Evidence of breakdown of the spin symmetry in diluted 2D electron gases

Recent claims of an experimental demonstration of spontaneous spin polarisation in dilute electron gases \cite{young99} revived long standing theoretical discussions \cite{ceper99,bloch}. In two dimensions, the stabilisation of a ferromagnetic fluid might be hindered by the occurrence of the metal-insulator transition at low densities \cite{abra79}. To circumvent localisation in the two-dimensional electron gas (2DEG) we investigated the low populated second electron subband, where the disorder potential is mainly screened by the high density of the first subband. This letter reports on the breakdown of the spin symmetry in a 2DEG, revealed by the abrupt enhancement of the exchange and correlation terms of the Coulomb interaction, as determined from the energies of the collective charge and spin excitations. Inelastic light scattering experiments and calculations within the time-dependent local spin-density approximation give strong evidence for the existence of a ferromagnetic ground state in the diluted regime.Comment: 4 pages, 4 figures, Revte

Reduction of the transverse effective charge of optical phonons in ZnO under pressure

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 96, 231906 (2010) and may be found at https://doi.org/10.1063/1.3447798."From Raman scattering on a-plane wurtzite ZnO crystals we obtained a decreasing splitting between longitudinal and transversal optical phonons with A1 and E1 symmetry as a function of hydrostatic pressure up to 5.5 GPa. Consequently, the transverse effective charge (e∗T) exhibits a strong reduction with increasing pressure, yielding 2.17–14.6×10−3 P/GPa and 2.04–13.7×10−3 P/GPa (in units of the elementary charge) for the A1 and E1 phonons, respectively. We find a clear systematic in the linear pressure coefficient of e∗T with bond polarity for the series of wide-band gap semiconductors SiC, AlN, GaN, and ZnO.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Síntesis hidrotermal de zeolita a partir de ceniza volante tipo F: influencia de la temperatura = Influence of temperature of alkaline hydrothermal treatment on the zeolite conversion of spanish coal class F fly ash

En este trabajo se presenta el papel que juega la temperatura durante el tratamiento hidrotermal en medio alcalino para convertir una ceniza volante de bajo contenido en cal (clase F, según la norma ASTM) en zeolita. Durante este tratamiento a la temperatura de 100 ºC se forma zeolita Na-P1 tipo gismondina (Na6Al6Si10O32.12H2O); al elevar la temperatura a 200 ºC, dicha zeolita se transforma en zeolita analcima C (Na(Si2Al)O6H2O) y en fase sodalita (1.08 Na2O.Al2O3.1.68SiO2.1.8H2O) junto con trazas de tobermorita-11Å (Ca5(OH)2Si6O16.4H2O). A esta temperatura y en estas condiciones se ha conseguido un 100% de reacción. Un estudio equivalente se ha llevado a cabo empleando agua como medio de referencia. La conversión de ceniza volante en zeolita se ha caracterizado mediante técnicas, como difracción de Rayos X (DRX), espectroscopia infrarroja por transformada de Fourier (FTIR) y análisis térmico (TG/ATD); asimismo los cambios en el área superficial se han llevado a cabo mediante la técnica BET-N

The Ferroelectric-Ferroelastic Debate about Metal Halide Perovskites

Metal halide perovskites (MHPs) are solution-processed materials with exceptional photoconversion efficiencies that have brought a paradigm shift in photovoltaics. The nature of the peculiar optoelectronic properties underlying such astounding performance is still controversial. The existence of ferroelectricity in MHPs and its alleged impact on photovoltaic activity have fueled an intense debate, in which unanimous consensus is still far from being reached. Here we critically review recent experimental and theoretical results with a two-fold objective: we argue that the occurrence of ferroelectric domains is incompatible with the A-site cation dynamics in MHPs and propose an alternative interpretation of the experiments based on the concept of ferroelasticity. We further underline that ferroic behavior in MHPs would not be relevant at room temperature or higher for the physics of photogenerated charge carriers, since it would be overshadowed by competing effects like polaron formation and ion migration

Starburst galaxies in the COSMOS field : clumpy star-formation at redshift 0 < z <0.5

This work has been funded by the Spanish MINECO, Grant ESTALLIDOS, AYA2013-47742-C4-2P and AYA2010-21887-C04-04. J.M.A. acknowledges support from the European Research Council Starting Grant SEDMorph (P.I. V. Wild). R.H.G. acknowledges the FPI grant from MINECO within ESTALLIDOS project.Context. At high redshift, starburst galaxies present irregular morphologies with 10-20% of their star formation occurring in giant clumps. These clumpy galaxies are considered the progenitors of local disk galaxies. To understand the properties of starbursts at intermediate and low redshift, it is fundamental to track their evolution and the possible link with the systems at higher z. Aims. We present an extensive, systematic, and multiband search and analysis of the starburst galaxies at redshift (0 1010. We classify galaxies into three main types, depending on their HST morphology: single knot (Sknot), single star-forming knot plus diffuse light (Sknot+diffuse), and multiple star-forming knots (Mknots/clumpy) galaxy. We found a fraction of Mknots/clumpy galaxy fclumpy = 0.24 considering out total sample of starburst galaxies up to z ∼ 0.5. The individual star-forming knots in our sample follows the same L(Hα) vs. size scaling relation as local giant HII regions. However, they slightly differ from the one provided using samples at high redshift. This result highlights the importance of spatially resolving the star-forming regions for this kind of study. Star-forming clumps in the central regions of Mknots galaxies are more massive, and present higher star formation rates, than those in the outskirts. This trend is less clear when we consider either the mass surface density or surface star formation rate. Sknot galaxies do show properties similar to both dwarf elliptical and irregulars in the surface brightness (μ) versus Mhost diagram in the B-band, and to spheroidals and ellipticals in the μ versus Mhost diagram in the V-band. Conclusions. The properties of our star-forming knots in Sknot+diffuse and Mknots/clumpy galaxies support the predictions of recent numerical simulations claiming that they have been produced by violent disk instabilities. We suggest that the evolution of these knots means that large and massive clumps at the galaxy centers represent the end product of the coalescence of surviving smaller clumps from the outskirts. Our results support this mechanism and make it unlikely that mergers are the reason behind the observed starburst knots. Sknot galaxies might be transitional phases of the Blue Compact Dwarfs (BCD) class, with their properties consistent with spheroidal-like, but blue structures.Publisher PDFPeer reviewe

Effects of fishing restrictions on the abundance, size structure and mortality rate of a western Mediterranean population of Scorpaena scrofa (Linnaeus, 1758)

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