12,741 research outputs found
Edge Electron Gas
The uniform electron gas, the traditional starting point for density-based
many-body theories of inhomogeneous systems, is inappropriate near electronic
edges. In its place we put forward the appropriate concept of the edge electron
gas.Comment: 4 pages RevTex with 7 ps-figures included. Minor changes in
title,text and figure
Resistivity and optical conductivity of cuprates within the t-J model
The optical conductivity and the d.c. resistivity
within the extended t-J model on a square lattice, as relevant to high-
cuprates, are reinvestigated using the exact-diagonalization method for small
systems, improved by performing a twisted boundary condition averaging. The
influence of the next-nearest-neighbor hopping is also considered. The
behaviour of results at intermediate doping is consistent with a
marginal-Fermi-liquid scenario and in the case of for follows
the power law with consistent
with experiments. At low doping for develops a
shoulder at , consistent with the observed mid-infrared
peak in experiments, accompanied by a shallow dip for . This
region is characterized by the resistivity saturation, whereas a more coherent
transport appears at producing a more pronounced decrease in
. The behavior of the normalized resistivity is within a
factor of 2 quantitatively consistent with experiments in cuprates.Comment: 8 pages, 10 figure
Theory of the Stark Effect for P donors in Si
We develop a multi-valley effective mass theory for substitutional donors in
silicon in an inhomogeneous environment. Valley-orbit coupling is treated
perturbatively. We apply the theory to the Stark effect in Si:P. The method
becomes more accurate at high fields, and it is designed to give correct
experimental binding energies at zero field. Unexpectedly, the ground state
energy for the donor electron is found to increase with electric field as a
consequence of spectrum narrowing of the 1s manifold. Our results are of
particular importance for the Kane quantum computer.Comment: published versio
A unified approach to nonlinearity, structural change and outliers
This paper demonstrates that the class of conditionally linear and Gaussianstate-space models offers a general and convenient framework for simultaneouslyhandling nonlinearity, structural change and outliers in time series. Manypopular nonlinear time series models, including threshold, smooth transitionand Markov-Switching models, can be written in state-space form. It is thenstraightforward to add components that capture parameter instability andintervention effects. We advocate a Bayesian approach to estimation andinference, using an efficient implementation of Markov Chain Monte Carlosampling schemes for such linear dynamic mixture models. The general modellingframework and the Bayesian methodology are illustrated by means of severalexamples. An application to quarterly industrial production growth rates forthe G7 countries demonstrates the empirical usefulness of the approach.Bayesian inference;threshold models;Markov-switching models;business cycle asymmetry;state-space models
Lattice deformations at martensite-martensite interfaces in Ni-Al
The atomic configurations at macrotwin interfaces between microtwinned martensite plates in material are investigated using high resolution transmission electron microscopy (HRTEM). The observed structures are interpreted in view of possible formation mechanisms of these interfaces. A distinction is made between cases in which the microtwins, originating from mutually perpendicular \{110\} austenite planes, enclose a final angle larger or smaller than , measured over the boundary. Two different configurations, one with crossing microtwins and the other with ending microtwins producing a step configuration are described. The latter is related with the existence of microtwin sequences with changing variant widths. Although both features appear irrespective of the material’s preparation technique, rapid solidification seems to prefer the step configuration. Depending on the actual case, tapering, bending and tip splitting of the small microtwin variants is observed. Sever lattice deformations and reorientations occur in a region of 5 – 10 nm around the interface while sequences of single plane ledges gradually bending the microtwins are found up to 50 nm away form the interface. These structures and deformations are interpreted in view of the need to accommodate any remaining stresses
Generalization of the density-matrix method to a non-orthogonal basis
We present a generalization of the Li, Nunes and Vanderbilt density-matrix
method to the case of a non-orthogonal set of basis functions. A representation
of the real-space density matrix is chosen in such a way that only the overlap
matrix, and not its inverse, appears in the energy functional. The generalized
energy functional is shown to be variational with respect to the elements of
the density matrix, which typically remains well localized.Comment: 11 pages + 2 postcript figures at the end (search for -cut here
The Decay Properties of the Finite Temperature Density Matrix in Metals
Using ordinary Fourier analysis, the asymptotic decay behavior of the density
matrix F(r,r') is derived for the case of a metal at a finite electronic
temperature. An oscillatory behavior which is damped exponentially with
increasing distance between r and r' is found. The decay rate is not only
determined by the electronic temperature, but also by the Fermi energy. The
theoretical predictions are confirmed by numerical simulations
Chat and instant messaging : the risks of secondary orality
The synchronous nature of chat and instant messaging (IM) make them unique among computer-enabled communications technologies in that their real-time exchange of data allows for rich media experiences, even though users can only use text symbols to trade messages. Chat and IM are also important in that they enable secondary orality, or the merger of the most beneficial aspects of orally-based cultures with the well-documented benefits of print and text. Where print in the modem day has fostered contemplative behavior and inward thought among human beings, chat and IM breathe vitality into print and, in a sense, allow print to be spoken. Chat and IM have provided well-documented benefits for business, academia and everyday human socialization. However, when the tools are used beyond these narrow contexts they not only lose their effectiveness; they also pose credible threats to society. Because chat and IM provide anonymity to their participants, the virtual communities they support are typically loosely governed, driven by stereotype, and replete with social deviance. Further, the more attractive online environments become, the less time and energy people will invest in the physical world, thereby threatening that the habitats of humans will ultimately wither and decay. Finally, as humans become less able to extricate themselves from their computer-enabled habitats, they will increasingly rely on the computer as a social prosthetic--if not evolve to the point where human beings and computers become indistinguishable
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