Optical absorption of divalent metal tungstates: Correlation between the band-gap energy and the cation ionic radius

We have carried out optical-absorption and reflectance measurements at room temperature in single crystals of AWO4 tungstates (A = Ba, Ca, Cd, Cu, Pb, Sr, and Zn). From the experimental results their band-gap energy has been determined to be 5.26 eV (BaWO4), 5.08 eV (SrWO4), 4.94 eV (CaWO4), 4.15 eV (CdWO4), 3.9-4.4 eV (ZnWO4), 3.8-4.2 eV (PbWO4), and 2.3 eV (CuWO4). The results are discussed in terms of the electronic structure of the studied tungstates. It has been found that those compounds where only the s electron states of the A2+ cation hybridize with the O 2p and W 5d states (e.g BaWO4) have larger band-gap energies than those where also p, d, and f states of the A2+ cation contribute to the top of the valence band and the bottom of the conduction band (e.g. PbWO4). The results are of importance in view of the large discrepancies existent in prevoiusly published data.Comment: 16 pages, 3 figures, 1 tabl

Non singular Hamiltonian systems and geodesic flows on surfaces with negative curvature

We extend here results for escapes in any given direction of the configuration space of a mechanical system with a non singular bounded at infinity homogeneus potential of degree -1, when the energy is positive. We use geometrical methods for analyzing the parallel and asymptotic escapes of this type of systems. By using Riemannian geometry methods we prove under suitable conditions on the potential that all the orbits escaping in a given direction are asymptotically parallel among themselves. We introduce a conformal Riemannian metric with negative curvature in the interior of the Hill's region for a fixed positive energy level and we consider the boundary as a singular part of the infinity. The associated geodesic flow has as solution curves those of the problem for a fixed energy. We perform the compactification of the region via the limiting directions of the geodesic flow, obtaining a closed unit disk with a quasi-complete metric of negative curvature

Erratum for "Non singular Hamiltonian systems and geodesic flows on surfaces with negative curvature"

Fe d'errade

Shoulder pain prevalence in competitive swimmers: A pilot study

Objetivos: conocer la prevalencia de dolor de hombro en nadadores de competición, sus características y su relación con factores antropométricos y deportivos. Métodos: estudio de prevalencia. Ciento cuarenta nadadores/as entre 12 y 24 años cumplimentaron un cuestionario durante una fase de entrenamiento específico. Resultados: el 25,7% revelaron padecer dolor de hombro. Se hallaron relaciones estadísticamente significativas entre el dolor de hombro y episodios previos de dolor (p<0,001), experiencia superior a tres años (p=0,014), Índice de Masa Corporal (p=0,015) y la especialidad estilo (p=0,008) y distancia (p=0,011). El dolor fue significativamente más intenso durante la actividad que en reposo (p<0,001). Conclusiones: el dolor de hombro en nadadores de competición entre 12 y 24 años parece ser un problema frecuente mayor Índice de Masa Corporal y a la especialidad del nadadorObjectives: to investigate the prevalence of shoulder pain in competitive swimmers and find out the characteristics of pain as well as its relation to anthropometric and sports factors. Methods: prevalence study. A hundred and forty competitive swimmers between 12 and 24 years old completed a questionnaire in a high intensity training phase. Results: 25.7% swimmers reported shoulder pain. There were significant statistical correlations between shoulder pain and previous episodes of pain (p<0.001), more than three years of experience (p=0.014), Body Mass Index (p=0.015) and stroke (p=0.008) and distance (p=0.011) specialty. Pain was statistically correlated with activity (p<0.001). Conclusions: shoulder pain seems to be a frequent and repetitive problem in competitive swimmers between 12 and 24 years old, which increases with years of practice. Furthermore, it seems to be associated with the activity, a higher Body Mass Index and the swimmers’ specialty

Job security and long-term investment: an experimental analysis

We investigate an experimental labor market setting in which we introduce the novel aspect that workers have the chance of investing money in a long-term project in order to increase their income. We find a strong relationship between what happens inside the labor market (worker's performance) and what happens outside the labor market (long-term investment). Contrary to the theoretical predictions with selfish preferences, we find that the mere presence of long-term projects acts as an effort-enforcement device; this effect seems to be driven by an increase in long-term employment relationships. In the other direction, long-term labor relationships seem to provide a safer environment for undertaking successful long-term projects. This article also considers three different types of experimental labor contracts. We find that performance-based dismissal barriers, whereby firms are required to retain workers if they have satisfied the effort level required by firms, lead to more long-term employment relationships and higher overall productivity. As theory predicts, the presence of renewable dismissal barriers makes it likely that workers will provide the desired effort level. Firms appear to correctly anticipate this, leading to greater social efficiency

The hidden advantage of delegation: pareto-improvements in a gift-exchange game

This paper analyzes the effect on performance and earnings of delegating the wage choice to employees. Our results how that such delegation significantly increases effort levels. Moreover, we observe a Pareto-improvement, as the earnings of both employers and employees increase when employers delegate than when they do not. Interestingly, we also find that the employees’ performance under delegation is higher than under non-delegation, even for similar wages. While there is strong evidence that behavior reflects strategic considerations, this result also holds for one-shot interactions. A possible non-strategic motivation explaining the positive reaction to delegation is a sense of enhanced responsibility

High-pressure structural investigation of several zircon-type orthovanadates

Room temperature angle-dispersive x-ray diffraction measurements on zircon-type EuVO4, LuVO4, and ScVO4 were performed up to 27 GPa. In the three compounds we found evidence of a pressure-induced structural phase transformation from zircon to a scheelite-type structure. The onset of the transition is near 8 GPa, but the transition is sluggish and the low- and high-pressure phases coexist in a pressure range of about 10 GPa. In EuVO4 and LuVO4 a second transition to a M-fergusonite-type phase was found near 21 GPa. The equations of state for the zircon and scheelite phases are also determined. Among the three studied compounds, we found that ScVO4 is less compressible than EuVO4 and LuVO4, being the most incompressible orthovanadate studied to date. The sequence of structural transitions and compressibilities are discussed in comparison with other zircon-type oxides.Comment: 34 pages, 2 Tables, 11 Figure

Quercomys bijmaigen. nov. sp. nov. (Gliridae, Rodentia, Mammalia) del Transito Oligoceno-Mioceno espanol

Quercomys bijmai gen. nov. sp. nov., a new dormouse of the subfamily Myomiminae, is described from the Oligocene/Miocene Boundary Interval of Santa Cilia (Huesca), Quel-1 and Fuenmayor-2 (La Rioja). Quercomys gen. nov. is characterized by its medium size, its hypsodont teeth, and its simple dental pattero. It is related to other genera, like Praearmantomys, Armantomys or Pseudodryomys, and its appearance is at a moment of high diversity of the family.Se describe Quercomys bijmai gen. nov. sp. nov. (Gliridae, Rodentia) del Tránsito OligocenoMioceno, de Santa Cilia (Huesca), Quel-1 y Fuenmayor-2 (La Rioja). Quercomys gen. nov. se caracteriza por sus dientes hipsodontos, talla media y modelo dental simple. Está relacionado con otros géneros como Praearmantomys, Armantomys o Pseudodryomys, y aparece en un momento de alta diversidad en la familia

First-order phase transition in a 2D random-field Ising model with conflicting dynamics

The effects of locally random magnetic fields are considered in a nonequilibrium Ising model defined on a square lattice with nearest-neighbors interactions. In order to generate the random magnetic fields, we have considered random variables $\{h\}$ that change randomly with time according to a double-gaussian probability distribution, which consists of two single gaussian distributions, centered at $+h_{o}$ and $-h_{o}$, with the same width $\sigma$. This distribution is very general, and can recover in appropriate limits the bimodal distribution ($\sigma\to 0$) and the single gaussian one ($ho=0$). We performed Monte Carlo simulations in lattices with linear sizes in the range $L=32 - 512$. The system exhibits ferromagnetic and paramagnetic steady states. Our results suggest the occurence of first-order phase transitions between the above-mentioned phases at low temperatures and large random-field intensities $h_{o}$, for some small values of the width $\sigma$. By means of finite size scaling, we estimate the critical exponents in the low-field region, where we have continuous phase transitions. In addition, we show a sketch of the phase diagram of the model for some values of $\sigma$.Comment: 13 pages, 9 figures, accepted for publication in JSTA

Complex high-pressure polymorphism of barium tungstate

We have studied BaWO 4 under compression at room temperature by means of x-ray diffraction and Raman spectroscopy. When compressed with neon as a pressure-transmitting medium (quasihydrostatic conditions), we found that BaWO 4 transforms from its low-pressure tetragonal structure into a much denser monoclinic structure. This result confirms our previous theoretical prediction based on ab initio calculations that the scheelite to BaWO 4-II transition occurs at room temperature if kinetic barriers are suppressed by pressure. However, our experiment without any pressure- transmitting medium has resulted in a phase transition to a completely different structure, suggesting nonhydrostaticity may be responsible for previously reported rich polymorphism in BaWO 4. The crystal structure of the low- and high-pressure phases from the quasihydrostatic experiments has been Rietveld refined. Additionally, for the tetragonal phase the effects of pressure on the unit-cell volume and lattice parameters are discussed. Finally, the pressure evolution of the Raman modes of different phases is reported and compared with previous studies. © 2012 American Physical Society.This research was supported by Spanish MEC (Grant No. MAT2010-21270-C04-01/04), MALTA Consolider Ingenio 2010 (Grant No. CSD2007-00045), and Vicerrectorado de Investigacion y Desarrollo of the Universitat Politecnica de Valencia (Grants No. UPV2011-0914 PAID-05-11 and No. UPV2011-0966 PAID-06-11). XRD data were collected at HPCAT, Advanced Photon Source (APS), Argonne National Laboratory. HPCAT is supported by CIW, CDAC, UNLV, and LLNL through funding from DOE-NNSA, DOE-BES, and NSF. APS is supported by DOE-BES under Contract No. DEAC02-06CH11357.Gomis Hilario, O.; Sans, JA.; Lacomba-Perales, R.; Errandonea, D.; Meng, Y.; Chervin, JC.; Polian, A. (2012). Complex high-pressure polymorphism of barium tungstate. 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