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The energy spectrum of the exciton and carrier states in a shallow GaAs/AlGaAs quantum well is experimentally studied by means of the spectroscopy of the nonradiative broadening of exciton resonances and the spectroscopy of the photoluminescence excitation. The observed peculiarities of the spectra are treated using the numerical solution of the one-dimensional Schrodinger equation for free carriers and three-dimensional equation for excitons in the quantum well. The conduction and valence band offsets in the shallow GaAs quantum well are determined.

Wave functions of heavy-hole excitons in GaAs/Al$_{0.3}$Ga$_{0.7}$As square quantum wells (QWs) of various widths are calculated by the direct numerical solution of a three-dimensional Schr\"odinger equation using a finite-difference scheme. These wave functions are then used to determine the exciton-exciton, exciton-electron and exciton-hole fermion exchange constants in a wide range of QW widths (5-150 nm). Additionally, the spin-dependent matrix elements of elastic exciton-exciton, exciton-electron and exciton-hole scattering are calculated. From these matrix elements, the collisional broadening of the exciton resonance is obtained within the Born approximation as a function of the areal density of excitons, electrons and holes respectively for QW widths of 5, 15, 30 and 50 nm. The obtained numerical results are compared with other theoretical works., Comment: 15 pages, 3 figures

In this work, we study the exchange interactions between two excitons in the GaAs/AlGaAs quantum wells of various widths. We numerically solved the Schrodinger equation for an exciton in a quantum well to find the two-exciton wave functions and to calculate the exchange integral. The results suggest that the strongest interactions between excitons occur in the quantum wells of widths of about 40–50 nm, with the exchange energy being of about of 9 μeV for an exciton density of 1/μm2.

Optically inactive or dark excitons play an important role in exciton and polariton devices. On one hand, they supply excitons to the light cone and feed the photoluminescence signal. On the other hand, they repel radiatively active excitons due to the exchange interaction and contribute to the formation of lateral potentials for exciton and polariton condensates. On top of this, they play an important role in scattering and energy relaxation dynamics of quasiparticles in semiconductors. So far, because of optical inaccessibility, studies were focused typically on one experimental technique, giving information about one quantity of dark excitons. Here we present a comprehensive study of the dark exciton reservoir in a high-quality 14-nm GaAs/AlGaAs quantum well using several experimental techniques. We develop a new method of nonradiative broadening spectroscopy of exciton resonances and combine it with nondegenerate pump-probe spectroscopy. The exciton and carrier dynamics in the reservoir is monitored via dynamic broadening of exciton resonances induced by exciton-exciton and exciton-carrier scattering. The dynamics is found to be strongly dependent on the optical excitation conditions. Based on the experimental results, we develop a model of dynamics in a reservoir of excitons and free carriers. The model allows us to describe the experimentally measured photoluminescence kinetics with no fitting parameters. We also demonstrate the optical control of the dark exciton density by means of an additional excitation that creates imbalance of free carriers depleting the reservoir. These results shed light onto the dynamics of the excitonic ``dark matter'' and pave the way to the high-precision engineering of optically induced potentials in exciton-polariton and integrated photonic devices. We expect that the observed results can be transferred also to other semiconductors so that the current quantum well serves as a high-quality model system.

In this paper, estimates are made of the Ruderman-Kittel interaction in a gallium arsenide crystal and in GaAs quantum wells of varying widths. The calculation results indicate that this interaction may be weaker than the magnetic dipole-dipole interaction by an order of magnitude. Also discovered was a relatively strong dependence of this interaction on the level of doping.

A many-body-theory approach has been developed to study positronium-atom interactions. As first applications, we calculate the elastic scattering and momentum-transfer cross sections and the pickoff annihilation rate ^{1}Z_{eff} for Ps collisions with He and Ne. For He the cross section is in agreement with previous coupled-state calculations, while comparison with experiment for both atoms highlights discrepancies between various sets of measured data. In contrast, the calculated ^{1}Z_{eff} (0.13 and 0.26 for He and Ne, respectively) are in excellent agreement with the measured values.

Synopsis Calculations of positron binding, scattering and annihilation using a model correlation potential and a basis of Gaussian states are carried out for a range of atoms and small molecules.

To analyze the elemental composition and topology of the extracellular compartments of the compound eye, the eyes of blowflies Calliphora vicina were rapidly frozen and ultrathin cryosections were freeze dried. Three zones of an ommatidium, peripheral cytosol of visual cells, rhabdomeres, and ommatidial cavities were analyzed by X-ray microprobe analysis. The ommatidial cavity was found to contain sodium and potassium in proportion similar to that in the blowfly hemolymph. Potassium-to-sodium ratio in a cytosol was typical for a cytosol. The rhabdomeres displayed an electrolyte content intermediate between the above compartments. Three topologically connected extracellular compartments were characterized by the experiments with tracers, monastral blue and lanthanum: (1) common intercellular space of ommatidia including peripheral clefts between the visual cells, both tracers entered this compartment; (2) the ommatidial cavity, which is not accessible for monastral blue, however, as revealed by our X-ray microanalysis, it was reachable for lanthanum; (3) rhabdomeric loops, which were accessible for lanthanum entering either via the cavity or from the common intercellular clefts. The above characteristics of the ionic content and topology of ommatidial compartments might suggest higher sodium and lower potassium content in the microvilli as compared with the cytosol. The rhabdomeric and “cavital” plasma membranes are assumed to be permeable for these ions so that a voltage of only 25–30 mV, negative inside, is probably formed across them, much lower than the known resting potential −60 mV across the peripheral plasma membrane of a visual cell.

Absolute two-photon detachment cross sections and photoelectron angular distribution are calculated for halogen negative ions within lowest-order perturbation theory. The Dyson equation method is used to obtain the outer np ground-state wave functions with proper asymptotic behavior exp (-k r), corresponding to correct (experimental) binding energies E=k^2/2. The latter is crucial for obtaining correct absolute values of the multiphoton cross sections (Gribakin and Kuchiev 1997 Phys. Rev.A55 3760). Comparisons with previous calculations and experimental data are performed., Comment: Latex, IOP stile, 22 pages, 8 figures

1. Providing the nest with vegetable food,Lethrus performs repeated excursions of about l m from the hole and returns home. 2. Homing by the straight line,Lethrus walks in compass direction toward the nest; the distance traveled is equal to the estimated distance to the hole, and in a case of faultLethrus begins the circling search.Lethrus is able to integrate over the complex path. 3. Lethrus sets direction relative to celestial cues: the sun and the polarized skylight. 4. Lethrus has two photoreceptor systems: the green-sensitive and UV-sensitive one; only an ultraviolet polarization pattern is used for polarotaxis. 5. The eye ofLethrus evolved from the superpositional prototype; it is adapted to diurnal vision by shortening of the clear zone and by thickening of the light-guiding tracts from the cone to the rhabdom. 6. Lethrus is different from other Lamellicorns possessing a typical multilobed rhabdom in thatLethrus has a crustacean-type rhabdom. It is assumed that this specialization of rhabdom provides the perfect analysis of polarized light and polarotaxis. 7. The complex orientational and nest behavior ofLethrus is not accompanied by enlargement of the brain and its presumed associative centres, the mushroom bodies. Among several scarabaeid beetles of comparable size (Amphimallon solstitialis, Geotrupes stercorosus)Lethrus apterus gains the first place in behavioral complexity but the last in number of sensilla on the antennal dumb, number of ommatidia, and the volume of mushroom globuli.