38 research outputs found
Cooling of radiative quantum-dot excitons by terahertz radiation: A spin-resolved Monte Carlo carrier dynamics model
We have developed a theoretical model to analyze the anomalous cooling of
radiative quantum dot (QD) excitons by THz radiation reported by Yusa et al
[Proc. 24th ICPS, 1083 (1998)]. We have made three-dimensional (3D) modeling of
the strain and the piezoelectric field and calculated the 3D density of states
of strain induced quantum dots. On the basis of this analysis we have developed
a spin dependent Monte Carlo model, which describes the carrier dynamics in
QD's when the intraband relaxation is modulated by THz radiation. We show that
THz radiation causes resonance transfer of holes from dark to radiative states
in strain-induced QD's. The transition includes a spatial transfer of holes
from the piezoelectric potential mimima to the deformation potential minimum.
This phenomenon strongly enhances the QD ground state luminescence at the
expense of the luminescence from higher states. Our model also reproduces the
delayed flash of QD ground state luminescence, activated by THz radiation even
s after the carrier generation. Our simulations suggest a more general
possibility to cool the radiative exciton subsystem in optoelectronic devices.Comment: 18 pages, 1 table, 8 figures, submitted to Physical Review B v2:
major conceptual changes. The article was extended considerably to suit
Physical Review B (instead of Physical Review Letters
Designable electron transport features in one-dimensional arrays of metallic nanoparticles: Monte Carlo study of the relation between shape and transport
We study the current and shot noise in a linear array of metallic
nanoparticles taking explicitly into consideration their discrete electronic
spectra. Phonon assisted tunneling and dissipative effects on single
nanoparticles are incorporated as well. The capacitance matrix which determines
the classical Coulomb interaction within the capacitance model is calculated
numerically from a realistic geometry. A Monte Carlo algorithm which
self-adapts to the size of the system allows us to simulate the single-electron
transport properties within a semiclassical framework. We present several
effects that are related to the geometry and the one-electron level spacing
like e.g. a negative differential conductance (NDC) effect. Consequently these
effects are designable by the choice of the size and arrangement of the
nanoparticles.Comment: 13 pages, 12 figure
Transport in metallic multi-island Coulomb blockade systems: A systematic perturbative expansion in the junction transparency
We study electronic transport through metallic multi-island Coulomb-blockade
systems. Based on a diagrammatic real-time approach, we develop a computer
algorithm that generates and calculates all transport contributions up to
second order in the tunnel-coupling strengths for arbitrary multi-island
systems. This comprises sequential and cotunneling, as well as terms
corresponding to a renormalization of charging energies and tunneling
conductances. Multi-island cotunneling processes with energy transfer between
different island are taken into account. To illustrate our approach we analyze
the current through an island in Coulomb blockade, that is electrostatically
coupled to a second island through which a large current is flowing. In this
regime both cotunneling processes involving one island only as well as
multi-island processes are important. The latter can be understood as
photon-assisted sequential tunneling in the blockaded island, where the photons
are provided by potential fluctuations due to sequential tunneling in the
second island. We compare results of our approach to a P(E)-theory for
photon-assisted tunneling in the weak coupling limit.Comment: 14 pages, 7 figures, published version; minor changes in Sec. IV
Sub-electron Charge Relaxation via 2D Hopping Conductors
We have extended Monte Carlo simulations of hopping transport in completely
disordered 2D conductors to the process of external charge relaxation. In this
situation, a conductor of area shunts an external capacitor
with initial charge . At low temperatures, the charge relaxation process
stops at some "residual" charge value corresponding to the effective threshold
of the Coulomb blockade of hopping. We have calculated the r.m.s value
of the residual charge for a statistical ensemble of capacitor-shunting
conductors with random distribution of localized sites in space and energy and
random , as a function of macroscopic parameters of the system. Rather
unexpectedly, has turned out to depend only on some parameter
combination: for negligible Coulomb interaction
and for substantial interaction. (Here
is the seed density of localized states, while is the
dielectric constant.) For sufficiently large conductors, both functions
follow the power law , but with different
exponents: for negligible and
for significant Coulomb interaction. We have been able to derive this law
analytically for the former (most practical) case, and also explain the scaling
(but not the exact value of the exponent) for the latter case. In conclusion,
we discuss possible applications of the sub-electron charge transfer for
"grounding" random background charge in single-electron devices.Comment: 12 pages, 5 figures. In addition to fixing minor typos and updating
references, the discussion has been changed and expande
A Numerical Study of Coulomb Interaction Effects on 2D Hopping Transport
We have extended our supercomputer-enabled Monte Carlo simulations of hopping
transport in completely disordered 2D conductors to the case of substantial
electron-electron Coulomb interaction. Such interaction may not only suppress
the average value of hopping current, but also affect its fluctuations rather
substantially. In particular, the spectral density of current
fluctuations exhibits, at sufficiently low frequencies, a -like increase
which approximately follows the Hooge scaling, even at vanishing temperature.
At higher , there is a crossover to a broad range of frequencies in which
is nearly constant, hence allowing characterization of the current
noise by the effective Fano factor F\equiv S_I(f)/2e \left. For
sufficiently large conductor samples and low temperatures, the Fano factor is
suppressed below the Schottky value (F=1), scaling with the length of the
conductor as . The exponent is significantly
affected by the Coulomb interaction effects, changing from when such effects are negligible to virtually unity when they are
substantial. The scaling parameter , interpreted as the average
percolation cluster length along the electric field direction, scales as when Coulomb interaction effects are negligible
and when such effects are substantial, in
good agreement with estimates based on the theory of directed percolation.Comment: 19 pages, 7 figures. Fixed minor typos and updated reference
A Numerical Study of Transport and Shot Noise at 2D Hopping
We have used modern supercomputer facilities to carry out extensive Monte
Carlo simulations of 2D hopping (at negligible Coulomb interaction) in
conductors with the completely random distribution of localized sites in both
space and energy, within a broad range of the applied electric field and
temperature , both within and beyond the variable-range hopping region. The
calculated properties include not only dc current and statistics of localized
site occupation and hop lengths, but also the current fluctuation spectrum.
Within the calculation accuracy, the model does not exhibit noise, so
that the low-frequency noise at low temperatures may be characterized by the
Fano factor . For sufficiently large samples, scales with conductor
length as , where , and
parameter is interpreted as the average percolation cluster length. At
relatively low , the electric field dependence of parameter is
compatible with the law which follows from directed
percolation theory arguments.Comment: 17 pages, 8 figures; Fixed minor typos and updated reference