859 research outputs found
Ferromagnetic imprinting of spin polarization in a semiconductor
We present a theory of the imprinting of the electron spin coherence and
population in an n-doped semiconductor which forms a junction with a
ferromagnet. The reflection of non-equilibrium semiconductor electrons at the
interface provides a mechanism to manipulate the spin polarization vector. In
the case of unpolarized excitation, this ballistic effect produces spontaneous
electron spin coherence and nuclear polarization in the semiconductor, as
recently observed by time-resolved Faraday rotation experiments. We investigate
the dependence of the spin reflection on the Schottky barrier height and the
doping concentration in the semiconductor and suggest control mechanisms for
possible device applications.Comment: 4 pages with 2 figure
Density-relaxation part of the self energy
A comment is made on the large-cluster limit of the self-energy correction for the quasiparticle energy gap in silicon clusters presented by Serdar Ogut, James R. Chelikowsky and Steven G. Louie in Phys. Rev. Lett. 79, 1770 (1997)
Collective oscillations driven by correlation in the nonlinear optical regime
We present an analytical and numerical study of the coherent exciton
polarization including exciton-exciton correlation. The time evolution after
excitation with ultrashort optical pulses can be divided into a slowly varying
polarization component and novel ultrafast collective modes. The frequency and
damping of the collective modes are determined by the high-frequency properties
of the retarded two-exciton correlation function, which includes Coulomb
effects beyond the mean-field approximation. The overall time evolution depends
on the low-frequency spectral behavior. The collective mode, well separated
from the slower coherent density evolution, manifests itself in the coherent
emission of a resonantly excited excitonic system, as demonstrated numerically.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Spin relaxation in low-dimensional systems
We review some of the newest findings on the spin dynamics of carriers and
excitons in GaAs/GaAlAs quantum wells. In intrinsic wells, where the optical
properties are dominated by excitonic effects, we show that exciton-exciton
interaction produces a breaking of the spin degeneracy in two-dimensional
semiconductors. In doped wells, the two spin components of an optically created
two-dimensional electron gas are well described by Fermi-Dirac distributions
with a common temperature but different chemical potentials. The rate of the
spin depolarization of the electron gas is found to be independent of the mean
electron kinetic energy but accelerated by thermal spreading of the carriers.Comment: 1 PDF file, 13 eps figures, Proceedings of the 1998 International
Workshop on Nanophysics and Electronics (NPE-98)- Lecce (Italy
Optimized Effective Potential for Extended Hubbard Model
Antiferromagnetic and charge ordered Hartree-Fock solutions of the one-band
Hubbard model with on-site and nearest-neighbor Coulomb repulsions are exactly
mapped onto an auxiliary local Kohn-Sham (KS) problem within a
density-functional theory. The mapping provides a new insight into the
interpretation of the KS equations. (i) With an appropriate choice of the basic
variable, there is a universal form of the KS potential, which is applicable
both for the antiferromagnetic and the charge ordered solutions. (ii) The
Kohn-Sham and Hartree-Fock eigenvalues are interconnected by a scaling
transformation. (iii) The band-gap problem is attributed to the derivative
discontinuity of the basic variable as the function of the electron number,
rather than a finite discontinuity of the KS potential. (iv) It is argued that
the conductivity gap and the energies of spin-wave excitations can be entirely
defined by the parameters of the ground state and the KS eigenvalues.Comment: 21 page, 3 figure
CaB_6: a new semiconducting material for spin electronics
Ferromagnetism was recently observed at unexpectedly high temperatures in
La-doped CaB_6. The starting point of all theoretical proposals to explain this
observation is a semimetallic electronic structure calculated for CaB_6 within
the local density approximation. Here we report the results of parameter-free
quasiparticle calculations of the single-particle excitation spectrum which
show that CaB_6 is not a semimetal but a semiconductor with a band gap of 0.8
eV. Magnetism in La_xCa_{1-x}B_6 occurs just on the metallic side of a Mott
transition in the La-induced impurity band.Comment: 4 pages, 1 postscript figur
Efficient total energy calculations from self-energy models
We propose a new method for calculating total energies of systems of interacting electrons, which requires little more computational resources than standard density-functional theories. The total energy is calculated within the framework of many-body perturbation theory by using an efficient model of the self-energy, that nevertheless retains the main features of the exact operator. The method shows promising performance when tested against quantum Monte Carlo results for the linear response of the homogeneous electron gas and structural properties of bulk silicon
Coulombian Disorder in Periodic Systems
We study the effect of unscreened charged impurities on periodic systems. We
show that the long wavelength component of the disorder becomes long ranged and
dominates static correlation functions. On the other hand, because of the
statistical tilt symmetry, dynamical properties such as pinning remain
unaffected. As a concrete example, we focus on the effect of Coulombian
disorder generated by charged impurities, on 3D charge density waves with non
local elasticity. We calculate the x-ray intensity and find that it is
identical to the one produced by thermal fluctuations in a disorder-free
smectic-A. We discuss the consequences of these results for experiments.Comment: 11 pages, 3 figures, revtex
Exact exchange-correlation potential of a ionic Hubbard model with a free surface
We use Lanczos exact diagonalization to compute the exact
exchange-correlation (xc) potential of a Hubbard chain with large binding
energy ("the bulk") followed by a chain with zero binding energy ("the
vacuum"). Several results of density functional theory in the continuum
(sometimes controversial) are verified in the lattice. In particular we show
explicitly that the fundamental gap is given by the gap in the Kohn-Sham
spectrum plus a contribution due to the jump of the xc-potential when a
particle is added. The presence of a staggered potential and a nearest-neighbor
interaction V allows to simulate a ionic solid. We show that in the ionic
regime in the small hopping amplitude limit the xc-contribution to the gap
equals V, while in the Mott regime it is determined by the Hubbard U
interaction. In addition we show that correlations generates a new potential
barrier at the surface
The Effective Particle-Hole Interaction and the Optical Response of Simple Metal Clusters
Following Sham and Rice [L. J. Sham, T. M. Rice, Phys. Rev. 144 (1966) 708]
the correlated motion of particle-hole pairs is studied, starting from the
general two-particle Greens function. In this way we derive a matrix equation
for eigenvalues and wave functions, respectively, of the general type of
collective excitation of a N-particle system. The interplay between excitons
and plasmons is fully described by this new set of equations. As a by-product
we obtain - at least a-posteriori - a justification for the use of the TDLDA
for simple-metal clusters.Comment: RevTeX, 15 pages, 5 figures in uufiles format, 1 figure avaible from
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