Instabilities in a Two-Component, Species Conserving Condensate

We consider a system of two species of bosons of equal mass, with interactions $U^{a}(|x|)$ and $U^{x}(|x|)$ for bosons of the same and different species respectively. We present a rigorous proof -- valid when the Hamiltonian does not include a species switching term -- showing that, when $U^{x}(|x|)>U^{a}(|x|)$, the ground state is fully "polarized" (consists of atoms of one kind only). In the unpolarized phase the low energy excitation spectrum corresponds to two linearly dispersing modes that are even a nd odd under species exchange. The polarization instability is signaled by the vani shing of the velocity of the odd modes.Comment: To appear in Phys. Rev.

Nucleosomes in gene regulation: theoretical approaches

This work reviews current theoretical approaches of biophysics and bioinformatics for the description of nucleosome arrangements in chromatin and transcription factor binding to nucleosomal organized DNA. The role of nucleosomes in gene regulation is discussed from molecular-mechanistic and biological point of view. In addition to classical problems of this field, actual questions of epigenetic regulation are discussed. The authors selected for discussion what seem to be the most interesting concepts and hypotheses. Mathematical approaches are described in a simplified language to attract attention to the most important directions of this field

Nonlinear optics and saturation behavior of quantum dot samples under continuous wave driving

The nonlinear optical response of self-assembled quantum dots is relevant to the application of quantum dot based devices in nonlinear optics, all-optical switching, slow light and self-organization. Theoretical investigations are based on numerical simulations of a spatially and spectrally resolved rate equation model, which takes into account the strong coupling of the quantum dots to the carrier reservoir created by the wetting layer states. The complex dielectric susceptibility of the ground state is obtained. The saturation is shown to follow a behavior in between the one for a dominantly homogeneously and inhomogeneously broadened medium. Approaches to extract the nonlinear refractive index change by fringe shifts in a cavity or self-lensing are discussed. Experimental work on saturation characteristic of InGa/GaAs quantum dots close to the telecommunication O-band (1.24-1.28 mm) and of InAlAs/GaAlAs quantum dots at 780 nm is described and the first demonstration of the cw saturation of absorption in room temperature quantum dot samples is discussed in detail

The role of volume contraction in the hypocalciuric action of chlorothiazide

The role of volume contraction in the hypocalciuric action of chlorothiazide. Thiazide-induced hypocalciuria usually requires two to four days of drug treatment for its full expression. The present experiments were designed to test the possibility that an immediate hypocalciuria could be produced in subjects whose extracellular volumes were already contracted. In previously untreated subjects, chlorothiazide (2 g/day) did not produce hypocalciuria on the first day of treatment. Hypocalciuria occurred on the second day and was somewhat enhanced on the third and fourth days of treatment. In subjects pretreated with furosemide (40 mg two or three times a day for two days), chlorothiazide produced hypocalciuria on the first day of its use. Under the influence of chlorothiazide, there was a strong, linear correlation between the changes in the excretion of calcium and sodium. The results are consistent with the idea that chlorothiazide enhances the ratio of calcium to sodium reabsorptions and that this tendency for absolute hypocalciuria can be overridden by a large natriuretic response to the drug. Volume contraction, which minimizes the natriuretic response to diuretics, allows a more immediate and pronounced hypocalciuric response to thiazides.Le rôle de la contraction des liquides extra-cellulaires dans la diminution de la calciurie par le chlorothiazide. L'hypocalciurie induite par les thiazides n'atteint son expression complète qu'après deux à quatre jours de traitement. Les expériences rapportées ont été réalisées pour évaluer la possibilité d'obtention d'une hypocalciurie immédiate chez les sujets dont le volume extracellulaire est diminué dès le départ. Chez des sujets non préalablement traitées, le chlorothiazide (2 g/jour) n'a pas déterminé d'hypocalciurie le premier jour de l'administration. Elle est survenue le second jour et a été renforcée les troisième et quatrième jours. Chez des sujets prétraités par la furosémide (40 mg deux ou trois fois par jour pendant deux jours) le chlorothiazide produit une hypocalciurie dès le premier jour de son administration. Sous l'influence du chlorothiazide existe une forte corrélation linéaire entre les modifications des excrétions du calcium et du sodium. Les résultats sont compatibles avec l'hypothèse suivant laquelle le chlorothiazide augmente la proportion de calcium réabsorbée par rapport au sodium et que cette tendance vers l'hypocalciurie absolue peut être annulée par une réponse natriurétique importante au thiazide. La contraction du volume extra-cellulaire, qui diminue la réponse natriurétique aux diurétiques, permet une hypocalciurie plus pré-coce et plus importante en réponse aux thiazides

<i>In Situ</i> Spectroscopic Ellipsometry Study of the Oxide Etching and Surface Damaging Processes on Silicon Under Hydrogen Plasma

ABSTRACTWe employed in situ ellipsometry in the monitoring of surface damage to monocrystalline silicon (Si) substrates under hydrogen plasma conditions. These measurements were complemented with spectroscopic ellipsometry and Raman spectroscopy, in order to characterize the surface conditions. It was found that heating the Si substrate to 700°C in the presence of molecular hydrogen produces etching of the native oxide layer, which is typically 10 Å thick. When the already hot and bare silicon surface is submitted to hydrogen plasma, it deteriorates very fast, becoming rough and full of voids. Modeling of the spectroscopic ellipsometry data was used to obtain a quantitative physical picture of the surface damage, in terms of roughness layer t ickness and void fraction. The results indicate that by the time a thin film starts to grow on these silicon surfaces, like in the chemical vapor deposition of diamond, the roughness produced by the hydrogen plasma has already determined to a large extent the rough nature of the film to be grown.</jats:p