60,666 research outputs found
Modeling and analysis of cascade solar cells
A brief review is given of the present status of the development of cascade solar cells. It is known that photovoltaic efficiencies can be improved through this development. The designs and calculations of the multijunction cells, however, are quite complicated. The main goal is to find a method which is a compromise between accuracy and simplicity for modeling a cascade solar cell. Three approaches are presently under way, among them (1) equivalent circuit approach, (2) numerical approach, and (3) analytical approach. Here, the first and the second approaches are discussed. The equivalent circuit approach using SPICE (Simulation Program, Integrated Circuit Emphasis) to the cascade cells and the cascade-cell array is highlighted. The methods of extracting parameters for modeling are discussed
Charged lepton mixing and oscillations from neutrino mixing in the early Universe
Charged lepton mixing as a consequence of neutrino mixing is studied for two
generations in the temperature regime in the
early Universe. We state the general criteria for charged lepton mixing,
critically reexamine aspects of neutrino equilibration and provide arguments to
suggest that neutrinos may equilibrate as mass eigenstates in the temperature
regime \emph{prior} to flavor equalization. We assume this to be the case, and
that neutrino mass eigenstates are in equilibrium with different chemical
potentials. Charged lepton self-energies are obtained to leading order in the
electromagnetic and weak interactions. The upper bounds on the neutrino
asymmetry parameters from CMB and BBN without oscillations, combined with the
fit to the solar and KamLAND data for the neutrino mixing angle, suggest that
for the two generation case there is resonant \emph{charged lepton} mixing in
the temperature range . In this range the charged lepton
oscillation frequency is of the same order as the electromagnetic damping rate.Comment: 17 pages, 2 figs, same results with more discussions on quantum Zeno
effect. To appear in Astroparticle Physic
The spatially homogeneous relativistic Boltzmann equation with a hard potential
In this paper, we study spatially homogeneous solutions of the Boltzmann
equation in special relativity and in Robertson-Walker spacetimes. We obtain an
analogue of the Povzner inequality in the relativistic case and use it to prove
global existence theorems. We show that global solutions exist for a certain
class of collision cross sections of the hard potential type in Minkowski space
and in spatially flat Robertson-Walker spacetimes.Comment: 21 page
Simulations of a classical spin system with competing superexchange and double-exchange interactions
Monte-Carlo simulations and ground-state calculations have been used to map
out the phase diagram of a system of classical spins, on a simple cubic
lattice, where nearest-neighbor pairs of spins are coupled via competing
antiferromagnetic superexchange and ferromagnetic double-exchange interactions.
For a certain range of parameters, this model is relevant for some magnetic
materials, such as doped manganites, which exhibit the remarkable colossal
magnetoresistance effect. The phase diagram includes two regions in which the
two sublattice magnetizations differ in magnitude. Spin-dynamics simulations
have been used to compute the time- and space-displaced spin-spin correlation
functions, and their Fourier transforms, which yield the dynamic structure
factor for this system. Effects of the double-exchange
interaction on the dispersion curves are shown.Comment: Latex, 3 pages, 3 figure
Does the BICEP2 Observation of Cosmological Tensor Modes Imply an Era of Nearly Planckian Energy Densities?
BICEP2 observations, interpreted most simply, suggest an era of inflation
with energy densities of order (, not far below the
Planck density. However, models of TeV gravity with large extra dimensions
might allow a very different interpretation involving much more modest energy
scales. We discuss the viability of inflation in such models, and conclude that
existing scenarios do not provide attractive alternatives to single field
inflation in four dimensions. Because the detection of tensor modes strengthens
our confidence that inflation occurred, it disfavors models of large extra
dimensions, at least for the moment.Comment: 4 pages, v3: version to appear in JHE
Effect of epitaxial strain on ferroelectric polarization in multiferroic BiFeO3 films
Multiferroic BiFeO3 epitaxial films with thickness ranging from 40 nm to 960
nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3
bottom electrodes. X-ray characterization shows that the structure evolves from
angularly-distorted tetragonal with c/a ~ 1.04 to more bulk-like distorted
rhombohedral (c/a ~ 1.01) as the strain relaxes with increasing thickness.
Despite this significant structural evolution, the ferroelectric polarization
along the body diagonal of the distorted pseudo-cubic unit cells, as calculated
from measurements along the normal direction, barely changes.Comment: Legend in Fig.3 corrected and et
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