9,713 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
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 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
Degenerate ground states and nonunique potentials: breakdown and restoration of density functionals
The Hohenberg-Kohn (HK) theorem is one of the most fundamental theorems of
quantum mechanics, and constitutes the basis for the very successful
density-functional approach to inhomogeneous interacting many-particle systems.
Here we show that in formulations of density-functional theory (DFT) that
employ more than one density variable, applied to systems with a degenerate
ground state, there is a subtle loophole in the HK theorem, as all mappings
between densities, wave functions and potentials can break down. Two weaker
theorems which we prove here, the joint-degeneracy theorem and the
internal-energy theorem, restore the internal, total and exchange-correlation
energy functionals to the extent needed in applications of DFT to atomic,
molecular and solid-state physics and quantum chemistry. The joint-degeneracy
theorem constrains the nature of possible degeneracies in general many-body
systems
A realistic model of superfluidity in the neutron star inner crust
A semi-microscopic self-consistent quantum approach developed recently to
describe the inner crust structure of neutron stars within the Wigner-Seitz
(WS) method with the explicit inclusion of neutron and proton pairing
correlations is further developed. In this approach, the generalized energy
functional is used which contains the anomalous term describing the pairing. It
is constructed by matching the realistic phenomenological functional by Fayans
et al. for describing the nuclear-type cluster in the center of the WS cell
with the one calculated microscopically for neutron matter. Previously the
anomalous part of the latter was calculated within the BCS approximation. In
this work corrections to the BCS theory which are known from the many-body
theory of pairing in neutron matter are included into the energy functional in
an approximate way. These modifications have a sizable influence on the
equilibrium configuration of the inner crust, i.e. on the proton charge Z and
the radius R_c of the WS cell. The effects are quite significant in the region
where the neutron pairing gap is larger.Comment: 24 pages, 14 figures; LaTeX, submitted to EPJ
Optical response of two-dimensional electron fluids beyond the Kohn regime: strong non-parabolic confinement and intense laser light
We investigate the linear and non-linear optical response of two-dimensional
(2D) interacting electron fluids confined by a strong non-parabolic potential.
We show that such fluids may exhibit higher-harmonic spectra under realistic
experimental conditions. Higher harmonics arise as the electrons explore
anharmonicities of the confinement potential (electron-electron interactions
reduce this non-linear effect). This opens the possibility of controlling the
optical functionality of such systems by engineering the confinement potential.
Our results were obtained within time-dependent density-functional theory,
employing the adiabatic local-density approximation. A classical hydrodynamical
model is in good agreement with the quantum-mechanical results.Comment: 4 pages, 4 figure
Band mass anisotropy and the intrinsic metric of fractional quantum Hall systems
It was recently pointed out that topological liquid phases arising in the
fractional quantum Hall effect (FQHE) are not required to be rotationally
invariant, as most variational wavefunctions proposed to date have been.
Instead, they possess a geometric degree of freedom corresponding to a shear
deformation that acts like an intrinsic metric. We apply this idea to a system
with an anisotropic band mass, as is intrinsically the case in many-valley
semiconductors such as AlAs and Si, or in isotropic systems like GaAs in the
presence of a tilted magnetic field, which breaks the rotational invariance. We
perform exact diagonalization calculations with periodic boundary conditions
(torus geometry) for various filling fractions in the lowest, first and second
Landau levels. In the lowest Landau level, we demonstrate that FQHE states
generally survive the breakdown of rotational invariance by moderate values of
the band mass anisotropy. At 1/3 filling, we generate a variational family of
Laughlin wavefunctions parametrized by the metric degree of freedom. We show
that the intrinsic metric of the Laughlin state adjusts as the band mass
anisotropy or the dielectric tensor are varied, while the phase remains robust.
In the n=1 Landau level, mass anisotropy drives transitions between
incompressible liquids and compressible states with charge density wave
ordering. In n>=2 Landau levels, mass anisotropy selects and enhances stripe
ordering with compatible wave vectors at partial 1/3 and 1/2 fillings.Comment: 9 pages, 8 figure
Physical mechanisms of interface-mediated intervalley coupling in Si
The conduction band degeneracy in Si is detrimental to quantum computing
based on spin qubits, for which a nondegenerate ground orbital state is
desirable. This degeneracy is lifted at an interface with an insulator as the
spatially abrupt change in the conduction band minimum leads to intervalley
scattering. We present a theoretical study of the interface-induced valley
splitting in Si that provides simple criteria for optimal fabrication
parameters to maximize this splitting. Our work emphasizes the relevance of
different interface-related properties to the valley splitting.Comment: 4 pages, revised versio
External field control of donor electron exchange at the Si/SiO2 interface
We analyze several important issues for the single- and two-qubit operations
in Si quantum computer architectures involving P donors close to a SiO2
interface. For a single donor, we investigate the donor-bound electron
manipulation (i.e. 1-qubit operation) between the donor and the interface by
electric and magnetic fields. We establish conditions to keep a donor-bound
state at the interface in the absence of local surface gates, and estimate the
maximum planar density of donors allowed to avoid the formation of a
2-dimensional electron gas at the interface. We also calculate the times
involved in single electron shuttling between the donor and the interface. For
a donor pair, we find that under certain conditions the exchange coupling (i.e.
2-qubit operation) between the respective electron pair at the interface may be
of the same order of magnitude as the coupling in GaAs-based two-electron
double quantum dots where coherent spin manipulation and control has been
recently demonstrated (for example for donors ~10 nm below the interface and
\~40 nm apart, J~10^{-4} meV), opening the perspective for similar experiments
to be performed in Si.Comment: 11 pages, 15 figures. Changes in Eq. 24 plus minor typo
Anomalous Mesozoic thermal regime, central Appalachian Piedmont: Evidence from sphene and zircon fission-track dating
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