217 research outputs found
Strain-induced energy band gap opening in two-dimensional bilayered silicon film
This work presents a theoretical study of the structural and electronic
properties of bilayered silicon films under in-plane biaxial strain/stress
using density functional theory. Atomic structures of the two-dimensional
silicon films are optimized by using both the local-density approximation and
generalized gradient approximation. In the absence of strain/stress, five
buckled hexagonal honeycomb structures of the bilayered silicon film have been
obtained as local energy minima and their structural stability has been
verified. These structures present a Dirac-cone shaped energy band diagram with
zero energy band gaps. Applying tensile biaxial strain leads to a reduction of
the buckling height. Atomically flat structures with zero bucking height have
been observed when the AA-stacking structures are under a critical biaxial
strain. Increase of the strain between 10.7% ~ 15.4% results in a band-gap
opening with a maximum energy band gap opening of ~168.0 meV obtained when
14.3% strain is applied. Energy band diagram, electron transmission efficiency,
and the charge transport property are calculated.Comment: 18 pages, 5 figures, 1 tabl
Dynamic Coupling of Piezoelectric Effects, Spontaneous Polarization, and Strain in Lattice-Mismatched Semiconductor Quantum-Well Heterostructures
A static and dynamic analysis of the combined and self-consistent influence of spontaneous polarization, piezoelectric effects, lattice mismatch, and strain effects is presented for a three-layer one-dimensional AlN/GaN wurtzite quantum-well structure with GaN as the central quantum-well layer . It is shown that, contrary to the assumption of Fonoberov and Balandin [J. Appl. Phys. 94, 7178 (2003); J. Vac. Sci. Technol. B 22, 2190 (2004)], even in cases with no current transport through the structure, the strain distributions are not well captured by minimization of the strain energy only and not, as is in principle required, the total free energy including electric and piezoelectric coupling and spontaneous polarization contributions. Furthermore, we have found that, when an ac signal is imposed through the structure, resonance frequencies exist where strain distributions are even more strongly affected by piezoelectric-coupling contributions depending on the amount of mechanical and electrical losses in the full material system
Properties of Two-Dimensional Silicon grown on Graphene Substrate
The structure and electrical properties of a two-dimensional (2D) sheet of
silicon on a graphene substrate are studied using first-principles
calculations. A new corrugated rectangular structure of silicon is proposed to
be the most energetically favorable structure. The shifting of the Fermi energy
level indicates self-doping. Calculation of electron density shows a weak
coupling between the silicon layer and graphene substrate. The 2D silicon sheet
turns to be metallic and has a much higher value of transmission efficiency
(TE) than the underlying graphene substrate.Comment: 5 Pages, 7 figure
Dynamics of a nanowire superlattice in an ac electric field
With a one-band envelope function theory, we investigate the dynamics of a
finite nanowire superlattice driven by an ac electric field by solving
numerically the time-dependent Schroedinger equation. We find that for an ac
electric field resonant with two energy levels located in two different
minibands, the coherent dynamics in nanowire superlattices is much more complex
as compared to the standard two-level description. Depending on the energy
levels involved in the transitions, the coherent oscillations exhibit different
patterns. A signature of barrier-well inversion phenomenon in nanowire
superlattices is also obtained.Comment: 14 pages, 4 figure
Recognition of Object Categories in Realistic Scenes
Classification of images maps the image content into a certain semantic term such as categories, domain,
object. Image classification should be able automatically check the existence of certain object (e.g. car,
animal, and scene) in the image content. This task is still challenging in computer vision since we have to
deal with the realistic image. The objective of this works is to discover the image classification methods
by mixturing the existing techniques with the aim of the best results in classification. In this work, we
implemented sparse coding method with spatial pyramid matching to classify the images. Beside gray
SIFT, four SIFT color descriptors were also used as a local descriptor. Linear Super Vector Machine (SVM)
is conducted for training and testing the images. The result of this work has shown that color descriptors
improve significantly the classification rate compared to gray SIFT
Spin-phonon coupling in single Mn doped CdTe quantum dot
The spin dynamics of a single Mn atom in a laser driven CdTe quantum dot is
addressed theoretically. Recent experimental
results\cite{Le-Gall_PRL_2009,Goryca_PRL_2009,Le-Gall_PRB_2010}show that it is
possible to induce Mn spin polarization by means of circularly polarized
optical pumping. Pumping is made possible by the faster Mn spin relaxation in
the presence of the exciton. Here we discuss different Mn spin relaxation
mechanisms. First, Mn-phonon coupling, which is enhanced in the presence of the
exciton. Second, phonon-induced hole spin relaxation combined with carrier-Mn
spin flip coupling and photon emission results in Mn spin relaxation. We model
the Mn spin dynamics under the influence of a pumping laser that injects
excitons into the dot, taking into account exciton-Mn exchange and phonon
induced spin relaxation of both Mn and holes. Our simulations account for the
optically induced Mn spin pumping.Comment: 17 pages, 11 figures, submitted to PR
Optoelectronics of Inverted Type-I CdS/CdSe Core/Crown Quantum Ring
Inverted type-I heterostructure core/crown quantum rings (QRs) are
quantum-efficient luminophores, whose spectral characteristics are highly
tunable. Here, we study the optoelectronic properties of type-I core/crown
CdS/CdSe QRs in the zincblende phase - over contrasting lateral size and crown
width. For this we inspect their strain profiles, transition energies,
transition matrix elements, spatial charge densities, electronic bandstructure,
band-mixing probabilities, optical gain spectra, maximum optical gains and
differential optical gains. Our framework uses an effective-mass envelope
function theory based on the 8-band kp method employing the valence
force field model for calculating the atomic strain distributions. The gain
calculations are based on the density-matrix equation and take into
consideration the excitonic effects with intraband scattering. Variations in
the QR lateral size and relative widths of core and crown (ergo the
composition) affect their energy levels, band-mixing probabilities, optical
transition matrix elements, emission wavelengths/intensity, etc. The optical
gain of QRs is also strongly dimension and composition dependent with further
dependency on the injection carrier density causing band-filling effect. They
also affect the maximum and differential gain at varying dimensions and
compositions.Comment: Published in AIP Journal of Applied Physics (11 pages, 7 figures
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