71,476 research outputs found
Beyond time-dependent exact-exchange: the need for long-range correlation
In the description of the interaction between electrons beyond the classical
Hartree picture, bare exchange often yields a leading contribution. Here we
discuss its effect on optical spectra of solids, comparing three different
frameworks: time-dependent Hartree-Fock, a recently introduced combined
density-functional and Green's functions approach applied to the bare exchange
self-energy, and time-dependent exact-exchange within time-dependent
density-functional theory (TD-EXX). We show that these three approximations
give rise to identical excitonic effects in solids; these effects are
drastically overestimated for semiconductors. They are partially compensated by
the usual overestimation of the quasiparticle band gap within Hartree-Fock. The
physics that lacks in these approaches can be formulated as screening. We show
that the introduction of screening in TD-EXX indeed leads to a formulation that
is equivalent to previously proposed functionals derived from Many-Body
Perturbation Theory. It can be simulated by reducing the long-range part of the
Coulomb interaction: this produces absorption spectra of semiconductors in good
agreement with experiment.Comment: 12 pages, 3 figures, 1 tabl
Approximating the coefficients in semilinear stochastic partial differential equations
We investigate, in the setting of UMD Banach spaces E, the continuous
dependence on the data A, F, G and X_0 of mild solutions of semilinear
stochastic evolution equations with multiplicative noise of the form dX(t) =
[AX(t) + F(t,X(t))]dt + G(t,X(t))dW_H(t), X(0)=X_0, where W_H is a cylindrical
Brownian motion on a Hilbert space H. We prove continuous dependence of the
compensated solutions X(t)-e^{tA}X_0 in the norms
L^p(\Omega;C^\lambda([0,T];E)) assuming that the approximating operators A_n
are uniformly sectorial and converge to A in the strong resolvent sense, and
that the approximating nonlinearities F_n and G_n are uniformly Lipschitz
continuous in suitable norms and converge to F and G pointwise. Our results are
applied to a class of semilinear parabolic SPDEs with finite-dimensional
multiplicative noise.Comment: Referee's comments have been incorporate
Ab initio study of reflectance anisotropy spectra of a sub-monolayer oxidized Si(100) surface
The effects of oxygen adsorption on the reflectance anisotropy spectrum (RAS)
of reconstructed Si(100):O surfaces at sub-monolayer coverage (first stages of
oxidation) have been studied by an ab initio DFT-LDA scheme within a
plane-wave, norm-conserving pseudopotential approach. Dangling bonds and the
main features of the characteristic RAS of the clean Si(100) surface are mostly
preserved after oxidation of 50% of the surface dimers, with some visible
changes: a small red shift of the first peak, and the appearance of a distinct
spectral structure at about 1.5 eV. The electronic transitions involved in the
latter have been analyzed through state-by-state and layer-by-layer
decompositions of the RAS. We suggest that new interplay between present
theoretical results and reflectance anisotropy spectroscopy experiments could
lead to further clarification of structural and kinetic details of the Si(100)
oxidation process in the sub-monolayer range.Comment: 21 pages, 8 figures. To be published in Physical Rev.
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An empirical study of unfair terms in online auction contracts in the UK: Evidence for the need for better enforcement mechanisms
This paper studies the terms of 28 online auction sites. It uncovers that, in this industry, unfair terms are common. The paper focusses on a small number of clauses but conclusively shows that enforcement in the UK is insufficient. The reasons for this insufficiency are explored and solutions proposed
On the density of systems of non-linear spatially homogeneous SPDEs
In this paper, we consider a system of second order non-linear stochastic
partial differential equations with spatial dimension , driven by a
-dimensional Gaussian noise, which is white in time and with some spatially
homogeneous covariance. The case of a single equation and a one-dimensional
noise, has largely been studied in the literature. The first aim of this paper
is to give a survey of some of the existing results. We will start with the
existence, uniqueness and H\"older's continuity of the solution. For this, the
extension of Walsh's stochastic integral to cover some measure-valued
integrands will be recalled. We will then recall the results concerning the
existence and smoothness of the density, as well as its strict positivity,
which are obtained using techniques of Malliavin calculus. The second aim of
this paper is to show how these results extend to our system of SPDEs. In
particular, we give sufficient conditions in order to have existence and
smoothness of the density on the set where the columns of the diffusion matrix
span . We then prove that the density is strictly positive in a point if
the connected component of the set where the columns of the diffusion matrix
span which contains this point has a non void intersection with the
support of the law of the solution. We will finally check how all these results
apply to the case of the stochastic heat equation in any space dimension and
the stochastic wave equation in dimension
Random-field Solutions to Linear Hyperbolic Stochastic Partial Differential Equations with Variable Coefficients
In this article we show the existence of a random-field solution to linear
stochastic partial differential equations whose partial differential operator
is hyperbolic and has variable coefficients that may depend on the temporal and
spatial argument. The main tools for this, pseudo-differential and Fourier
integral operators, come from microlocal analysis. The equations that we treat
are second-order and higher-order strictly hyperbolic, and second-order weakly
hyperbolic with uniformly bounded coefficients in space. For the latter one we
show that a stronger assumption on the correlation measure of the random noise
might be needed. Moreover, we show that the well-known case of the stochastic
wave equation can be embedded into the theory presented in this article.Comment: 40 pages, final version, Stochastic Processes and their Applications
(2017
Simultaneous description of four positive and four negative parity bands
The extended coherent state model is further extended in order to describe
two dipole bands of different parities. The formalism provides a consistent
description of eight rotational bands. A unified description for spherical,
transitional and deformed nuclei is possible. Projecting out the angular
momentum and parity from a sole state, the band acquires a
magnetic character, while the electric properties prevail for the other band.
Signatures for a static octupole deformation in some states of the dipole bands
are pointed out. Some properties which distinguish between the dipole band
states and states of the same parity but belonging to other bands are
mentioned. Interesting features concerning the decay properties of the two
bands are found. Numerical applications are made for Gd, Yb,
Th, Ra, U and Pu, and the results are
compared with the available data.Comment: 36 pages, 13 figures, 12 table
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