674 research outputs found
Spin separation in digital ferromagnetic heterostructures
In a study of the ferromagnetic phase of a multilayer digital ferromagnetic
semiconductor in the mean-field and effective-mass approximations, we find the
exchange interaction to have the dominant energy scale of the problem,
effectively controlling the spatial distribution of the carrier spins in the
digital ferromagnetic heterostructures. In the ferromagnetic phase, the
majority and minority carriers tend to be in different regions of the space
(spin separation). Hence, the charge distribution of carriers also changes
noticeably from the ferromagnetic to the paramagnetic phase. An example of a
design to exploit these phenomena is given.Comment: 4 pages, 3 figures. Submitted to Phys. Rev.
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
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
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
Spin relaxation of conduction electrons in bulk III-V semiconductors
Spin relaxation time of conduction electrons through the Elliot-Yafet,
D'yakonov-Perel and Bir-Aronov-Pikus mechanisms is calculated theoretically for
bulk GaAs, GaSb, InAs and InSb of both - and -type. Relative importance
of each spin relaxation mechanism is compared and the diagrams showing the
dominant mechanism are constructed as a function of temperature and impurity
concentrations. Our approach is based upon theoretical calculation of the
momentum relaxation rate and allows understanding of the interplay between
various factors affecting the spin relaxation over a broad range of temperature
and impurity concentration.Comment: an error in earlier version correcte
Theoretical Study of Cubic Structures Based on Fullerene Carbon Clusters: CC and (C
We study a new hypothetical form of solid carbon \csc, with a unit cell which
is composed of the \cs \ fullerene cluster and an additional single carbon atom
arranged in the zincblende structure. Using {\it ab initio} calculations, we
show that this new form of solid carbon has lower energy than hyperdiamond, the
recently proposed form composed of \cs \ units in the diamond structure. To
understand the bonding character of of these cluster-based solids, we analyze
the electronic structure of \csc \ and of hyperdiamond and compare them to the
electronic states of crystalline cubic diamond.Comment: 15 pages, latex, no figure
Nitrogen-Functionalized Graphene Nanoflakes (GNFs:N): Tunable Photoluminescence and Electronic Structures
This study investigates the strong photoluminescence (PL) and X-ray excited
optical luminescence observed in nitrogen-functionalized 2D graphene nanoflakes
(GNFs:N), which arise from the significantly enhanced density of states in the
region of {\pi} states and the gap between {\pi} and {\pi}* states. The
increase in the number of the sp2 clusters in the form of pyridine-like N-C,
graphite-N-like, and the C=O bonding and the resonant energy transfer from the
N and O atoms to the sp2 clusters were found to be responsible for the blue
shift and the enhancement of the main PL emission feature. The enhanced PL is
strongly related to the induced changes of the electronic structures and
bonding properties, which were revealed by the X-ray absorption near-edge
structure, X-ray emission spectroscopy, and resonance inelastic X-ray
scattering. The study demonstrates that PL emission can be tailored through
appropriate tuning of the nitrogen and oxygen contents in GNFs and pave the way
for new optoelectronic devices.Comment: 8 pages, 6 figures (including toc figure
Sustained elevation of Epstein–Barr virus antibody levels preceding clinical onset of nasopharyngeal carcinoma
We have monitored Epstein–Barr virus (EBV) IgA antibody levels of 39 nasopharyngeal carcinoma (NPC) cases for up to 15 years before clinical onset of NPC, and assessed preclinical serologic status of another 68 cases. Our results identify a serologic window preceding diagnosis when antibody levels are raised and sustained. This window can persist for as long as 10 years, with a mean duration estimated to as 37±28 months. Ninety-seven of these 107 NPC cases exhibited such a window. Cases that did not may reflect individual antibody response to EBV. Serologic screening at enrollment identified those cases who had already entered the window and became clinically manifested earlier (median=28 months) than those who entered the window after enrollment (median=90 months). The former account for 19 of 21 cases diagnosed within 2 years of screening. Nasopharyngeal carcinoma risk levels among seropositive subjects were also highest during this period. Both prediction rates and risk levels declined thereafter; cases detected at later times were composed of increasing proportions of individuals who entered the serological window after screening. Our findings establish EBV antibody as an early marker of NPC and suggest that repeated screening to monitor cases as they enter this window has considerable predictive value, with practical consequences for cancer treatment
Exact Kohn-Sham exchange kernel for insulators and its long-wavelength behavior
We present an exact expression for the frequency-dependent Kohn-Sham
exact-exchange (EXX) kernel for periodic insulators, which can be employed for
the calculation of electronic response properties within time-dependent (TD)
density-functional theory. It is shown that the EXX kernel has a
long-wavelength divergence behavior of the exact full exchange-correlation
kernel and thus rectifies one serious shortcoming of the adiabatic
local-density approximation and generalized-gradient approximations kernels. A
comparison between the TDEXX and the GW-approximation-Bethe-Salpeter-equation
approach is also made.Comment: two column format 6 pages + 1 figure, to be publisehd in Physical
Review
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