6,692 research outputs found
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
Spectrometer for Hard X-Ray Free Electron Laser Based on Diffraction Focusing
X-ray free electron lasers (XFELs) generate sequences of ultra-short,
spatially coherent pulses of x-ray radiation. We propose the diffraction
focusing spectrometer (DFS), which is able to measure the whole energy spectrum
of the radiation of a single XFEL pulse with an energy resolution of . This is much better than for most modern x-ray
spectrometers. Such resolution allows one to resolve the fine spectral
structure of the XFEL pulse. The effect of diffraction focusing occurs in a
single crystal plate due to dynamical scattering, and is similar to focusing in
a Pendry lens made from the metamaterial with a negative refraction index. Such
a spectrometer is easier to operate than those based on bent crystals. We show
that the DFS can be used in a wide energy range from 5 keV to 20 keV.Comment: 9 pages, 8 figures, 2 table
Thermal analysis of submicron nanocrystalline diamond films
The thermal properties of sub-μm nanocrystalline diamond films in the range of 0.37–1.1 μm grown by hot filament CVD, initiated by bias enhanced nucleation on a nm-thin Si-nucleation layer on various substrates, have been characterized by scanning thermal microscopy. After coalescence, the films have been outgrown with a columnar grain structure. The results indicate that even in the sub-μm range, the average thermal conductivity of these NCD films approaches 400 W m− 1 K− 1. By patterning the films into membranes and step-like mesas, the lateral component and the vertical component of the thermal conductivity, k<sub>lateral</sub> and k<sub>vertical</sub>, have been isolated showing an anisotropy between vertical conduction along the columns, with k<sub>vertical</sub> ≈ 1000 W m− 1 K− 1, and a weaker lateral conduction across the columns, with k<sub>lateral</sub> ≈ 300 W m− 1 K− 1
The range of the tangential Cauchy-Riemann system on a CR embedded manifold
We prove that every compact, pseudoconvex, orientable, CR manifold of \C^n,
bounds a complex manifold in the sense. In particular, the
tangential Cauchy-Riemann system has closed range
Effective mass theory of monolayer \delta-doping in the high-density limit
Monolayer \delta-doped structures in silicon have attracted renewed interest
with their recent incorporation into atomic-scale device fabrication strategies
as source and drain electrodes and in-plane gates. Modeling the physics of
\delta-doping at this scale proves challenging, however, due to the large
computational overhead associated with ab initio and atomistic methods. Here,
we develop an analytical theory based on an effective mass approximation. We
specifically consider the Si:P materials system, and the limit of high donor
density, which has been the subject of recent experiments. In this case,
metallic behavior including screening tends to smooth out the local disorder
potential associated with random dopant placement. While smooth potentials may
be difficult to incorporate into microscopic, single-electron analyses, the
problem is easily treated in the effective mass theory by means of a jellium
approximation for the ionic charge. We then go beyond the analytic model,
incorporating exchange and correlation effects within a simple numerical model.
We argue that such an approach is appropriate for describing realistic,
high-density, highly disordered devices, providing results comparable to
density functional theory, but with greater intuitive appeal, and lower
computational effort. We investigate valley coupling in these structures,
finding that valley splitting in the low-lying \Gamma band grows much more
quickly than the \Gamma-\Delta band splitting at high densities. We also find
that many-body exchange and correlation corrections affect the valley splitting
more strongly than they affect the band splitting
Donor Electron Wave Functions for Phosphorus in Silicon: Beyond Effective Mass Theory
We calculate the electronic wave-function for a phosphorus donor in silicon
by numerical diagonalisation of the donor Hamiltonian in the basis of the pure
crystal Bloch functions. The Hamiltonian is calculated at discrete points
localised around the conduction band minima in the reciprocal lattice space.
Such a technique goes beyond the approximations inherent in the effective-mass
theory, and can be modified to include the effects of altered donor impurity
potentials, externally applied electro-static potentials, as well as the
effects of lattice strain. Modification of the donor impurity potential allows
the experimentally known low-lying energy spectrum to be reproduced with good
agreement, as well as the calculation of the donor wavefunction, which can then
be used to calculate parameters important to quantum computing applications.Comment: 10 pages, 5 figure
Exact-exchange density-functional theory for quasi-two-dimensional electron gases
A simple exact-exchange density-functional method for a quasi-two-dimensional
electron gas with variable density is presented. An analytical expression for
the exact-exchange potential with only one occupied subband is provided,
without approximations. When more subbands are occupied the exact-exchange
potential is obtained numerically. The theory shows that, in contradiction with
LDA, the exact-exchange potential exhibits discontinuities and the system
suffers a zero-temperature first-order transition each time a subband is
occupied. Results suggesting that the translational symmetry might be
spontaneously broken at zero temperature are presented. An extension of the
theory to finite temperatures allows to describe a drop in the intersubband
spacing in good quantitative agreement with recent experiments.Comment: 14 pages, 3 figure
The Taylor Estimate of Recoverable Strains in Shape-Memory Polycrystals
Shape-memory behavior ls the ability of ccrwin materials to recover, on heating, apparently plastic deformation sustained below a critical temperature. Some materials have good shape-memory behavior as single crystals but little or none as polycrystals, while others have good shape-memory behavior even as polycrystals. Bhattacharya and Kohn (1996. 1997) have proposed a framework to understand this difference. They use energy minimization and the Taylor estimate to argue that the recoverable strains in a polycrystal depend not only on the texture of the polycrystal and the transformation, but critically on the change in symmetry during the underlying martensitic phase transformation. Their results agree with the experimental observations. Shu and Bhattacharya (1997) have also used the
Taylor estimate to study the effect of texture in polycrys- tals of Nickel-Titanium and Copper based shape-memory alloys. The use of the Taylor estimate was evaluated in some detail in Bhattacharya and Kohn ( 1997) and more recently in Shu and Bhattacharya (1997) and Shu (1997). In this short report, we summarize the model of recoverable strain and discuss some results that allow us to evaluate the Taylor estimate
Multiscale simulations in simple metals: a density-functional based methodology
We present a formalism for coupling a density functional theory-based quantum
simulation to a classical simulation for the treatment of simple metallic
systems. The formalism is applicable to multiscale simulations in which the
part of the system requiring quantum-mechanical treatment is spatially confined
to a small region. Such situations often arise in physical systems where
chemical interactions in a small region can affect the macroscopic mechanical
properties of a metal. We describe how this coupled treatment can be
accomplished efficiently, and we present a coupled simulation for a bulk
aluminum system.Comment: 15 pages, 7 figure
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