284 research outputs found
Kohn-Sham potential with discontinuity for band gap materials
We model a Kohn-Sham potential with a discontinuity at integer particle
numbers derived from the GLLB approximation of Gritsenko et al. We evaluate the
Kohn-Sham gap and the discontinuity to obtain the quasiparticle gap. This
allows us to compare the Kohn-Sham gaps to those obtained by accurate many-body
perturbation theory based optimized potential methods. In addition, the
resulting quasiparticle band gap is compared to experimental gaps. In the GLLB
model potential, the exchange-correlation hole is modeled using a GGA energy
density and the response of the hole to density variations is evaluated by
using the common-denominator approximation and homogeneous electron gas based
assumptions. In our modification, we have chosen the PBEsol potential as the
GGA to model the exchange hole, and add a consistent correlation potential. The
method is implemented in the GPAW code, which allows efficient parallelization
to study large systems. A fair agreement for Kohn-Sham and the quasiparticle
band gaps with semiconductors and other band gap materials is obtained with a
potential which is as fast as GGA to calculate.Comment: submitted to Physical Review
Nanowire terahertz quantum cascade lasers
International audienceQuantum cascade lasers made of nanowire axial heterostructures are proposed. The dissipative quantum dynamics of their carriers is theoretically investigated using non-equilibrium Green functions. Their transport and gain properties are calculated for varying nanowire thickness, from the classical-wire regime to the quantum-wire regime. Our calculation shows that the lateral quantum confinement provided by the nanowires allows an increase of the maximum operation temperature and a strong reduction of the current density threshold compared to conventional terahertz quantum cascade laser
Dynamics of photoexcited carriers in graphene
The nonequilibrium dynamics of carriers and phonons in graphene is
investigated by solving the microscopic kinetic equations with the
carrier-phonon and carrier-carrier Coulomb scatterings explicitly included. The
Fermi distribution of hot carriers are found to be established within 100 fs
and the temperatures of electrons in the conduction and valence bands are very
close to each other, even when the excitation density and the equilibrium
density are comparable, thanks to the strong inter-band Coulomb scattering.
Moreover, the temporal evolutions of the differential transmission obtained
from our calculations agree with the experiments by Wang et al. [Appl. Phys.
Lett. 96, 081917 (2010)] and Hale et al. [Phys. Rev. B 83, 121404 (2011)] very
well, with two distinct differential transmission relaxations presented. We
show that the fast relaxation is due to the rapid carrier-phonon thermalization
and the slow one is mainly because of the slow decay of hot phonons. In
addition, it is found that the temperatures of the hot phonons in different
branches are different and the temperature of hot carriers can be even lower
than that of the hottest phonons. Finally, we show that the slow relaxation
rate exhibits a mild valley in the excitation density dependence and is
linearly dependent on the probe-photon energy.Comment: 9 pages, 4 figure
Tunable optical Aharonov-Bohm effect in a semiconductor quantum ring
By applying an electric field perpendicular to a semiconductor quantum ring
we show that it is possible to modify the single particle wave function between
quantum dot (QD)-like to ring-like. The constraints on the geometrical
parameters of the quantum ring to realize such a transition are derived. With
such a perpendicular electric field we are able to tune the Aharanov-Bohm (AB)
effect for both single particles and for excitons. The tunability is in both
the strength of the AB-effect as well as in its periodicity. We also
investigate the strain induce potential inside the self assembled quantum ring
and the effect of the strain on the AB effect
Are the Tails of Percolation Thresholds Gaussians ?
The probability distribution of percolation thresholds in finite lattices
were first believed to follow a normal Gaussian behaviour. With increasing
computer power and more efficient simulational techniques, this belief turned
to a stretched exponential behaviour, instead. Here, based on a further
improvement of Monte Carlo data, we show evidences that this question is not
yet answered at all.Comment: 7 pages including 3 figure
Emission spectrum of quasi-resonant laterally coupled quantum dots
We calculate the emission spectrum of neutral and charged excitons in a pair
of laterally coupled InGaAs quantum dots with nearly degenerate energy levels.
As the interdot distance decreases, a number of changes take place in the
emission spectrum which can be used as indications of molecular coupling. These
signatures ensue from the stronger tunnel-coupling of trions as compared to
that of neutral excitons.Comment: 7 pages, 7 figure
The Tails of the Crossing Probability
The scaling of the tails of the probability of a system to percolate only in
the horizontal direction was investigated numerically for correlated
site-bond percolation model for .We have to demonstrate that the
tails of the crossing probability far from the critical point have shape
where is the correlation
length index, is the probability of a bond to be closed. At
criticality we observe crossover to another scaling . Here is a scaling index describing the
central part of the crossing probability.Comment: 20 pages, 7 figures, v3:one fitting procedure is changed, grammatical
change
A Compact Linear Programming Relaxation for Binary Sub-modular MRF
We propose a novel compact linear programming (LP) relaxation for binary
sub-modular MRF in the context of object segmentation. Our model is obtained by
linearizing an -norm derived from the quadratic programming (QP) form of
the MRF energy. The resultant LP model contains significantly fewer variables
and constraints compared to the conventional LP relaxation of the MRF energy.
In addition, unlike QP which can produce ambiguous labels, our model can be
viewed as a quasi-total-variation minimization problem, and it can therefore
preserve the discontinuities in the labels. We further establish a relaxation
bound between our LP model and the conventional LP model. In the experiments,
we demonstrate our method for the task of interactive object segmentation. Our
LP model outperforms QP when converting the continuous labels to binary labels
using different threshold values on the entire Oxford interactive segmentation
dataset. The computational complexity of our LP is of the same order as that of
the QP, and it is significantly lower than the conventional LP relaxation
Simple eigenvalue-self-consistent Δ ¯ G W 0 .
We show that a rigid scissors-like GW self-consistency approach, labeled here Δ ¯ G W 0 , can be trivially implemented at zero additional cost for large scale one-shot G 0 W 0 calculations. The method significantly improves one-shot G 0 W 0 and for large systems is very accurate. Δ ¯ G W 0 is similar in spirit to evGW 0 where the self-consistency is only applied on the eigenvalues entering Green's function, while both W and the eigenvectors of Green's function are held fixed. Δ ¯ G W 0 further assumes that the shift of the eigenvalues is rigid scissors-like so that all occupied states are shifted by the same amount and analogously for all the unoccupied states. We show that this results in a trivial modification of the time-dependent G 0 W 0 self-energy, enabling an a posteriori self-consistency cycle. The method is applicable for our recent stochastic-GW approach, thereby enabling self-consistent calculations for giant systems with thousands of electrons. The accuracy of Δ ¯ G W 0 increases with the system size. For molecules, it is up to 0.4-0.5 eV away from coupled-cluster single double triple (CCSD(T)), but for tetracene and hexacene, it matches the ionization energies from both CCSD(T) and evGW 0 to better than 0.05 eV. For solids, as exemplified here by periodic supercells of semiconductors and insulators with 6192 valence electrons, the method matches evGW 0 quite well and both methods are in good agreement with the experiment
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