2,076 research outputs found
Negative high-frequency differential conductivity in semiconductor superlattices
We examine the high-frequency differential conductivity response properties
of semiconductor superlattices having various miniband dispersion laws. Our
analysis shows that the anharmonicity of Bloch oscillations (beyond
tight-binding approximation) leads to the occurrence of negative high-frequency
differential conductivity at frequency multiples of the Bloch frequency. This
effect can arise even in regions of positive static differential conductivity.
The influence of strong electron scattering by optic phonons is analyzed. We
propose an optimal superlattice miniband dispersion law to achieve
high-frequency field amplification
Spin Josephson effect in ferromagnet/ferromagnet tunnel junctions
We consider the tunnel spin current between two ferromagnetic metals from a
perspective similar to the one used in superconductor/superconductor tunnel
junctions. We use fundamental arguments to derive a Josephson-like spin tunnel
current . Here the phases are
associated with the planar contribution to the magnetization,
. The crucial step in our
analysis is the fact that the -component of the spin is canonically
conjugate to the phase of the planar contribution: . This is
analogous to the commutation relation in superconductors, where
is the phase associated to the superconducting order parameter and
is the Cooper pair number operator. We briefly discuss the experimental
consequences of our theoretical analysis.Comment: LaTex, seven pages, no figures; version to appear in Europhys. Lett.;
in order to make room for a more extended microscopic analysis, the
phenomenological discussion contained in v2 was remove
Continuous Neel to Bloch Transition as Thickness Increases: Statics and Dynamics
We analyze the properties of Neel and Bloch domain walls as a function of
film thickness h, for systems where, in addition to exchange, the dipole-dipole
interaction must be included. The Neel to Bloch phase transition is found to be
a second order transition at hc, mediated by a single unstable mode that
corresponds to oscillatory motion of the domain wall center. A uniform
out-of-plane rf-field couples strongly to this critical mode only in the Neel
phase. An analytical Landau theory shows that the critical mode frequency
varies as the square root of (hc - h) just below the transition, as found
numerically.Comment: 4 pages, 4 figure
Pharmacology in special populations: extreme age
The article deals the definition, history of development in Russia, the main provisions, problems and prospects of development of pharmacology of extreme ages. The features of the nomenclature of medicines and the applicability of different dosage forms in different age periods are shown. Recommendations on rational use of medicines in patients of special populations — extreme ages are given. The risks of responsible self-treatment and polypharmacology are indicated. The data of international monitoring of drug safety in patients of extreme ages are presented. The article is addressed to pharmacologists, clinical pharmacologists, drug developers and doctors of medical specialties
Weak localization of holes in high-mobility heterostructures
Theory of weak localization is developed for two-dimensional holes in
semiconductor heterostructures. Ballistic regime of weak localization where the
backscattering occurs from few impurities is studied with account for
anisotropic momentum scattering of holes. The transition from weak localization
to anti-localization is demonstrated for long dephasing times. For stronger
dephasing the conductivity correction is negative at all hole densities due to
non-monotonous dependence of the spin relaxation time on the hole wavevector.
The anomalous temperature dependent correction to the conductivity is
calculated. We show that the temperature dependence of the conductivity is
non-monotonous at moderate hole densities.Comment: 5 pages, 4 figure
Exciton spin decay modified by strong electron-hole exchange interaction
We study exciton spin decay in the regime of strong electron-hole exchange
interaction. In this regime the electron spin precession is restricted within a
sector formed by the external magnetic field and the effective exchange fields
triggered by random spin flips of the hole. Using Hanle effect measurements, we
demonstrate that this mechanism dominates our experiments in CdTe/(Cd,Mg)Te
quantum wells. The calculations provide a consistent description of the
experimental results, which is supported by independent measurements of the
parameters entering the model.Comment: 5 pages, 3 figure
Quantum analogue of the spin-flop transition for a spin pair
Quantum (step-like) magnetization curves are studies for a spin pair with
antiferromagnetic coupling in the presence of a magnetic field parallel to the
easy axis of the magnetic anisotropy. The consideration is done both
analytically and numerically for a wide range of the anisotropy constants and
spins up to . Depending on the origin of the anisotropy
(exchange or single-ion), the magnetization curve can demonstrate the jumps
more than unity and the concentration of the unit jumps in a narrow range of
the field. We also point the region of the problem parameters, where the
behavior is quasiclassical for , and where system is substantially
quantum in the limit .Comment: 5 pages, 5 figure
Improvement of methods for semi-finished carbon product tapping from the basic oxygen furnace (BOF)
The article considers the experience of optimizing the slag adjustment mode in the Basic Oxygen Furnace (BOF) and reducing the proportion of oxide non-metallic impurities in steel. The article presents the results of a research on the separation of slag from metal during the starting period of carbon semi-product tapping, on the elimination of an emergency situation when the hole is shut up at any degree of slag viscosity, the use of the device at any tap hole diameter after the heat, the simplicity of the design. The use of the device makes it possible to increase the durability of the taphole lining, reduce repair costs, and improve the quality of steel by reducing non-metallic impurities. The innovative devices proposed by the author can be useful for metallurgical enterprises
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