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The electron-electron two-stream instability has been studied in two-valley n-GaAs embedded with nanoparticle (NP) cluster. We considered polar optical phonon as the dominant intravalley scattering mechanism and drifted Maxwellian to approximate the distribution of carriers between each valley. We derived the dispersion relation for electrostatic wave using hydrodynamic model of plasmas. While investigating temporal behaviour, we found two novel modes of propagation. These two novel modes are found to be evanescent by nature and propagate with equal phase velocities, but in opposite directions. The pre-existing modes retained their profile structures even in the presence of NP cluster. On the other hand, in the case of convective instability, no new modes are obtained but the propagation and gain profiles of the preexisting modes are significantly modified by the presence of NP cluster in the host medium. Key words: two-stream instability, semiconductor plasma, many valley semiconductor. Nous etudions l'instabilite a deux faisceaux d'electrons dans un n-GaAs a deux vallees incorpore avec un agregat de nanoparticules (NP). Nous considerons les phonons optiques comme le mecanisme dominant de diffusion intra-vallee et une maxwellienne decalee pour approximer la distribution des porteurs dans chaque vallee. Nous utilisons un modele hydrodynamique du plasma pour deriver la relation de dispersion de l'onde electrostatique. En analysant le comportement temporel, nous identifions deux nouveaux modes de propagation. Nous trouvons que ces deux nouveaux modes sont evanescents par nature et se propagent avec la meme vitesse de phase, mais en directions opposees. Les modes preexistants conservent leur structure, meme en presence de l'agregat de NP. D'autre part, s'il y a instabilite convective, les nouveaux modes n'apparaissent pas et les profils de gain des modes preexistants sont significativement modifies a cause de la presence de l'agregat NP dans le milieu hote. [Traduit par la Redaction] Mots-cles: instabilite a deux faisceaux, plasma de semi-conducteur, semi-conducteur a multiples vallees., 1. Introduction Two-stream instability is a well studied phenomenon in plasma physics. It can be induced by an energetic particle stream injected in the plasma, or setting a current along [...]

Depth is used often as a proxy for gradients in energetic resources on coral reefs and for predicting patterns of community energy use. With increasing depth, loss of light can lead to a reduced reliance on autotrophy and an increased reliance on heterotrophy by mixotrophic corals. However, the generality of such trophic zonation varies across contexts. By combining high-resolution oceanographic measurements with isotopic analyses ([delta].sup.13C, [delta].sup.15N) of multiple producer and consumer levels across depths (10-30 m) at a central Pacific oceanic atoll, we show trophic zonation in mixotrophic corals can be both present and absent within the same reef system. Deep-water internal waves that deliver cool particulate-rich waters to shallow reefs occurred across all sites (2.5-5.6 events week.sup.-1 at 30 m) but the majority of events remained depth-restricted (4.3-9.7% recorded at 30 m propagated to 10 m). In the absence of other particulate delivery, mixotrophs increased their relative degree of heterotrophy with increasing depth. However, where relatively long-lasting downwelling events (1.4-3.3 times the duration of any other site) occurred simultaneously, mixotrophs displayed elevated and consistent degrees of heterotrophy regardless of depth. Importantly, these long-lasting surface pulses were of a lagoonal origin, an area of rich heterotrophic resource supply. Under such circumstances, we hypothesize heterotrophic resource abundance loses its direct linkage with depth and, with resources readily available at all depths, trophic zonation is no longer present. Our results show that fine-scale intra-island hydrographic regimes and hydrodynamic connectivity between reef habitats contribute to explaining the context specific nature of coral trophic depth zonation in shallow reef ecosystems., Author(s): Gareth J. Williams [sup.1] [sup.2] , Stuart A. Sandin [sup.2] , Brian J. Zgliczynski [sup.2] , Michael D. Fox [sup.2] , Jamison M. Gove [sup.3] , Justin S. Rogers [...]