503 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
A Framework for Symmetric Part Detection in Cluttered Scenes
The role of symmetry in computer vision has waxed and waned in importance
during the evolution of the field from its earliest days. At first figuring
prominently in support of bottom-up indexing, it fell out of favor as shape
gave way to appearance and recognition gave way to detection. With a strong
prior in the form of a target object, the role of the weaker priors offered by
perceptual grouping was greatly diminished. However, as the field returns to
the problem of recognition from a large database, the bottom-up recovery of the
parts that make up the objects in a cluttered scene is critical for their
recognition. The medial axis community has long exploited the ubiquitous
regularity of symmetry as a basis for the decomposition of a closed contour
into medial parts. However, today's recognition systems are faced with
cluttered scenes, and the assumption that a closed contour exists, i.e. that
figure-ground segmentation has been solved, renders much of the medial axis
community's work inapplicable. In this article, we review a computational
framework, previously reported in Lee et al. (2013), Levinshtein et al. (2009,
2013), that bridges the representation power of the medial axis and the need to
recover and group an object's parts in a cluttered scene. Our framework is
rooted in the idea that a maximally inscribed disc, the building block of a
medial axis, can be modeled as a compact superpixel in the image. We evaluate
the method on images of cluttered scenes.Comment: 10 pages, 8 figure
Real-time deep hair matting on mobile devices
Augmented reality is an emerging technology in many application domains.
Among them is the beauty industry, where live virtual try-on of beauty products
is of great importance. In this paper, we address the problem of live hair
color augmentation. To achieve this goal, hair needs to be segmented quickly
and accurately. We show how a modified MobileNet CNN architecture can be used
to segment the hair in real-time. Instead of training this network using large
amounts of accurate segmentation data, which is difficult to obtain, we use
crowd sourced hair segmentation data. While such data is much simpler to
obtain, the segmentations there are noisy and coarse. Despite this, we show how
our system can produce accurate and fine-detailed hair mattes, while running at
over 30 fps on an iPad Pro tablet.Comment: 7 pages, 7 figures, submitted to CRV 201
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
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
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
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