Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field

The effect of an external electric field F on the excitonic photoluminescence (PL) spectra of a symmetric coupled double quantum well (DQW) is investigated both theoretically and experimentally. We show that the variational method in a two-particle electron-hole wave function approximation gives a good agreement with measurements of PL on a narrow DQW in a wide interval of F including flat-band regime. The experimental data are presented for an MBE-grown DQW consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers. The bias voltage is applied along the growth direction. Spatially direct and indirect excitonic transitions are identified, and the radius of the exciton and squeezing of the exciton in the growth direction are evaluated variationally. The excitonic binding energies, recombination energies, oscillator strengths, and relative intensities of the transitions as functions of the applied field are calculated. Our analysis demonstrates that this simple model is applicable in case of narrow DQWs not just for a qualitative description of the PL peak positions but also for the estimation of their individual shapes and intensities.Comment: 5 pages, 4 figures (accepted in Phys. Rev. B

Effect of isotopic mass on the photoluminescence spectra of beta zinc sulfide

Zinc sulfide is a wide bandgap semiconductor which crystallizes in either the wurtzite modification (a-ZnS), the zincblende modification (b-ZnS) or as one of several similar tetrahedrally coordinated polytypes. In this work, we report a photoluminescence study of different samples of isotopically pure b-ZnS crystals, and crystals with the natural isotopic abundances, at 15 and 77 K. The derivatives of the free and bound exciton energies on isotopic mass have been obtained. They allow us to estimate the contribution of the zinc and sulfur vibrations to the bandgap renormalization energy by electron-phonon interaction. A two-oscillator model based on the zinc and sulfur renormalization energies has been used to account for the temperature dependence of the bandgap energy in ZnS. The results are compared with those found for other tetrahedrally coordinated semiconductors.Comment: 19 pages, 4 Postscript figures, sent to Solid State Communication

Nematoil: a novel secreted lipid that coats the outer surface of the dauer larva of Pristionchus pacificus

The dauer larva formed by multiple nematodes is a specialized developmentally arrested stage for survival and dispersal from unfavorable environment. The survival abilities of dauer larvae are determined by their specific morphology, metabolism, and enhanced stress resistance. A major morphological feature of dauer larvae is the remodeled body surface - they are effectively sealed off by a dauer-specific cuticle that restricts the chemical exchange with the environment. Although the dauer formation is genetically well investigated in several nematode models, there is no much information about the chemical means by which dauer larvae resist to the various kinds of environmental stress. We have found that the nematode Pristionchus pacificus synthesizes dauer stage specific lipids that form a hydrophobic film covering the entire outer surface of the animal. Detailed observation showed that the synthesis and the secretion of the lipids are simultaneously executed late in dauer differentiation, shortly preceded by the molt to dauer larva. The hydrophobic film is a complex mixture of several lipids and advanced chemical analysis revealed that its major component is a very long-chain polyunsaturated wax ester that we name Nematoil. The lipid coat alters the surface properties of the animals - they tend to congregate in tight “dauer clumps” consisting of up to hundreds of individuals, which supposedly enhances their impermeability. Thus, P. pacificus dauer larvae have the biochemical means to enhance their stress response by counteracting collectively