149 research outputs found
INFLUENCE OF PLASMON-PHONON COUPLING ON THE ELECTRON-IMPURITY INTERACTION IN THE FREE-CARRIER ABSORPTION*
An experimental and theoretical evidence that the electron-impurity scattering is modified in the presence of the electromagnetic field oscillating with the frequency corresponding to the optical phonon energy is given. The coupled plasmon-phonon mode related feature in the reflectivity spectrum can be used to determine the upper edge of the LO phonon frequency band in highly doped materials. PACS numbers: 71.45. Gm, 78.30.Fs, 72.30.+q The infrared absorption in semiconductors is mainly caused by phonons and free carriers. In many cases these mechanisms can be treated separately, each of them giving its own contribution to the optical dielectric function. The simple Drude-Zener theory assuming frequency independent damping conStants of free-carrier plasma and phonon excitations, satisfactorily describes most of the experimental reflectivity and transmission spectra in the infrared. A deviation from thiS model can be expected when the light frequency approaches the optical phonon energy band. Strong mixing of the electron plasma and polar phonon modes leading to the collective motion of electrons and lattice ions causes a significant modification of the effective scattering of electrons by impurities Most studies of the role of the dynamic screening in the free-carrier absorption were devoted to the question of plasmon excitation in the process of electron-impurity scattering (for the references see, e.g.
Interlayer Exchange Coupling in (Ga,Mn)As-based Superlattices
The interlayer coupling between (Ga,Mn)As ferromagnetic layers in
all-semiconductor superlattices is studied theoretically within a tight-binding
model, which takes into account the crystal, band and magnetic structure of the
constituent superlattice components. It is shown that the mechanism originally
introduced to describe the spin correlations in antiferromagnetic EuTe/PbTe
superlattices, explains the experimental results observed in ferromagnetic
semiconductor structures, i.e., both the antiferromagnetic coupling between
ferromagnetic layers in IV-VI (EuS/PbS and EuS/YbSe) superlattices as well as
the ferromagnetic interlayer coupling in III-V ((Ga,Mn)As/GaAs) multilayer
structures. The model allows also to predict (Ga,Mn)As-based structures, in
which an antiferromagnetic interlayer coupling could be expected.Comment: 4 pages, 3 figure
(Ga,Mn)As based superlattices and the search for antiferromagnetic interlayer coupling
Antiferromagnetic interlayer coupling in dilute magnetic semiconductor
superlattices could result in the realisation of large magnetoresistance
effects analogous to the giant magnetoresistance seen in metallic multilayer
structures. In this paper we use a mean-field theory of carrier induced
ferromagnetism to explore the multidimensional parameter space available in
(Ga,Mn)As based superlattice systems. Based on these investigations we examine
the feasibility of creating a superlattice that exhibits antiferromagnetic
coupling and suggest potentially viable recipes.Comment: 11 pages, 9 figure
Ferromagnetic GaMnAs/GaAs superlattices - MBE growth and magnetic properties
We have studied the magnetic properties of (GaMnAs)m/(GaAs)n superlattices
with magnetic GaMnAs layers of thickness between 8 and 16 molecular layers (ML)
(23-45 \AA), and with nonmagnetic GaAs spacers from 4 ML to 10 ML (11-28 \AA).
While previous reports state that GaMnAs layers thinner than 50 \AA are
paramagnetic in the whole Mn composition range achievable using MBE growth (up
to 8% Mn), we have found that short period superlattices exhibit a
paramagnetic-to-ferromagnetic phase transition with a transition temperature
which depends on both the thickness of the magnetic GaMnAs layer and the
nonmagnetic GaAs spacer. The neutron scattering experiments have shown that the
magnetic layers in superlattices are ferromagnetically coupled for both thin
(below 50 \AA) and thick (above 50 \AA) GaMnAs layers.Comment: Proceedings of 4th International Workshop on Molecular Beam Epitaxy
and Vapour Phase Epitaxy Growth Physics and Technology, September 23 - 28
(2001), Warszawa, Poland, to appear in Thin Solid Films. 24 pages, 8 figure
FREE-CARRIER PLASMONS AS A NOVEL TOOL IN SEMICONDUCTOR PHYSICS*
It is demonstrated that free-carrier plasmons, being well defined collective excitations of the electron gas in the range of small wave vectors, can serve as a sensitive tool to investigate the optical processes related to the small momentum transfers. As an example the system HgSe:Fe is analysed both experimentally and theoretically. It is well known that the excitation of the free-carrier plasma in the light absorption process is possible only in the presence of defects breaking the translational invariance of the system. Due to the overall momentum conservation requirement there must exist a momentum source to make the photon absorption *This work is supported in part by CPBP 01.06. (141
Ferromagnetism in Diluted Magnetic Semiconductor Heterojunction Systems
Diluted magnetic semiconductors (DMSs), in which magnetic elements are
substituted for a small fraction of host elements in a semiconductor lattice,
can become ferromagnetic when doped. In this article we discuss the physics of
DMS ferromagnetism in systems with semiconductor heterojunctions. We focus on
the mechanism that cause magnetic and magnetoresistive properties to depend on
doping profiles, defect distributions, gate voltage, and other system
parameters that can in principle be engineered to yield desired results.Comment: 12 pages, 7 figures, review, special issue of Semicon. Sci. Technol.
on semiconductor spintronic
The nature of the intranight variability of radio-quiet quasars
We select a sample of 10 radio-quiet quasars with confirmed intranight
optical variability and with available X-ray data. We compare the variability
properties and the broad band spectral constraints to the predictions of
intranight variability by three models: (i) irradiation of an accretion disk by
a variable X-ray flux (ii) an accretion disk instability (iii) the presence of
a weak blazar component. We concluded that the third model, e.g. the blazar
component model, is the most promising if we adopt a cannonball model for the
jet variable emission. In this case, the probability of detecting the
intranight variability is within 20-80%, depending on the ratio of the disk to
the jet optical luminosity. Variable X-ray irradiation mechanism is also
possible but only under additional requirement: either the source should have a
very narrow Hbeta line or occasional extremely strong flares should appear at
very large disk radii.Comment: MNRAS (in press
Definitive Evidence of Interlayer Coupling Between (Ga,Mn)As Layers Separated by a Nonmagnetic Spacer
We have used polarized neutron reflectometry to study the structural and
magnetic properties of the individual layers in a series of
(Al,Be,Ga)As/(Ga,Mn)As/GaAs/(Ga,Mn)As multilayer samples. Structurally, we
observe that the samples are virtually identical except for the GaAs spacer
thickness (which varies from 3-12 nm), and confirm that the spacers contain
little or no Mn. Magnetically, we observe that for the sample with the thickest
spacer layer, modulation doping by the(Al,Be,Ga)As results in (Ga,Mn)As layers
with very different temperature dependent magnetizations. However, as the
spacer layer thickness is reduced, the temperature dependent magnetizations of
the top an bottom (Ga,Mn)As layers become progressively more similar - a trend
we find to be independent of the crystallographic direction along which spins
are magnetized. These results definitively show that (Ga,Mn)As layers can
couple across a non-magnetic spacer, and that such coupling depends on spacer
thickness.Comment: Submitted to Physical Review
Ferromagnetic semiconductors
The current status and prospects of research on ferromagnetism in
semiconductors are reviewed. The question of the origin of ferromagnetism in
europium chalcogenides, chromium spinels and, particularly, in diluted magnetic
semiconductors is addressed. The nature of electronic states derived from 3d of
magnetic impurities is discussed in some details. Results of a quantitative
comparison between experimental and theoretical results, notably for Mn-based
III-V and II-VI compounds, are presented. This comparison demonstrates that the
current theory of the exchange interactions mediated by holes in the valence
band describes correctly the values of Curie temperatures T_C magnetic
anisotropy, domain structure, and magnetic circular dichroism. On this basis,
chemical trends are examined and show to lead to the prediction of
semiconductor systems with T_C that may exceed room temperature, an expectation
that are being confirmed by recent findings. Results for materials containing
magnetic ions other than Mn are also presented emphasizing that the double
exchange involving hoping through d states may operate in those systems.Comment: 18 pages, 8 figures; special issue of Semicon. Sci. Technol. on
semiconductor spintronic
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