220 research outputs found
Spin orientation of a two-dimensional electron gas by a high-frequency electric field
Coupling of spin states and space motion of conduction electrons due to
spin-orbit interaction opens up possibilities for manipulation of the electron
spins by electrical means. It is shown here that spin orientation of a
two-dimensional electron gas can be achieved by excitation of the carriers with
a linearly polarized high-frequency electric field. In (001)-grown quantum well
structures excitation with in-plane ac electric field induces orientation of
the electron spins along the quantum well normal, with the spin sign and the
magnitude depending on the field polarization.Comment: 5 pages, 1 figur
The Bell Theorem as a Special Case of a Theorem of Bass
The theorem of Bell states that certain results of quantum mechanics violate
inequalities that are valid for objective local random variables. We show that
the inequalities of Bell are special cases of theorems found ten years earlier
by Bass and stated in full generality by Vorob'ev. This fact implies precise
necessary and sufficient mathematical conditions for the validity of the Bell
inequalities. We show that these precise conditions differ significantly from
the definition of objective local variable spaces and as an application that
the Bell inequalities may be violated even for objective local random
variables.Comment: 15 pages, 2 figure
Generation of spin currents and spin densities in systems with reduced symmetry
We show that the spin-current response of a semiconductor crystal to an
external electric field is considerably more complex than previously assumed.
While in systems of high symmetry only the spin-Hall components are allowed, in
systems of lower symmetry other non-spin-Hall components may be present. We
argue that, when spin-orbit interactions are present only in the band
structure, the distinction between intrinsic and extrinsic contributions to the
spin current is not useful. We show that the generation of spin currents and
that of spin densities in an electric field are closely related, and that our
general theory provides a systematic way to distinguish between them in
experiment. We discuss also the meaning of vertex corrections in systems with
spin-orbit interactions.Comment: 4 page
Origin of magnetoelectric behavior in BiFeO
The magnetoelectric behavior of BiFeO has been explored on the basis of
accurate density functional calculations. The structural, electronic, magnetic,
and ferroelectric properties of BiFeO are predicted correctly without
including strong correlation effect in the calculation. Moreover, the
experimentally-observed elongation of cubic perovskite-like lattice along the
[111] direction is correctly reproduced. At high pressure we predicted a
pressure-induced structural transition and the total energy calculations at
expanded lattice show two lower energy ferroelectric phases, closer in energy
to the ground state phase. Band-structure calculations show that BiFeO will
be an insulator in A- and G-type antiferromagnetic phases and a metal in other
magnetic configurations. Chemical bonding in BiFeO has been analyzed using
various tools and electron localization function analysis shows that
stereochemically active lone-pair electrons at the Bi sites are responsible for
displacements of the Bi atoms from the centro-symmetric to the
noncentrosymmetric structure and hence the ferroelectricity. A large
ferroelectric polarization (88.7 C/cm) is predicted in accordance
with recent experimental findings. The net polarization is found to mainly (
98%) originate from Bi atoms. Moreover the large scatter in experimentally
reported polarization values is due to the large anisotropy in the spontaneous
polarization.Comment: 19 pages, 12 figures, 4 table
FIRST-PRINCIPLE STUDY OF THE TRITIUM BEHAVIOR IN A FLiBe MOLTEN SALT MIXTURE
This article presents the results of ab initio MD modeling and research of FLiBe molten salt at different temperatures and compositions. As a result, were obtained energetic characteristics of the system, made conclusions about the behavior of tritium and helium
Principals of the theory of light reflection and absorption by low-dimensional semiconductor objects in quantizing magnetic fields at monochromatic and pulse excitations
The bases of the theory of light reflection and absorption by low-dimensional
semiconductor objects (quantum wells, wires and dots) at both monochromatic and
pulse irradiations and at any form of light pulses are developed. The
semiconductor object may be placed in a stationary quantizing magnetic field.
As an example the case of normal light incidence on a quantum well surface is
considered. The width of the quantum well may be comparable to the light wave
length and number of energy levels of electronic excitations is arbitrary. For
Fourier-components of electric fields the integral equation (similar to the
Dyson-equation) and solutions of this equation for some individual cases are
obtained.Comment: 14 page
Influence of Anomalous Dispersion on Optical Characteristics of Quantum Wells
Frequency dependencies of optical characteristics (reflection, transmission
and absorption of light) of a quantum well are investigated in a vicinity of
interband resonant transitions in a case of two closely located excited energy
levels. A wide quantum well in a quantizing magnetic field directed normally to
the quantum-well plane, and monochromatic stimulating light are considered.
Distinctions between refraction coefficients of barriers and quantum well, and
a spatial dispersion of the light wave are taken into account. It is shown that
at large radiative lifetimes of excited states in comparison with nonradiative
lifetimes, the frequency dependence of the light reflection coefficient in the
vicinity of resonant interband transitions is defined basically by a curve,
similar to the curve of the anomalous dispersion of the refraction coefficient.
The contribution of this curve weakens at alignment of radiative and
nonradiative times, it is practically imperceptible at opposite ratio of
lifetimes . It is shown also that the frequency dependencies similar to the
anomalous dispersion do not arise in transmission and absorption coefficients.Comment: 10 pages, 6 figure
Symmetries of a class of nonlinear fourth order partial differential equations
In this paper we study symmetry reductions of a class of nonlinear fourth
order partial differential equations \be u_{tt} = \left(\kappa u + \gamma
u^2\right)_{xx} + u u_{xxxx} +\mu u_{xxtt}+\alpha u_x u_{xxx} + \beta u_{xx}^2,
\ee where , , , and are constants. This
equation may be thought of as a fourth order analogue of a generalization of
the Camassa-Holm equation, about which there has been considerable recent
interest. Further equation (1) is a ``Boussinesq-type'' equation which arises
as a model of vibrations of an anharmonic mass-spring chain and admits both
``compacton'' and conventional solitons. A catalogue of symmetry reductions for
equation (1) is obtained using the classical Lie method and the nonclassical
method due to Bluman and Cole. In particular we obtain several reductions using
the nonclassical method which are no} obtainable through the classical method
BAs and boride III-V alloys
Boron arsenide, the typically-ignored member of the III-V arsenide series
BAs-AlAs-GaAs-InAs is found to resemble silicon electronically: its Gamma
conduction band minimum is p-like (Gamma_15), not s-like (Gamma_1c), it has an
X_1c-like indirect band gap, and its bond charge is distributed almost equally
on the two atoms in the unit cell, exhibiting nearly perfect covalency. The
reasons for these are tracked down to the anomalously low atomic p orbital
energy in the boron and to the unusually strong s-s repulsion in BAs relative
to most other III-V compounds. We find unexpected valence band offsets of BAs
with respect to GaAs and AlAs. The valence band maximum (VBM) of BAs is
significantly higher than that of AlAs, despite the much smaller bond length of
BAs, and the VBM of GaAs is only slightly higher than in BAs. These effects
result from the unusually strong mixing of the cation and anion states at the
VBM. For the BAs-GaAs alloys, we find (i) a relatively small (~3.5 eV) and
composition-independent band gap bowing. This means that while addition of
small amounts of nitrogen to GaAs lowers the gap, addition of small amounts of
boron to GaAs raises the gap (ii) boron ``semi-localized'' states in the
conduction band (similar to those in GaN-GaAs alloys), and (iii) bulk mixing
enthalpies which are smaller than in GaN-GaAs alloys. The unique features of
boride III-V alloys offer new opportunities in band gap engineering.Comment: 18 pages, 14 figures, 6 tables, 61 references. Accepted for
publication in Phys. Rev. B. Scheduled to appear Oct. 15 200
ОСОБЕННОСТИ ФОРМИРОВАНИЯ ТОНКИХ ПЛЕНОК КРЕМНИЯ, ОСАЖДАЕМЫХ МАГНЕТРОННЫМ РАСПЫЛЕНИЕМ
The surface morphology and optical properties of Si coatings formed by magnetron sputtering were studied using atomic force microscopy, scanning electron microscopy, and spectrophotometry methods. The possibility to influence the surface morphology of coating (filamentous structures and/or round holes) and the location of maxima and minima in reflectance and transmittance via a controllable variation of magnetron sputtering regimes (substrate temperature and bias potential) is shown. Методами атомно-силовой и сканирующей электронной микроскопии, а также спектрофотометрии исследованы морфология поверхности и оптические характеристики тонких Si-покрытий, сформированных методом магнетронного распыления. Показано, что при контролируемой вариации технологических параметров магнетронного распыления таких, как температура подложки и потенциал смещения, можно менять морфологию поверхности пленок Si. Для некоторых режимов осаждения обнаружено появление на поверхности нитевидных структур и/или круглых углублений, изменения положения минимумов и максимумов в оптических спектрах отражения и пропускания.
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