Raman and infrared studies of La_1-ySr_yMn_1-xM_xO_3 (M=Cr, Co, Cu, Zn, Sc or Ga): Oxygen disorder and local vibrational modes

We present results of our study of polarized Raman scattering and infrared reflectivity of rhombohedral ceramic La_1-ySr_yMn_1-xM_xO_3 manganites in the temperature range between 77 and 320K. In our samples, a part of the Mn atoms is substituted by M = Cr, Co, Cu, Zn, Sc, or Ga with x in the range 0 -- 0.1. The hole concentration was kept at the optimal value of about 32% by tuning the Sr content y. We have monitored the distortions of the oxygen sublattice by the presence of broad bands in the Raman spectra, the increase of d.c. resistivity extracted from the infrared reflectivity, and the change of the critical temperature of the ferromagnetic transition. Our results support the idea, that these properties are mainly determined by the radius of the substituent ion, its electronic and magnetic structure playing only a minor role. Furthermore, the Raman spectra exhibit an additional A_g-like high frequency mode attributed to the local breathing vibration of oxygens surrounding the substituent ion. Its frequency and intensity strongly depend on the type of the substituent. In the Co-substituted sample, the mode anomalously softens when going from 300 to 77K. The frequency of the bulk A_{1g} mode depends linearly on the angle of the rhombohedral distortion.Comment: 11 pages, 11 Figures, best-fit values table added, a discussion adde

Aplicación de la función densidad electrónica para diferenciar la estructura de la corrensita y de la clorita-clorita hinchable

El estudio por difracción de rayos X de interestratificados de minerales de la arcilla es de gran utilidad en la investigación geológica de los materiales arcillosos. El analisis de los espectros de difracción de interestratificados regulares utilizando los metodos de "tanteo" de determinación de estructuras cristalinas asi como los basados en la transformada de Fourier, permite obtener una información mas relacionada con el interestratificado que con la muestra arcillosa global en el caso de estudiar muestras no puras, mejor que los métodos quimicos en los que se basan un gran número de conclusiones de tipo estructural. La aplicación de la transformada de Fourier para la función de mezcla asi como de la función densidad electrónica pone de manifiesto que las estructuras de los interestratificados regulares de cloritaclorita hinchable y de clorita-esmectita son distintas, a pesar de ser utilizada indistintamente por algunos autores la denominación de corrensita para ambos interestratificados

Males respond to female begging signals of need: A handicapping experiment in the pied flycatcher, Ficedula hypoleuca

The 'female nutrition' hypothesis proposes that food provided by males during incubation is an important energy source for females in bird species in which females alone incubate. Females should be able to communicate their needs through begging signals to mates and males may compensate for the energetic limitations of females through their feeding visits, owing to their overlapping reproductive interests. To test whether female begging during incubation is an honest signal of energetic need and whether mates respond to it we experimentally handicapped female pied flycatchers at the beginning of incubation by clipping two primary flight feathers on each wing. Experimental manipulation led females to intensify begging displays arising from condition impairment and males accordingly increased their incubation feeding rates. Female begging intensity explained more than half of the variation in male incubation feeding rate, thereby showing that female nutrition is the main factor explaining male incubation feeding. Moreover, handicapped females consumed a higher proportion of male food deliveries during the first few days after hatching and weighed less at the end of the nestling period than control females. Handicapping had no influence on female incubation behaviour, hatching and breeding success, nestling and male condition or female nestling provisioning. The provisioning rates of males in the late nestling stage were higher in experimental nests. This is the first experimental study showing that males adjust incubation feeding rates to behavioural displays of need by their mates. The ability of females to modify their begging displays according to need may be an important adaptation that allows females to maintain a good energetic condition during incubation. © 2014 The Association for the Study of Animal Behaviour.This study was financed by project CGL2010-19233-C03-02 to J.M. from Spanish MICINN.Peer Reviewe

Modulation of the electronic properties of GaN films by surface acoustic waves

We report on the interaction between photogenerated electron-hole pairs and surface acoustic waves (SAW) in GaN films grown on sapphire substrates. The spatial separation of photogenerated carriers by the piezoelectric field of the SAW is evidenced by the quenching of the photoluminescence (PL) intensity. The quenching levels in GaN are significantly smaller than those measured in GaAs under similar conditions. The latter is attributed to the lower exciton ionization efficiency and carrier separation probabilities mediated by the piezoelectric effect. The PL spectra also evidence energy shifts and broadenings of the electronic transitions, which are attributed to the band gap modulation by the SAW strain [email protected]

Surface acoustic Bloch oscillations, the Wannier-Stark ladder and Landau-Zener tunneling in a solid

We present the experimental observation of Bloch oscillations, the Wannier-Stark ladder, and Landau-Zener tunneling of surface acoustic waves in perturbed grating structures on a solid substrate. A model providing a quantitative description of our experimental observations, including multiple Landau-Zener transitions of the anticrossed surface acoustic Wannier-Stark states, is developed. The use of a planar geometry for the realization of the Bloch oscillations and Landau-Zener tunneling allows a direct access to the elastic field distribution. The vertical surface displacement has been measured by interferometry

Raman spectroscopy and photoluminescence of ZnTe thin films grown on GaAs

We report resonant Raman scattering and photoluminescence (PL) measurements on two ZnTe thin films grown by molecular-beam epitaxy on GaAs substrates with thicknesses around 0.5 and 1.0 μm. The data have been obtained by using the different excitation energies of an Ar+ laser to distinguish Raman from PL and analyze resonant effects. The characteristic features of the low-temperature PL spectra are the light and heavy free exciton emissions, split due to the thermal strain effect, followed by several phonon replicas of these lines. Moreover, longitudinal and transversal polariton splittings of heavy excitons are clearly observed. Their reduced masses have been obtained from the exciton binding energies. Room and low-temperature Raman spectra show, besides the typical longitudinal optical (LO) multiphonon emissions, forbidden zone-center transverse optical (TO)+(n−1)LO phonon combinations, which yield an accurate value for the LO and TO phonon energies. The breakdown of the selection rules is attributed to surface [email protected] ; [email protected]

Growth and optical characterization of indirect-gap AlxGa1−xAs alloys

Nonintentionally doped AlxGa1−xAs layers with 0.38 x 0.84 were grown on (100) GaAs substrates by liquid phase epitaxy (LPE) under near-equilibrium conditions. The crystalline quality of the samples was studied by photoluminescence at 2 K and room temperature Raman spectroscopy. The peculiar behavior in the photoluminescence intensities of the indirect bound exciton line and the donor–acceptor pair transition is explained from the evolution of the silicon donor binding energy according to the aluminum composition. It was also possible to observe the excitonic transition corresponding to the AlxGa1−xAs/GaAs interface, despite the disorder and other factors which are normally involved when growing high-aluminum-content layers by this technique. Furthermore, Raman measurements show the quadratic variations of longitudinal optical phonon frequencies with aluminum concentration in good agreement with previous experimental results. In this work we show that high quality indirect-gap AlxGa1−xAs samples can be grown by LPE under near-equilibrium [email protected]

Effects of Gamma Irradiation on the Kinetics of the Adsorption and Desorption of Hydrogen in Carbon Microfibres

In this study, three types of carbon fibres were used, they were ex-polyacrylonitrile carbon fibres with high bulk modulus, ex-polyacrylonitrile fibres with high strength, and vapour grown carbon fibres. All the samples were subjected to a hydrogen adsorption process at room temperature in an over-pressured atmosphere of 25 bars. The adsorption process was monitored through electrical resistivity measurements. As conditioning of the fibres, a chemical activation by acid etching followed by ¿-ray irradiation with 60Co radioisotopes was performed. The surface energy was deter-mined by means of the sessile drop test. Both conditioning treatments are supplementary; the chemical activation works on the outer surface and the ¿-irradiation works in the bulk material as well. Apparently, the most significant parameter for hydrogen storage is the crystallite size. From this point of view, the mostPeer Reviewe