Investigation of acceptor levels and hole scattering mechanisms in p-gallium selenide by means of transport measurements under pressure

The effect of pressure on acceptor levels and hole scattering mechanisms in p-GaSe is investigated through Hall effect and resistivity measurements under quasi-hydrostatic conditions up to 4 GPa. The pressure dependence of the hole concentration is interpreted through a carrier statistics equation with a single (nitrogen) or double (tin) acceptor whose ionization energies decrease under pressure due to the dielectric constant increase. The pressure effect on the hole mobility is also accounted for by considering the pressure dependencies of both the phonon frequencies and the hole-phonon coupling constants involved in the scattering rates.Comment: 13 pages, Latex, 4 ps figures. to appear in High Pressure Research 69 (1997

A collective effort to identify and quantify geo-energy risks

The increasing global demand for energy and the imminent need to reduce carbon emissions in our planet has led mankind to find new solutions. Some in the energy industry have taken special interest in geothermal reservoirs, a resource with the potential to provide large amounts of renewable energy. Meanwhile, the storage of carbon dioxide in underground geological formations presents a fantastic opportunity to discard CO2 and mitigate global warming. This study links efforts from academic institutions, industry energy operators, industrial partners and research institutes to answer fundamental scientific questions that can help us understand the subsurface and generate better exploitation practices. We examine the geology of reservoirs used for geothermal energy extraction and carbon dioxide capture. We use a combination of field geology, photogrammetry, mineral analysis and experimental rock mechanics to understand fracture networks and fluid flow paths of two geologically diverse reservoirs in Europe: 1) the Hengill geothermal system in south-west Iceland, and 2) the Carnmenellis granite geothermal system in Cornwall (UK). These results aim to provide experimental data to refine numerical models predicting fluid flow and contribute to the quantification of the associated risks of exploiting the subsurface

Análisis probabilístico de la eficiencia de los niveles de referencia para la evaluación de la contaminación de suelos

The developing of regulations for contaminated soils in Spain is taking place under considerations of specific uses for soil and also taking in to account environmental risk. Based on basic principles of risk assessment, a methodology to derive threshold concentrations for substances in soil has been proposed by IGME (Spanish Geological Survey). Regard these threshold concentrations an instrumental issue is to assess their efficiency (trend to produce few false positives and negatives by using them in soil pollution assessment). A probabilistic analysis for efficiency of these screening concentrations has been done by means of a Monte Carlo simulation exercise. Taking into account simulation results, threshold values show a high efficiency with negligible probabilities to yield false positives and probabilities around 0.1 for false positives.El desarrollo de reglamentación para suelos contaminados se viene llevando a cabo teniendo en consideración tanto los usos del suelo como el riesgo ambiental. De acuerdo a los principios elementales de la valoración de riesgos, el IGME (Instituto Geológico y Minero de España) ha planteado una propuesta para la determinación de concentraciones umbral en suelos. En relación con estas concentraciones una cuestión de la máxima importancia es valorar su eficiencia (tendencia a dar lugar a pocos falsos positivos o negativos en su uso en la evaluación de la contaminación de suelos). Con el fin de valorar este extremo se ha realizado un análisis probabilístico mediante métodos de simulación de Monte Carlo. Los resultados obtenidos permiten concluir que los niveles umbral propuestos presentan una elevada eficiencia con una probalidad de falsos positivos prácticamente nula y de falsos negativos alrededor de 0.1

Tin-related double acceptors in gallium selenide single crystals

Gallium selenide single crystals doped with different amounts of tin are studied through resistivity and Hall effect measurements in the temperature range from 30 to 700 K. At low doping concentration tin is shown to behave as a double acceptor impurity in gallium selenide with ionization energies of 155 and 310 meV. At higher doping concentration tin also introduces deep donor levels, but the material remains p-type in the whole studied range of tin doping concentrations. The deep character of donors in gallium selenide is discussed by comparison of its conduction band structure to that of indium selenide under pressure. The double acceptor center is proposed to be a tin atom in interlayer position, with a local configuration that is similar to that of tin diselenide. The hole mobility exhibits an anomalous dependence on the tin content, attaining its maximum value in the ingot with 0.2% nominal tin content. This is proposed to be related to impurity pairing effects giving rise to thermal shallow acceptors with low ionization energy and low carrier scattering cross section, making the hole mobility to be controlled by phonon scattering mechanisms even for relatively high impurity [email protected] ; [email protected]

Scaling law for the transient behavior of type-II neuron models

We study the transient regime of type-II biophysical neuron models and determine the scaling behavior of relaxation times $\tau$ near but below the repetitive firing critical current, $\tau \simeq C (I_c-I)^{-\Delta}$. For both the Hodgkin-Huxley and Morris-Lecar models we find that the critical exponent is independent of the numerical integration time step and that both systems belong to the same universality class, with $\Delta = 1/2$. For appropriately chosen parameters, the FitzHugh-Nagumo model presents the same generic transient behavior, but the critical region is significantly smaller. We propose an experiment that may reveal nontrivial critical exponents in the squid axon.Comment: 6 pages, 9 figures, accepted for publication in Phys. Rev.

Calculation of quantum discord for qubit-qudit or N qubits

Quantum discord, a kind of quantum correlation, is defined as the difference between quantum mutual information and classical correlation in a bipartite system. It has been discussed so far for small systems with only a few independent parameters. We extend here to a much broader class of states when the second party is of arbitrary dimension d, so long as the first, measured, party is a qubit. We present two formulae to calculate quantum discord, the first relating to the original entropic definition and the second to a recently proposed geometric distance measure which leads to an analytical formulation. The tracing over the qubit in the entropic calculation is reduced to a very simple prescription. And, when the d-dimensional system is a so-called X state, the density matrix having non-zero elements only along the diagonal and anti-diagonal so as to appear visually like the letter X, the entropic calculation can be carried out analytically. Such states of the full bipartite qubit-qudit system may be named "extended X states", whose density matrix is built of four block matrices, each visually appearing as an X. The optimization involved in the entropic calculation is generally over two parameters, reducing to one for many cases, and avoided altogether for an overwhelmingly large set of density matrices as our numerical investigations demonstrate. Our results also apply to states of a N-qubit system, where "extended X states" consist of (2^(N+2) - 1) states, larger in number than the (2^(N+1) - 1) of X states of N qubits. While these are still smaller than the total number (2^(2N) - 1) of states of N qubits, the number of parameters involved is nevertheless large. In the case of N = 2, they encompass the entire 15-dimensional parameter space, that is, the extended X states for N = 2 represent the full qubit-qubit system.Comment: 6 pages, 1 figur

Electronic structure of the double perovskite Ba2Er(Nb,Sb)O 6

In this work, we present a detailed study of the structural and the electronic structure of the double perovskite Ba2Er(Nb,Sb)O6. All calculations were performed with the Full-Potential Linear Augmented Plane Wave method (FP-LAPW) based on the Density Functional Theory (DFT). From the minimization of energy as a function of volume using the Murnaghan's state equation has been obtained the equilibrium lattice parameter and the bulk modulus of these compounds. The study of the electronic structure was based in the analysis of the electronic density of states (DOS), and the density of charge, showing that these compounds have a total magnetic moment of 3.0 μB per formula unit due to Er atoms.Instituto de Física La Plat

Osteopoiquilosis y síndrome de Buschke-Ollendörf: a propósito de un caso

Presentamos una paciente de 22 años que tras estudios radiológicos rutinarios, muestra una displasia ósea condensante, completamente asintomática e inócua, que afectaba principalmente pelvis y extremidades distales, mientras que las costillas, cráneo y columna dorsolumbar estaban indemnes. Asimismo, aparecen nevus elásticos y fibromas en la piel. Esta entidad no puede confundirse con un carcinoma metastásico esclerosante, esclerosis tuberosa u otras displasias como la melorreostosis.A female patient, 22 years oíd, showing characteristic sclerotic bone images on radiographic examinations is reported. The main locations were on the distal limbs and pelvis. She has also elastic nevi as freckles in the skin. This entity was not stablished as inhered process. The differential diagnosis lies between sclerotic metastasis, neurofibromatosis (Von Recklinghausen), Bourneville disease and other displastyc lesions

GOOGLE EARTH ENGINE: APPLICATION OF ALGORITHMS FOR REMOTE SENSING OF CROPS IN TUSCANY (ITALY)

Abstract. Remote sensing has become an important mean to assess crop areas, specially for the identification of crop types. Google Earth Engine (GEE) is a free platform that provides a large number of satellite images from different constellations. Moreover, GEE provides pixel-based classifiers, which are used for mapping agricultural areas. The objective of this work is to evaluate the performance of different classification algorithms such as Minimum Distance (MD), Random Forest (RF), Support Vector Machine (SVM), Classification and Regression Trees (CART) and Na¨ıve Bayes (NB) on an agricultural area in Tuscany (Italy). Four different scenarios were implemented in GEE combining different information such as optical and Synthetic Aperture Radar (SAR) data, indices and time series. Among the five classifiers used the best performers were RF and SVM. Integrating Sentinel-1 (S1) and Sentinel-2 (S2) slightly improves the classification in comparison to the only S2 image classifications. The use of time series substantially improves supervised classifications. The analysis carried out so far lays the foundation for the integration of time series of SAR and optical data