285 research outputs found
Spin waves in magnetic quantum wells with Coulomb interaction and exchange coupling
We theoretically describe the spin excitation spectrum of a two dimensional
electron gas embedded in a quantum well with localized magnetic impurities.
Compared to the previous work, we introduce equations that allow to consider
the interplay between the Coulomb interaction of delocalized electrons and the
exchange coupling between electrons and magnetic impurities. Strong
qualitative changes are found : mixed waves propagate below the single particle
continuum, an anticrossing gap is open at a specific wavevector and the kinetic
damping due to the electron motion strongly influences the coupling strength
between electrons and impurities spins
Ultimate photo-induced Kerr rotation achieved in semiconductor microcavities
Photoinduced Kerr rotation by more than radians is demonstrated in
planar quantum well microcavity in the strong coupling regime. This result is
close to the predicted theoretical maximum of . It is achieved by
engineering microcavity parameters such that the optical impedance matching
condition is reached at the smallest negative detuning between exciton
resonance and the cavity mode. This ensures the optimum combination of the
exciton induced optical non-linearity and the enhancement of the Kerr angle by
the cavity. Comprehensive analysis of the polarization state of the light in
this regime shows that both renormalization of the exciton energy and the
saturation of the excitonic resonance contribute to the observed optical
nonlinearities.Comment: Shortened version prepared to submit in Phys. Rev. Letter
Flame Enhancement and Quenching in Fluid Flows
We perform direct numerical simulations (DNS) of an advected scalar field
which diffuses and reacts according to a nonlinear reaction law. The objective
is to study how the bulk burning rate of the reaction is affected by an imposed
flow. In particular, we are interested in comparing the numerical results with
recently predicted analytical upper and lower bounds. We focus on reaction
enhancement and quenching phenomena for two classes of imposed model flows with
different geometries: periodic shear flow and cellular flow. We are primarily
interested in the fast advection regime. We find that the bulk burning rate v
in a shear flow satisfies v ~ a*U+b where U is the typical flow velocity and a
is a constant depending on the relationship between the oscillation length
scale of the flow and laminar front thickness. For cellular flow, we obtain v ~
U^{1/4}. We also study flame extinction (quenching) for an ignition-type
reaction law and compactly supported initial data for the scalar field. We find
that in a shear flow the flame of the size W can be typically quenched by a
flow with amplitude U ~ alpha*W. The constant alpha depends on the geometry of
the flow and tends to infinity if the flow profile has a plateau larger than a
critical size. In a cellular flow, we find that the advection strength required
for quenching is U ~ W^4 if the cell size is smaller than a critical value.Comment: 14 pages, 20 figures, revtex4, submitted to Combustion Theory and
Modellin
On the existence of traveling waves in the 3D Boussinesq system
We extend earlier work on traveling waves in premixed flames in a
gravitationally stratified medium, subject to the Boussinesq approximation. For
three-dimensional channels not aligned with the gravity direction and under the
Dirichlet boundary conditions in the fluid velocity, it is shown that a
non-planar traveling wave, corresponding to a non-zero reaction, exists, under
an explicit condition relating the geometry of the crossection of the channel
to the magnitude of the Prandtl and Rayleigh numbers, or when the advection
term in the flow equations is neglected.Comment: 15 pages, to appear in Communications in Mathematical Physic
Polariton Dispersion Law in Periodic Bragg and Near-Bragg Multiple Quantum Well Structures
The structure of polariton spectrum is analyzed for periodic multiple quantum
well structures with periods at or close to Bragg resonance condition at the
wavelength of the exciton resonance. The results obtained used to discuss
recent reflection and luminescent experiments by M. H\"{u}bner et al [Phys.
Rev. Lett. {\bf 83}, 2841 (1999)] carried out with long multiple quantum well
structures. It is argued that the discussion of quantum well structures with
large number of wells is more appropriate in terms of normal modes of infinite
periodic structures rather then in terms of super- and sub- radiant modes.Comment: replaced with a new version, an error in one of the equations is
correcte
Дослідження стійкості геміклональних популяційних систем гібридогенного комплексу зелених жаб за допомогою імітаційного моделювання
The concept of stability used in ecology and mathematics is analyzed. Stability is interpreted as the ability of the system to remain in a restricted zone of a phase space that corresponds to a certain type of systems. This approach is applied to describe changes in the structure of hemiclonal population systems of hybridogenous complex of water frogs (Pelophylax esculentus complex). Simulation model of these population systems built on a recursive difference equations in MS Excel is used. The dynamics of transitions in the part of phase space of the hemiclonal populationsystems of water frogs is described. Typology of stabile states of biosystems is analyzed.Проанализировано понятие устойчивости в экологии и математике. Обосновывается трактовка устойчивости как способности системы оставаться в ограниченной зоне фазового пространства, соответствующей определенному типу рассматриваемых систем. Этот подход применен для описания изменений состава гемиклональных популяционных систем гибридогенного комплекса зеленых лягушек (Pelophylax esculentus complex). Использована имитационная модель, построенная на рекуррентных разностных уравнениях в MS Excel. Описана динамика переходов в части фазового пространства гемиклональных популяционных систем зеленых лягушек. Проанализирована типология состояний устойчивости биосистем. Проаналізовано поняття стійкості в екології та математиці. Обґрунтовується трактування стійкості як здатності системи залишатися в обмеженій зоні фазового простору, що відповідає певному типу систем, які розглядаються. Цей підхід застосований для опису змін складу геміклональних популяційних систем гібридогенного комплексу зелених жаб (Pelophylax esculentus complex). Використано імітаційну модель, побудовану на рекурентних різницевих рівняннях в MS Excel. Описано динаміку переходів у частині фазового простору геміклональних популяційних систем зелених жаб. Проаналізовано типологію станів стійкості біосистем.
Lecture-Conversation in Methodological Instrumentation of Russian as a Foreign Language Teacher
The article justifies the urgency of development of lectures-conversations when presenting the theoretical material of the training courses to foreigners. Attention is drawn to the difficulties of foreign students in listening the lecture in its traditional understanding as a monologue of the teacher. The importance of a methodical understanding of the theoretical presentation of the material in question-answer form is emphasised. Scientific novelty and practical significance of research results is seen in the fact that the issues are described from the point of view of the purpose of their performance to the students, the positive effect of the issue, the shortcomings of the question. It is assumed that within this form of classes the teacher can introduce new material, systematize theoretical information studied by students, consolidate students’ knowledge in the subject area, take into account communicative and personal aspects of adaptation of foreign students in Russian educational environment. On the original author’s material of lecture-conversation on theoretical grammar the kinds of questions are demonstrated that are appropriate to use in dialogue with foreign students. Methodological training for teachers in the formulation of various types of questions is proposed. It is concluded that the systematic use of lecture-conversation in teaching practice will contribute to overcoming the communicative and psychological difficulties of foreign students listening to lectures in a foreign language (Russian) that meets the modern trends of multicultural education
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