Nuclear spin warm-up in bulk n-GaAs

We show that the spin-lattice relaxation in n-type insulating GaAs is dramatically accelerated at low magnetic fields. The origin of this effect, that cannot be explained in terms of well-known diffusion-limited hyperfine relaxation, is found in the quadrupole relaxation, induced by fluctuating donor charges. Therefore, quadrupole relaxation, that governs low field nuclear spin relaxation in semiconductor quantum dots, but was so far supposed to be harmless to bulk nuclei spins in the absence of optical pumping can be studied and harnessed in much simpler model environment of n-GaAs bulk crystal.Comment: 5 pages, 4 figure

Polariton Local States in Periodic Bragg Multiple Quantum Well Structures

We analytically study optical properties of several types of defects in Bragg multiple quantum well structures. We show that a single defect leads to two local polariton modes in the photonic band gap. These modes lead to peculiarities in reflection and transmission spectra. Detailed recommendations for experimental observation of the studied effects are given.Comment: 3 pages, 1 figure, RevTex, Submitted to Opt. Let

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

Flame Evolution During Type Ia Supernovae and the Deflagration Phase in the Gravitationally Confined Detonation Scenario

We develop an improved method for tracking the nuclear flame during the deflagration phase of a Type Ia supernova, and apply it to study the variation in outcomes expected from the gravitationally confined detonation (GCD) paradigm. A simplified 3-stage burning model and a non-static ash state are integrated with an artificially thickened advection-diffusion-reaction (ADR) flame front in order to provide an accurate but highly efficient representation of the energy release and electron capture in and after the unresolvable flame. We demonstrate that both our ADR and energy release methods do not generate significant acoustic noise, as has been a problem with previous ADR-based schemes. We proceed to model aspects of the deflagration, particularly the role of buoyancy of the hot ash, and find that our methods are reasonably well-behaved with respect to numerical resolution. We show that if a detonation occurs in material swept up by the material ejected by the first rising bubble but gravitationally confined to the white dwarf (WD) surface (the GCD paradigm), the density structure of the WD at detonation is systematically correlated with the distance of the deflagration ignition point from the center of the star. Coupled to a suitably stochastic ignition process, this correlation may provide a plausible explanation for the variety of nickel masses seen in Type Ia Supernovae.Comment: 14 pages, 10 figures, accepted to the Astrophysical Journa

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

Nuclear spin-lattice relaxation in p-type GaAs

Spin-lattice relaxation of the nuclear spin system in p-type GaAs is studied using a three-stage experimental protocol including optical pumping and measuring the difference of the nuclear spin polarization before and after a dark interval of variable length. This method allows us to measure the spin-lattice relaxation time $T_1$ of optically pumped nuclei "in the dark", that is, in the absence of illumination. The measured $T_1$ values fall into the sub-second time range, being three orders of magnitude shorter than in earlier studied n-type GaAs. The drastic difference is further emphasized by magnetic-field and temperature dependences of $T_1$ in p-GaAs, showing no similarity to those in n-GaAs. This unexpected behavior is explained within a developed theoretical model involving quadrupole relaxation of nuclear spins, which is induced by electric fields within closely spaced donor-acceptor pairs.Comment: 9 pages, 8 figure

Hyperbolic Discounting and Its Consequences: Empirical Analysis on the Base of Surveys

During this cross-cultural research regarding time preferences the credibility of hyperbolic discounting model is examined. This paper is based on the comparison between the exponential discounting model and hyperbolic model and proposal of different modifications to the existing concepts. Different kind of anomalies, which bring up the necessity of the extension of the hyperbolic discounting model, are mentioned. This paper is aimed at exploring the discount model, the process of decision-making and the formation of the individuals’ subjective discount rate from the standpoint of behavioral economics by including in the model such psychological factors as cultural and economic environment and risk aversion. These supplements lead to more complex and credible discount model. The term of procrastination is considered as a direct implementation of hyperbolic discounting. Furthermore, the potential drawbacks occurred during the experimental part are highlighted and possible solutions are proposed. Empirical analysis is based on the surveys.У ході цього крос-культурного дослідження тимчасових переваг перевіряється достовірність моделі гіперболічного дисконтування. У даній статті порівнюються моделі експоненціального дисконтування і гіперболічна модель і пропонуються різні модифікації існуючих концепцій. Згадуються різні види аномалій, які викликають необхідність розширення моделі гіперболічного дисконтування. Ця стаття спрямована на вивчення моделі дисконтування, процесу прийняття рішень і формування суб'єктивної ставки дисконтування індивідів з точки зору поведінкової економіки, шляхом включення в модель таких психологічних чинників, як культурне й економічне середовище, а також несхильність до ризику. Ці доповнення призводять до більш складної і такої моделі дисконтування, що заслуговує довіри. Прокрастинація розглядається як пряма реалізація гіперболічного дисконтування. Крім того, виділено потенційні недоліки, що виникли в ході експериментальної частини, і запропоновано можливі рішення. Емпіричний аналіз заснований на опитуваннях.В ходе этого кросс-культурного исследования временных предпочтений проверяется достоверность модели гиперболического дисконтирования. В данной статье сравниваются модели экспоненциального дисконтирования и гиперболическая модель и предлагаются различные модификации существующих концепций. Упоминаются различные виды аномалий, которые вызывают необходимость расширения модели гиперболического дисконтирования. Эта статья направлена на изучение модели дисконтирования, процесса принятия решений и формирования субъективной ставки дисконтирования индивидов с точки зрения поведенческой экономики, путем включения в модель таких психологических факторов, как культурная и экономическая среда, а также нерасположенность к риску. Эти дополнения приводят к более сложной и заслуживающей доверия модели дисконтирования. Прокрастинация рассматривается как прямая реализация гиперболического дисконтирования. Кроме того, выделены потенциальные недостатки, возникшие в ходе экспериментальной части, и предложены возможные решения. Эмпирический анализ основан на опросах

Spin Texture in a Cold Exciton Gas

We report on the observation of a spin texture in a cold exciton gas in a GaAs/AlGaAs coupled quantum well structure. The spin texture is observed around the exciton rings. The observed phenomena include: a ring of linear polarization, a vortex of linear polarization with polarization perpendicular to the radial direction, an anisotropy in the exciton flux, a skew of the exciton fluxes in orthogonal circular polarizations and a corresponding four-leaf pattern of circular polarization, a periodic spin texture, and extended exciton coherence in the region of the polarization vortex. The data indicate a transport regime where the spin polarization is locked to the direction of particle propagation and scattering is suppressed.Comment: version 2 contains updated supplementary materia

Nucleosynthesis in thermonuclear supernovae with tracers: convergence and variable mass particles

Nucleosynthetic yield predictions for multi-dimensional simulations of thermonuclear supernovae generally rely on the tracer particle method to obtain isotopic information of the ejected material for a given supernova simulation. We investigate how many tracer particles are required to determine converged integrated total nucleosynthetic yields. For this purpose, we conduct a resolution study in the number of tracer particles for different hydrodynamical explosion models at fixed spatial resolution. We perform hydrodynamic simulations on a co-expanding Eulerian grid in two dimensions assuming rotational symmetry for both pure deflagration and delayed detonation Type Ia supernova explosions. Within a given explosion model, we vary the number of tracer particles to determine the minimum needed for the method to give a robust prediction of the integrated yields of the most abundant nuclides. For the first time, we relax the usual assumption of constant tracer particle mass and introduce a radially vary- ing distribution of tracer particle masses. We find that the nucleosynthetic yields of the most abundant species (mass fraction > 10E-5) are reasonably well predicted for a tracer number as small as 32 per axis and direction - more or less independent of the explosion model. We conclude that the number of tracer particles that were used in extant published works appear to have been sufficient as far as integrated yields are concerned for the most copiously produced nuclides. Additionally we find that a suitably chosen tracer mass distribution can improve convergence for nuclei produced in the outer layer of the supernova where the constant tracer mass prescription suffers from poor spatial resolution.Comment: 9 pages, 5 figures, accepted for publication in MNRA