27,281 research outputs found
Polarimetry at the ILC
At the ILC, the luminosity-weighted average polarization at the IP needs to
be determined at the permille-level. In order to reach this goal, the combined
information from the polarimeter and the collision data is required. In this
study, a unified approach will be presented, which for the first time combines
the cross section measurements with the expected constraints from the
polarimeters. Hereby, the statistical and systematical uncertainties are taken
into account, including their correlations. This study shows that a fast spin
flip frequency is required because it easily reduces the systematic
uncertainty, while a non-perfect helicity reversal can be compensated for
within the unified approach. The final goal is to provide a realistic
estimation of the luminosity-weighted average polarization at the IP to be used
in the physic analyses.Comment: 8 pages, conference proceedings LCWS 2016 Morioka Japa
Illumination strategies for intensity-only imaging
We propose a new strategy for narrow band, active array imaging of localized
scat- terers when only the intensities are recorded and measured at the array.
We consider a homogeneous medium so that wave propagation is fully coherent. We
show that imaging with intensity-only measurements can be carried out using the
time reversal operator of the imaging system, which can be obtained from
intensity measurements using an appropriate illumination strategy and the
polarization identity. Once the time reversal operator has been obtained, we
show that the images can be formed using its singular value decomposition
(SVD). We use two SVD-based methods to image the scatterers. The proposed
approach is simple and efficient. It does not need prior information about the
sought image, and guarantees exact recovery in the noise-free case.
Furthermore, it is robust with respect to additive noise. Detailed numerical
simulations illustrate the performance of the proposed imaging strategy when
only the intensities are captured
Robust determination of maximally-localized Wannier functions
We propose an algorithm to determine Maximally Localized Wannier Functions
(MLWFs). This algorithm, based on recent theoretical developments, does not
require any physical input such as initial guesses for the Wannier functions,
unlike popular schemes based on the projection method. We discuss how the
projection method can fail on fine grids when the initial guesses are too far
from MLWFs. We demonstrate that our algorithm is able to find localized Wannier
functions through tests on two-dimensional systems, simplified models of
semiconductors, and realistic DFT systems by interfacing with the Wannier90
code. We also test our algorithm on the Haldane and Kane-Mele models to examine
how it fails in the presence of topological obstructions
Anharmonic free energies and phonon dispersions from the stochastic self-consistent harmonic approximation: application to platinum and palladium hydrides
Harmonic calculations based on density-functional theory are generally the
method of choice for the description of phonon spectra of metals and
insulators. The inclusion of anharmonic effects is, however, delicate as it
relies on perturbation theory requiring a considerable amount of computer time,
fast increasing with the cell size. Furthermore, perturbation theory breaks
down when the harmonic solution is dynamically unstable or the anharmonic
correction of the phonon energies is larger than the harmonic frequencies
themselves.We present a stochastic implementation of the self-consistent
harmonic approximation valid to treat anharmonicity at any temperature in the
non-perturbative regime. The method is based on the minimization of the free
energy with respect to a trial density matrix described by an arbitrary
harmonic Hamiltonian. The minimization is performed with respect to all the
free parameters in the trial harmonic Hamiltonian, namely, equilibrium
positions, phonon frequencies and polarization vectors. The gradient of the
free energy is calculated following a stochastic procedure. The method can be
used to calculate thermodynamic properties, dynamical properties and anharmonic
corrections to the Eliashberg function of the electron-phonon coupling. The
scaling with the system size is greatly improved with respect to perturbation
theory. The validity of the method is demonstrated in the strongly anharmonic
palladium and platinum hydrides. In both cases we predict a strong anharmonic
correction to the harmonic phonon spectra, far beyond the perturbative limit.
In palladium hydrides we calculate thermodynamic properties beyond the
quasiharmonic approximation, while in PtH we demonstrate that the high
superconducting critical temperatures at 100 GPa predicted in previous
calculations based on the harmonic approximation are strongly suppressed when
anharmonic effects are included.Comment: 17 pages, 5 figure
Charge reversal of colloidal particles
A theory is presented for the effective charge of colloidal particles in
suspensions containing multivalent counterions. It is shown that if colloids
are sufficiently strongly charged, the number of condensed multivalent
counterion can exceed the bare colloidal charge leading to charge reversal.
Charge renormalization in suspensions with multivalent counterions depends on a
subtle interplay between the solvation energies of the multivalent counterions
in the bulk and near the colloidal surface. We find that the effective charge
is {\it not} a monotonically decreasing function of the multivalent salt
concentration. Furthermore, contrary to the previous theories, it is found that
except at very low concentrations, monovalent salt hinders the charge reversal.
This conclusion is in agreement with the recent experiments and simulations
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