations and dynamic heterogeneities: a computer study of ion dynamics
We analyse the correlated back and forth dynamics and dynamic
heterogeneities, i.e. the presence of fast and slow ions, for a lithium
metasilicate system via computer simulations. For this purpose we define, in
analogy to previous work in the field of glass transition, appropriate
three-time correlation functions. They contain information about the dynamics
during two successive time intervals. First we apply them to simple model
systems in order to clarify their information content. Afterwards we use this
formalism to analyse the lithium trajectories. A strong back-dragging effect is
observed, which also fulfills the time-temperature superposition principle.
Furthermore, it turns out that the back-dragging effect is long-ranged and
exceeds the nearest neighbor position. In contrast, the strength of the dynamic
heterogeneities does not fulfill the time-temperature superposition principle.
The lower the temperature, the stronger the mobility difference between fast
and slow ions. The results are then compared with the simple model systems
considered here as well as with some lattice models of ion dynamics.Comment: 12 pages, 10 figure
Complex lithium ion dynamics in simulated LiPO3 glass studied by means of multi-time correlation functions
Molecular dynamics simulations are performed to study the lithium jumps in
LiPO3 glass. In particular, we calculate higher-order correlation functions
that probe the positions of single lithium ions at several times. Three-time
correlation functions show that the non-exponential relaxation of the lithium
ions results from both correlated back-and-forth jumps and the existence of
dynamical heterogeneities, i.e., the presence of a broad distribution of jump
rates. A quantitative analysis yields that the contribution of the dynamical
heterogeneities to the non-exponential depopulation of the lithium sites
increases upon cooling. Further, correlated back-and-forth jumps between
neighboring sites are observed for the fast ions of the distribution, but not
for the slow ions and, hence, the back-jump probability depends on the
dynamical state. Four-time correlation functions indicate that an exchange
between fast and slow ions takes place on the timescale of the jumps
themselves, i.e., the dynamical heterogeneities are short-lived. Hence, sites
featuring fast and slow lithium dynamics, respectively, are intimately mixed.
In addition, a backward correlation beyond the first neighbor shell for highly
mobile ions and the presence of long-range dynamical heterogeneities suggest
that fast ion migration occurs along preferential pathways in the glassy
matrix. In the melt, we find no evidence for correlated back-and-forth motions
and dynamical heterogeneities on the length scale of the next-neighbor
distance.Comment: 12 pages, 13 figure
Spatio-temporal dynamics of quantum-well excitons
We investigate the lateral transport of excitons in ZnSe quantum wells by
using time-resolved micro-photoluminescence enhanced by the introduction of a
solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps,
respectively. Strong deviation from classical diffusion is observed up to 400
ps. This feature is attributed to the hot-exciton effects, consistent with
previous experiments under cw excitation. The coupled transport-relaxation
process of hot excitons is modelled by Monte Carlo simulation. We prove that
two basic assumptions typically accepted in photoluminescence investigations on
excitonic transport, namely (i) the classical diffusion model as well as (ii)
the equivalence between the temporal and spatial evolution of the exciton
population and of the measured photoluminescence, are not valid for
low-temperature experiments.Comment: 8 pages, 6 figure
Phonon-induced dephasing of localized optical excitations
The dynamics of strongly localized optical excitations in semiconductors is studied including electron-phonon interaction. The coupled microscopic equations of motion for the interband polarization and the carrier distribution functions contain coherent and incoherent contributions. While the coherent part is solved through direct numerical integration, the incoherent one is treated by means of a generalized Monte Carlo simulation. The approach is illustrated for a simple model system. The temperature and excitation energy dependence of the optical dephasing rate is analyzed and the results are compared to those of alternative approaches
Energy position of the transport path in disordered organic semiconductors
The concept of transport energy is the most transparent theoretical approach to describe hopping transport in disordered systems with steeply energy dependent density of states (DOS), in particular in organic semiconductors with Gaussian DOS. This concept allows one to treat hopping transport in the framework of a simple multiple-trapping model, replacing the mobility edge by a particular energy level called the transport energy. However, there is no consensus among researchers on the position of this transport level. In this article, we suggest a numerical procedure to find out the energy level most significantly contributing to charge transport in organic semiconductors. The procedure is based on studying the effects of DOS modifications on the charge carrier mobility in straightforward computer simulations. We also show why the most frequently visited energy, computed in several numerical studies to determine the transport energy, is not representative for charge transport
Influence of disorder on electrically and optically detected electron spin nutation
A numerical study of the influence of disorder in semiconductors on spin-Rabi nutation observed with pulsed electrically or optically detected magnetic-resonance techniques (pEDMR and pODMR, respectively) is presented. It is shown that transient nutation signals of disordered spin ensembles differ from ordered ensembles as inhomogeneously broadened Lande-factor distributions are presented. In contrast to ordered systems, the magnitudes of spin-Rabi nutation and spin-Rabi beat nutation change significantly with a strong dependence of their ratio on the correlation of the Lande factors within the nearest-neighbor spin pairs. An interpretation of these results is given and their application for the investigation of disorder using pEDMR and pODMR is discussed