11,823 research outputs found
Total destruction of invariant tori for the generalized Frenkel-Kontorova model
We consider generalized Frenkel-Kontorova models on higher dimensional
lattices. We show that the invariant tori which are parameterized by continuous
hull functions can be destroyed by small perturbations in the topology
with
Optical Dielectric Functions of III-V Semiconductors in Wurtzite Phase
Optical properties of semiconductors can exhibit strong polarization
dependence due to crystalline anisotropy. A number of recent experiments have
shown that the photoluminescence intensity in free standing nanowires is
polarization dependent. One contribution to this effect is the anisotropy of
the dielectric function due to the fact that most nanowires crystalize in the
wurtzite form. While little is known experimentally about the band structures
wurtzite phase III-V semiconductors, we have previously predicted the bulk band
structure of nine III-V semiconductors in wurtzite phase.Here, we predict the
frequency dependent dielectric functions for nine non-Nitride wurtzite phase
III-V semiconductors (AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs and InSb).
Their complex dielectric functions are calculated in the dipole approximation
by evaluating the momentum matrix elements on a dense grid of special k-points
using empirical pseudopotential wave functions. Corrections to the momentum
matrix elements accounting for the missing core states are made using a scaling
factor which is determined by using the optical sum rules on the calculated
dielectric functions for the zincblende polytypes. The dielectric function is
calculated for polarizations perpendicular and parallel to the c-axis of the
crystal
Predicted band structures of III-V semiconductors in wurtzite phase
While non-nitride III-V semiconductors typically have a zincblende structure,
they may also form wurtzite crystals under pressure or when grown as
nanowhiskers. This makes electronic structure calculation difficult since the
band structures of wurtzite III-V semiconductors are poorly characterized. We
have calculated the electronic band structure for nine III-V semiconductors in
the wurtzite phase using transferable empirical pseudopotentials including
spin-orbit coupling. We find that all the materials have direct gaps. Our
results differ significantly from earlier {\it ab initio} calculations, and
where experimental results are available (InP, InAs and GaAs) our calculated
band gaps are in good agreement. We tabulate energies, effective masses, and
linear and cubic Dresselhaus zero-field spin-splitting coefficients for the
zone-center states. The large zero-field spin-splitting coefficients we find
may lead to new functionalities for designing devices that manipulate spin
degrees of freedom
The effect of clouds on the earth's radiation balance
The effect of global cloudiness on the radiation balance at the top of the atmosphere is studied in general circulation model experiments. Wintertime simulations were conducted with clouds that had realistic optical properties, and were compared with simulations in which the clouds were transparent to either solar or thermal radiation. Clouds increase the net balance by limiting longwave loss to space, but decrease it by reflecting solar radiation. It is found that the net result of cloudiness is to maintain net radiation which is less than would be realized under clear conditions: Clouds cause the net radiation at the top of the atmosphere to increase due to longwave absorption, but to decrease even more due to cloud reflectance of solar radiation
Towards an optical potential for rare-earths through coupled channels
The coupled-channel theory is a natural way of treating nonelastic channels,
in particular those arising from collective excitations, defined by nuclear
deformations. Proper treatment of such excitations is often essential to the
accurate description of reaction experimental data. Previous works have applied
different models to specific nuclei with the purpose of determining
angular-integrated cross sections. In this work, we present an extensive study
of the effects of collective couplings and nuclear deformations on integrated
cross sections as well as on angular distributions in a consistent manner for
neutron-induced reactions on nuclei in the rare-earth region. This specific
subset of the nuclide chart was chosen precisely because of a clear static
deformation pattern. We analyze the convergence of the coupled-channel
calculations regarding the number of states being explicitly coupled. Inspired
by the work done by Dietrich \emph{et al.}, a model for deforming the spherical
Koning-Delaroche optical potential as function of quadrupole and hexadecupole
deformations is also proposed. We demonstrate that the obtained results of
calculations for total, elastic and inelastic cross sections, as well as
elastic and inelastic angular distributions correspond to a remarkably good
agreement with experimental data for scattering energies above around a few
MeV.Comment: 7 pages, 6 figures. Submitted to the proceedings of the XXXVI
Reuni\~ao de Trabalho de F\'{\i}sica Nuclear no Brasil (XXXVI Brazilian
Workshop on Nuclear Physics), held in Maresias, S\~ao Paulo, Brazil in
September 2013, which should be published on AIP Conference Proceeding
Series. arXiv admin note: substantial text overlap with arXiv:1311.1115,
arXiv:1311.042
Infinite index extensions of local nets and defects
Subfactor theory provides a tool to analyze and construct extensions of
Quantum Field Theories, once the latter are formulated as local nets of von
Neumann algebras. We generalize some of the results of [LR95] to the case of
extensions with infinite Jones index. This case naturally arises in physics,
the canonical examples are given by global gauge theories with respect to a
compact (non-finite) group of internal symmetries. Building on the works of
Izumi, Longo, Popa [ILP98] and Fidaleo, Isola [FI99], we consider generalized
Q-systems (of intertwiners) for a semidiscrete inclusion of properly infinite
von Neumann algebras, which generalize ordinary Q-systems introduced by Longo
[Lon94] to the infinite index case. We characterize inclusions which admit
generalized Q-systems of intertwiners and define a braided product among the
latter, hence we construct examples of QFTs with defects (phase boundaries) of
infinite index, extending the family of boundaries in the grasp of [BKLR16].Comment: 50 page
Impact of ultrafast electronic damage in single particle x-ray imaging experiments
In single particle coherent x-ray diffraction imaging experiments, performed
at x-ray free-electron lasers (XFELs), samples are exposed to intense x-ray
pulses to obtain single-shot diffraction patterns. The high intensity induces
electronic dynamics on the femtosecond time scale in the system, which can
reduce the contrast of the obtained diffraction patterns and adds an isotropic
background. We quantify the degradation of the diffraction pattern from
ultrafast electronic damage by performing simulations on a biological sample
exposed to x-ray pulses with different parameters. We find that the contrast is
substantially reduced and the background is considerably strong only if almost
all electrons are removed from their parent atoms. This happens at fluences of
at least one order of magnitude larger than provided at currently available
XFEL sources.Comment: 15 pages, 3 figures submitted to PR
Primary cell uses neither liquid nor fused electrolytes
Dry, solid state primary battery cell establishes an electrode reaction by a charge transfer mechanism without liquid phase ionization of electrolyte compounds. The charge transfer complex is sufficiently conductive to permit the passage of useful current
Electronic states in heterostructures formed by ultranarrow layers
Low-energy electronic states in heterosrtuctures formed by ultranarrow layer
(single or several monolayers thickness) are studied theoretically. The host
material is described within the effective mass approximation and effect of
ultranarrow layers is taken into account within the framework of the transfer
matrix approach. Using the current conservation requirement and the inversion
symmetry of ultranarrow layer, the transfer matrix is written through two
phenomenological parameters. The binding energy of localized state, the
reflection (transmission) coefficient for the single ultranarrow layer case,
and the energy spectrum of superlattice are determined by these parameters.
Spectral dependency of absorption in superlattice due to photoexcitation of
electrons from localized states into minibands is strongly dependent on the
ultranarrow layers characteristics. Such a dependency can be used for
verification of the transfer matrix parameters.Comment: 7 pages, 7 figure
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