2,310 research outputs found
Electric field control of spin-orbit splittings in GaAs/AlGaAs coupled quantum wells
Electron spin dynamics is investigated in n-i-n GaAs/AlGaAs coupled quantum
wells. The electron spin dephasing time is measured as a function of an
external electrical bias under resonant excitation of the 1sHH intrawell
exciton using a time-resolved Kerr rotation technique. It is found a strong
electron spin dephasing time anisotropy caused by an interference of the
structure inversion asymmetry and the bulk inversion asymmetry. This anisotropy
is shown to be controlled by an electrical bias. A theoretical analysis of
electron spin dephasing time anisotropy is developed. The ratio of Rashba and
Dresselhaus spin splittings is studied as a function of applied bias.Comment: 4 pages, 3 figure
Orbital, precessional and flaring variability of Cygnus X-1
We present the results of a 2.5-year multiwavelength monitoring programme of
Cygnus X-1, making use of hard and soft X-ray data, optical spectroscopy,
UBVJHK photometry and radio data. In particular we confirm that the 5.6-day
orbital period is apparent in all wavebands and note the existence of a
wavelength-dependence to the modulation, in the sense that higher energies
reach minimum first. We also find a strong modulation at a period of 142 +/- 7
days, which we suggest is due to precession and/or radiative warping of the
accretion disc. Strong modulation of the hard and soft X-ray flux at this long
period may not be compatible with simple models of an optically thin accretion
flow and corona in the low state. We present the basic components required for
more detailed future modelling of the system - including a partially optically
thick jet, quasi-continuous in the low state, the base of which acts as the
Comptonising corona. In addition, we find that there are a number of flares
which appear to be correlated in at least two wavebands and generally in more.
We choose two of these flares to study in further detail and find that the hard
and soft X-rays are well-correlated in the first and that the soft X-rays and
radio are correlated in the second. In general, the optical and infrared show
similar behaviour to each other but are not correlated with the X-rays or
radio.Comment: Accepted for publication in MNRAS, 2 figures in colou
Superfluidity of "dirty" indirect excitons and magnetoexcitons in two-dimensional trap
The superfluid phase transition of bosons in a two-dimensional (2D) system
with disorder and an external parabolic potential is studied. The theory is
applied to experiments on indirect excitons in coupled quantum wells. The
random field is allowed to be large compared to the dipole-dipole repulsion
between excitons. The slope of the external parabolic trap is assumed to change
slowly enough to apply the local density approximation (LDA) for the superfluid
density, which allows us to calculate the Kosterlitz-Thouless temperature
at each local point of the trap. The superfluid phase occurs
around the center of the trap () with the normal phase outside
this area. As temperature increases, the superfluid area shrinks and disappears
at temperature . Disorder acts to deplete the condensate; the
minimal total number of excitons for which superfluidity exists increases with
disorder at fixed temperature. If the disorder is large enough, it can destroy
the superfluid entirely. The effect of magnetic field is also calculated for
the case of indirect excitons. In a strong magnetic field , the superfluid
component decreases, primarily due to the change of the exciton effective mass.Comment: 13 pages, 3 figure
The acoustic model of oscillations of gas combustion in coaxial pipes
© Published under licence by IOP Publishing Ltd. Organization of pulse combustion mode is one of the possible solutions to the problem of energy efficiency installations using hydrocarbon fuel. For grate combustion of solid fuels, in particular, solid industrial wastes are considered to be promising coaxial system, allowing the admission of secondary air to the combustion zone. In this paper we proposed an acoustic model of oscillations of gas when burning solid fuel in the system is coaxially arranged pipes with natural air supply. The description of the motion of the gas in the system during one period of oscillation
Self-exited gas oscillations in Helmholtz resonator type combustor
When the length of the resonance tube is much less and order of the acoustic wavelength is studied. Experimental set-up with a multiport burner is considered. The "energy balance" method is used to calculate the frequency and amplitude of the gas oscillations. The results of measurements and calculations show that increasing the length of the resonance tube causes the excitation oscillation with a first lower frequency. Further, there are no oscillations. When the length of the resonance tube becomes the order of the wavelength, the second resonant frequency is excited. The calculation results are in good agreement with the experimental data
Coherent spin dynamics of an interwell excitonic gas in GaAs/AlGaAs coupled quantum wells
The spin dynamics of an interwell excitons gas has been investigated in n-i-n
GaAs/AlGaAs coupled quantum wells (CQWs). In these heterostructures the
electron and the hole are spatially separated in neighboring quantum wells by a
narrow AlAs barrier, when an electric field is applied. The time evolution
kinetics of the interwell exciton photoluminescence has been measured under
resonant excitation of the 1sHH intrawell exciton, using a pulsed tunable
laser. The formation of a collective exciton phase in time and the temperature
dependence of its spin relaxation rate have been studied. The spin relaxation
rate of the interwell excitons is strongly reduced in the collective phase.
This observation provides evidence for the coherence of the indirect excitons
collective phase at temperatures below a critical .Comment: 8 pages, 9 figure
Frequencies of gas oscillations in a pipe with a concentrated heat source
It is known that the location of the heat source significantly affects the frequency of acoustic oscillations in the channels. The case of a step change in the sound speed is investigated. In this article, linear distribution of sound speed in hot gas is considered. The well-known equations are used to calculate frequencies of the gas oscillations. The analysis shows that the movement of the flame from the down up in an open tube causes a nonmonotonic change in the resonant frequency. The calculation results are in good agreement with the experimental data
Mössbauer spectroscopy with a high velocity resolution applied for the study of meteoritic iron-bearing minerals
Mössbauer spectroscopy with a high velocity resolution was applied for study of iron-bearing minerals in different meteorites. The possibility of technique to reveal small variations in Mössbauer hyperfine parameters of the 57Fe in the non-equivalent M1 and M2 sites in olivines from Farmington L5 and Tsarev L5 ordinary chondrites and from Omolon and Seymchan pallasites was demonstrated. The necessity of accounting for the Fe and Ni occupation probabilities in the local microenvironments for non-equivalent sites M1, M2 and M3 in schreibersite, an iron nickel phosphide from Sikhote-Alin iron meteorite, in the fit of its Mössbauer spectra was shown. Variations in Mössbauer parameters of metal samples from visually different areas at the saw-cut surface of Chinga iron meteorite fragment with unknown origin were observed; these variations may be related to different metal phase composition and local variations of Ni concentration in the metal phases in these areas. © 2012 Elsevier B.V. All rights reserved
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