365 research outputs found
Reducing decoherence of the confined exciton state in a quantum dot by pulse-sequence control
We study the phonon-induced dephasing of the exciton state in a quantum dot
excited by a sequence of ultra-short pulses. We show that the multiple-pulse
control leads to a considerable improvement of the coherence of the optically
excited state. For a fixed control time window, the optimized pulsed control
often leads to a higher degree of coherence than the control by a smooth single
Gaussian pulse. The reduction of dephasing is considerable already for 2-3
pulses.Comment: Final version (moderate changes
Compensation device of breakaway torque of reaction wheel assembly
Статья посвящена разработке устройства компенсации момента трогания в управляющихдвигателях-маховиках. В статье анализируются проблемы, связанные с возникновением явления момента трогания в двигателях. Автором предлагаются пути решения данной проблемы, а так же рассматриваются преимущества и недостатки каждого из них. В результате было предложено устройство, которое позволяет снизить момент трогания управляющих двигателей-маховиков и исключить зону нечувствительности к малым управляющим сигналам, описана его структура и представлен краткий алгоритм работы такого устройства. Оценка результатов была произведена на испытательном стенде, представляющем собой имитационную модель управляющего двигателя-маховика.The article is devoted to the development of compensation device of breakaway torque of reaction wheel assembly. The article analyses the problems associated with the emergence of phenomenon of breakaway torque in reaction wheel assembly. The author suggests ways of solving this problem, as well as discusses the advantages anddisadvantages of each of them. As a result, it was suggested the device which reduces breakaway torque of reaction wheel assembly and exclude the dead zone in range of small signals, described its structure and provides an algorithm for such a device. Evaluation of the results was carried out on a test bench, which represent a simulationmodel of reaction wheel assembly
Exciton-LO-phonon dynamics in InAs/GaAs quantum dots: Effects of zone-edge phonon damping
The dynamics of an exciton-LO-phonon system after an ultrafast optical
excitation in an InAs/GaAs quantum dot is studied theoretically. Influence of
anharmonic phonon damping and its interplay with the phonon dispersion is
analyzed. The signatures of the zone-edge decay process in the absorption
spectrum and time evolution are highlighted, providing a possible way of
experimental investigation on phonon anharmonicity effects.Comment: 10 pages, 2 figure
Theory of exciton-exciton correlation in nonlinear optical response
We present a systematic theory of Coulomb interaction effects in the
nonlinear optical processes in semiconductors using a perturbation series in
the exciting laser field. The third-order dynamical response consists of
phase-space filling correction, mean-field exciton-exciton interaction, and
two-exciton correlation effects expressed as a force-force correlation
function. The theory provides a unified description of effects of bound and
unbound biexcitons, including memory-effects beyond the Markovian
approximation. Approximations for the correlation function are presented.Comment: RevTex, 35 pages, 10 PostScript figs, shorter version submitted to
Physical Review
Coherent Propagation of Polaritons in Semiconductor Heterostructures: Nonlinear Pulse Transmission in Theory and Experiment
The influence of coherent optical nonlinearities on polariton propagation
effects is studied within a theory-experiment comparison. A novel approach that
combines a microscopic treatment of the boundary problem in a sample of finite
thickness with excitonic and biexcitonic nonlinearities is introduced.
Light-polarization dependent spectral changes are analyzed for single-pulse
transmission and pump-probe excitation
Геолого-геофизическая и фациально-петрофизическая характеристика пласта Ю[1]{3} Казанского нефтегазоконденсатного месторождения
Выделение границ высокопродуктивной зоны, выделение карбонатных прослоев и граница их распространения, построение карты фаций, расчёт петрофизической модели на основе геофизических исследований скважины, оценка влияния распространения карбонатных тел на разработку.Identification of high-productive zone boundaries, identification of carbonate interlayers and boundary of their distribution, construction of facies map, calculation of petrophysical model on the basis of geophysical studies of the well, assessment of the influence of carbonate bodies distribution on the development
Two-band superconductors: Extended Ginzburg-Landau formalism by a systematic expansion in small deviation from the critical temperature
We derive the extended Ginzburg-Landau (GL) formalism for a clean s-wave
two-band superconductor by employing a systematic expansion of the free-energy
functional and the corresponding matrix gap equation in powers of the small
deviation from the critical temperature tau = 1-T/T_c. The two lowest orders of
this expansion produce the equation for T_c and the GL theory. It is shown that
in agreement with previous studies, the two-band GL theory maps onto the
single-band GL model and thus fails to describe the difference in the spatial
profiles of the two band condensates. We prove that except for some very
special cases, this difference appears already in the leading correction to the
GL theory, which constitutes the extended GL formalism. We derive linear
differential equations that determine the leading corrections to the band order
parameters and magnetic field, discuss the validity of these equations, and
consider examples of an important interplay between the band condensates.
Finally, we present numerical results for the thermodynamic critical magnetic
field and temperature-dependent band gaps (at zero field), which are in a very
good agreement with those obtained from the full BCS approach in a wide
temperature range. To this end, we emphasize the advantages of our extended GL
theory in comparison with the often used two-component GL-like model based on
an unreconstructed two-band generalization of the Gor'kov derivation
Ultrafast Coulomb-induced dynamics of 2D magnetoexcitons
We study theoretically the ultrafast nonlinear optical response of quantum
well excitons in a perpendicular magnetic field. We show that for
magnetoexcitons confined to the lowest Landau levels, the third-order
four-wave-mixing (FWM) polarization is dominated by the exciton-exciton
interaction effects. For repulsive interactions, we identify two regimes in the
time-evolution of the optical polarization characterized by exponential and
{\em power law} decay of the FWM signal. We describe these regimes by deriving
an analytical solution for the memory kernel of the two-exciton wave-function
in strong magnetic field. For strong exciton-exciton interactions, the decay of
the FWM signal is governed by an antibound resonance with an
interaction-dependent decay rate. For weak interactions, the continuum of
exciton-exciton scattering states leads to a long tail of the time-integrated
FWM signal for negative time delays, which is described by the product of a
power law and a logarithmic factor. By combining this analytic solution with
numerical calculations, we study the crossover between the exponential and
non-exponential regimes as a function of magnetic field. For attractive
exciton-exciton interaction, we show that the time-evolution of the FWM signal
is dominated by the biexcitonic effects.Comment: 41 pages with 11 fig
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