31,769 research outputs found
Ionization of hydrogen atoms by electron impact at 1eV, 0.5eV and 0.3eV above threshold
We present here triple differential cross sections for ionization of hydrogen
atoms by electron impact at 1eV, 0.5eV and 0.3eV energy above threshold,
calculated in the hyperspherical partial wave theory. The results are in very
good agreement with the available semiclassical results of Deb and Crothers
\cite{DC02} for these energies. With this, we are able to demonstrate that the
hyperspherical partial wave theory yields good cross sections from 30 eV
\cite{DPC03} down to near threshold for equal energy sharing kinematics.Comment: 6 pages, 9 figure
Adiabatic quantum pumping in an Aharonov-Bohm loop and in a Si-like nanowire: Role of interference in real space and in momentum space
We study the consequences of interference effects on the current generated by
adiabatic quantum pumping in two distinct one-dimensional (1D) lattice model.
The first model contains an Aharonov-Bohm (AB) loop within a tight-binding
chain of lattice sites. The static AB phase is shown to strongly affect
interference between the two arms of the loop, serving as an on-off switch and
regulator for the pumped current. The second model simulates pumping in
semiconductors with indirect band-gaps, by utilizing a tight-binding chain with
next-nearest-neighbor coupling. The model exhibits signatures of interference
between degenerate conduction band states with different Fermi wavevectors.Comment: 7 pages, 7 figure
Mean squared error of empirical predictor
The term ``empirical predictor'' refers to a two-stage predictor of a linear
combination of fixed and random effects. In the first stage, a predictor is
obtained but it involves unknown parameters; thus, in the second stage, the
unknown parameters are replaced by their estimators. In this paper, we consider
mean squared errors (MSE) of empirical predictors under a general setup, where
ML or REML estimators are used for the second stage. We obtain second-order
approximation to the MSE as well as an estimator of the MSE correct to the same
order. The general results are applied to mixed linear models to obtain a
second-order approximation to the MSE of the empirical best linear unbiased
predictor (EBLUP) of a linear mixed effect and an estimator of the MSE of EBLUP
whose bias is correct to second order. The general mixed linear model includes
the mixed ANOVA model and the longitudinal model as special cases
M\"{o}ller and Bhabha scattering in the noncommutative standard model
We study the M\"{o}ller and Bhabha scattering in the noncommutative extension
of the standard model(SM) using the Seiberg-Witten maps of this to first order
of the noncommutative parameter . We look at the angular
distribution to explore the noncommutativity of space-time at
around TeV and find that the distribution deviates
significantly from the one obtained from the commutative version of the
standard model.Comment: 15 pages, 14 eps figures.Text is modified a little and version to
appear in Phys.Rev.
Transport properties of diluted magnetic semiconductors: Dynamical mean field theory and Boltzmann theory
The transport properties of diluted magnetic semiconductors (DMS) are
calculated using dynamical mean field theory (DMFT) and Boltzmann transport
theory. Within DMFT we study the density of states and the dc-resistivity,
which are strongly parameter dependent such as temperature, doping, density of
the carriers, and the strength of the carrier-local impurity spin exchange
coupling. Characteristic qualitative features are found distinguishing weak,
intermediate, and strong carrier-spin coupling and allowing quantitative
determination of important parameters defining the underlying ferromagnetic
mechanism. We find that spin-disorder scattering, formation of bound state, and
the population of the minority spin band are all operational in DMFT in
different parameter range. We also develop a complementary Boltzmann transport
theory for scattering by screened ionized impurities. The difference in the
screening properties between paramagnetic () and ferromagnetic ()
states gives rise to the temperature dependence (increase or decrease) of
resistivity, depending on the carrier density, as the system goes from the
paramagnetic phase to the ferromagnetic phase. The metallic behavior below
for optimally doped DMS samples can be explained in the Boltzmann theory
by temperature dependent screening and thermal change of carrier spin
polarization.Comment: 15 pages, 15 figure
Controlled Flow of Spin-Entangled Electrons via Adiabatic Quantum Pumping
We propose a method to dynamically generate and control the flow of
spin-entangled electrons, each belonging to a spin-singlet, by means of
adiabatic quantum pumping. The pumping cycle functions by periodic time
variation of localized two-body interactions. We develop a generalized approach
to adiabatic quantum pumping as traditional methods based on scattering matrix
in one dimension cannot be applied here. We specifically compute the flow of
spin-entangled electrons within a Hubbard-like model of quantum dots, and
discuss possible implementations and identify parameters that can be used to
control the singlet flow.Comment: 4 pages, 3 figure
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