244 research outputs found
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
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
Surface Analysis of OFE-Copper X-Band Accelerating Structures and Possible Correlation to RF Breakdown Events
X-band accelerator structures meeting the Next Linear Collider (NLC) design
requirements have been found to suffer vacuum surface damage caused by radio
frequency (RF) breakdown, when processed to high electric-field gradients.
Improved understanding of these breakdown events is desirable for the
development of structure designs, fabrication procedures, and processing
techniques that minimize structure damage. RF reflected wave analysis and
acoustic sensor pickup have provided breakdowns localization in RF structures.
Particle contaminations found following clean autopsy of four RF-processed
travelling wave structures, have been catalogued and analyzed. Their influence
on RF breakdown, as well as that of several other material-based properties,
will be discussed.Comment: 21 pages, 8 figures, 4 tables, Submitted to JVST A as a proceeding of
the 50th AVS conference (Baltimore, MD, 2-7 Nov 2003
On the nature of steady states of spin distributions in the presence of spin-orbit interactions
In the presence of spin-orbit interactions, the steady state established for
spin distributions in an electric field is qualitatively different from the
steady state for charge distributions. This is primarily because the steady
state established for spin distributions involves spin precession due to
spin-orbit coupling. We demonstrate in this work that the spin density matrix
in an external electric field acquires two corrections with different
dependencies on the characteristic momentum scattering time. One part is
associated with conserved spins, diverges in the clean limit and is responsible
for the establishment of a steady-state spin density in electric fields.
Another part is associated with precessing spins, is finite in the clean limit
and is responsible for the establishment of spin currents in electric fields.
Scattering between these distributions has important consequences for spin
dynamics and spin-related effects in general, and explains some recent puzzling
observations, which are captured by our unified theory.Comment: 10 pages, 1 figur
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
Electric-field switchable magnetization via the Dzyaloshinskii-Moriya interaction: FeTiO_3 versus BiFeO_3
In this article we review and discuss a mechanism for coupling between
electric polarization and magnetization that can ultimately lead to
electric-field switchable magnetization. The basic idea is that a ferroelectric
distortion in an antiferromagnetic material can "switch on" the
Dzyaloshinskii-Moriya interaction which leads to a canting of the
antiferromagnetic sublattice magnetizations, and thus to a net magnetization.
This magnetization M is coupled to the polarization P via a trilinear free
energy contribution of the form P(M x L), where L is the antiferromagnetic
order parameter. In particular, we discuss why such an invariant is present in
R3c FeTiO_3 but not in the isostructural multiferroic BiFeO_3. Finally, we
construct symmetry groups that in general allow for this kind of
ferroelectrically-induced weak ferromagnetism.Comment: 15 pages, 3 images, to appear in J. Phys: Condens. Matter Focus Issue
on Multiferroic
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
Rolled-Up Nanotech: Illumination-Controlled Hydrofluoric Acid Etching of AlAs Sacrificial Layers
<p>Abstract</p> <p>The effect of illumination on the hydrofluoric acid etching of AlAs sacrificial layers with systematically varied thicknesses in order to release and roll up InGaAs/GaAs bilayers was studied. For thicknesses of AlAs below 10 nm, there were two etching regimes for the area under illumination: one at low illumination intensities, in which the etching and releasing proceeds as expected and one at higher intensities in which the etching and any releasing are completely suppressed. The “etch suppression” area is well defined by the illumination spot, a feature that can be used to create heterogeneously etched regions with a high degree of control, shown here on patterned samples. Together with the studied self-limitation effect, the technique offers a way to determine the position of rolled-up micro- and nanotubes independently from the predefined lithographic pattern.</p
Impurity breakdown and terahertz luminescence in n-GaN epilayers under external electric field
We report on the observation and experimental studies of impurity breakdown and terahertz luminescence in n-GaN epilayers under external electric field. The terahertz electroluminescence is observed in a wide range of doping levels (at noncompensated donor density from 4.5×10[sup 16] to 3.4×10[sup 18] cm[sup −3]). Spectra of terahertz luminescence and photoconductivity are studied by means of Fourier transform spectrometry. Distinctive features of the spectra can be assigned to intracenter electron transitions between excited and ground states of silicon and oxygen donors and to hot electron transitions to the donor states.Peer reviewe
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