Detection and characterization of hydraulically active fractures in a carbonate aquifer: results from self-potential, temperature and fluid electrical conductivity logging in the Combioula hydrothermal system in the southwestern Swiss Alps

A geophysical and geochemical study has been conducted in a fractured carbonate aquifer located at Combioula in the southwestern Swiss Alps with the objective to detect and characterize hydraulically active fractures along a 260-m-deep borehole. Hydrochemical analyses, borehole diameter, temperature and fluid electrical conductivity logging data were integrated in order to relate electrokinetic self-potential signals to groundwater flow inside the fracture network. The results show a generally good, albeit locally variable correlation of variations of the self-potential signals with variations in temperature, fluid electrical conductivity and borehole diameter. Together with the hydrochemical evidence, which was found to be critical for the interpretation of the self-potential data, these measurements not only made it possible to detect the hydraulically active fractures but also to characterize them as zones of fluid gain or fluid loss. The results complement the available information from the corresponding litholog and illustrate the potential of electrokinetic self-potential signals in conjunction with temperature, fluid electrical conductivity and hydrochemical analyses for the characterization of fractured aquifers, and thus may offer a perspective for an effective quantitative characterization of this increasingly important class of aquifers and geothermal reservoir

Thermal stability of in-grown vacancy defects in GaN grown by hydride vapor phase epitaxy

We have used positron annihilation spectroscopy to study the thermal behavior of different native vacancy defects typical of freestanding GaN grown by hydride vapor phase epitaxy under high pressure annealing at different annealing temperatures. The results show that the VGa‐ON pairs dissociate and the Ga vacancies anneal out from the bulk of the material at temperatures 1500–1700K. A binding energy of Eb=1.6(4)eV can be determined for the pair. Thermal formation of Ga vacancies is observed at the annealing temperatures above 1700K, indicating that Ga vacancies are created thermally at the high growth temperature, but their ability to form complexes such as VGa‐ON determines the fraction of vacancy defects surviving the cooling down. The formation energy of the isolated Ga vacancy is experimentally determined.Peer reviewe

Enhancement of piezoelectricity in a mixed ferroelectric

We use first-principles density-functional total energy and polarization calculations to calculate the piezoelectric tensor at zero temperature for both cubic and simple tetragonal ordered supercells of Pb_3GeTe_4. The largest piezoelectric coefficient for the tetragonal configuration is enhanced by a factor of about three with respect to that of the cubic configuration. This can be attributed to both the larger strain-induced motion of cations relative to anions and higher Born effective charges in the tetragonal case. A normal mode decomposition shows that both cation ordering and local relaxation weaken the ferroelectric instability, enhancing piezoelectricity.Comment: 5 pages, revtex, 2 eps figure

Effect of high-temperature annealing on the residual strain and bending of freestanding GaN films grown by hydride vapor phase epitaxy

The effect of high-temperature high-pressure annealing on the residual strain, bending, and point defect redistribution of freestanding hydride vapor phase epitaxial GaN films was studied. The bending was found to be determined by the difference in the in-plane lattice parameters in the two faces of the films. The results showed a tendency of equalizing the lattice parameters in the two faces with increasing annealing temperature, leading to uniform strain distribution across the film thickness. A nonmonotonic behavior of structural parameters with increasing annealing temperature was revealed and related to the change in the point defect content under the high-temperature treatment.Peer reviewe

Temperature-dependence of exciton radiative recombination in (Al,Ga)N/GaN quantum wells grown on a-plane GaN substrates

This article presents the dynamics of excitons in a-plane (Al,Ga)N/GaN single quantum wells of various thicknesses grown on bulk GaN substrates. For all quantum well samples, recombination is observed to be predominantly radiative in the low-temperature range. At higher temperatures, the escape of charge carriers from the quantum well to the (Al,Ga)N barriers is accompanied by a reduction in internal quantum efficiency. Based on the temperature-dependence of time-resolved photoluminescence experiments, we also show how the local disorder affects the exciton radiative lifetime at low temperature and the exciton non-radiative lifetime at high temperature.We acknowledge financial support from the Swiss National Science Foundation through Project No. 129715 and from the Polish National Science Center (Project DEC-2011/ 03/B/ST3/02647). The work was partially supported by the European Union within European Regional Development Fund through Innovative Economy Grant No. POIG.01.01.02-00-008/08. P.C. also acknowledges financing from the European Union Seventh Framework Program under grant agreement No. 265073

Overview of the 2010 Carbonaceous Aerosols and Radiative Effects Study (CARES)

Substantial uncertainties still exist in the scientific understanding of the possible interactions between urban and natural (biogenic) emissions in the production and transformation of atmospheric aerosol and the resulting impact on climate change. The US Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program's Carbonaceous Aerosol and Radiative Effects Study (CARES) carried out in June 2010 in Central Valley, California, was a comprehensive effort designed to improve this understanding. The primary objective of the field study was to investigate the evolution of secondary organic and black carbon aerosols and their climate-related properties in the Sacramento urban plume as it was routinely transported into the forested Sierra Nevada foothills area. Urban aerosols and trace gases experienced significant physical and chemical transformations as they mixed with the reactive biogenic hydrocarbons emitted from the forest. Two heavily-instrumented ground sites – one within the Sacramento urban area and another about 40 km to the northeast in the foothills area – were set up to characterize the evolution of meteorological variables, trace gases, aerosol precursors, aerosol size, composition, and climate-related properties in freshly polluted and "aged" urban air. On selected days, the DOE G-1 aircraft was deployed to make similar measurements upwind and across the evolving Sacramento plume in the morning and again in the afternoon. The NASA B-200 aircraft, carrying remote sensing instruments, was also deployed to characterize the vertical and horizontal distribution of aerosols and aerosol optical properties within and around the plume. This overview provides: (a) the scientific background and motivation for the study, (b) the operational and logistical information pertinent to the execution of the study, (c) an overview of key observations and initial findings from the aircraft and ground-based sampling platforms, and (d) a roadmap of planned data analyses and focused modeling efforts that will facilitate the integration of new knowledge into improved representations of key aerosol processes and properties in climate models.United States. Dept. of Energy. Atmospheric System Research Program (Contract DE-AC06-76RLO 1830)United States. National Oceanic and Atmospheric AdministrationUnited States. National Aeronautics and Space Administration. HQ Science Mission Directorate Radiation Sciences ProgramUnited States. National Aeronautics and Space Administration. CALIPSO ProgramUnited States. Dept. of Energy. Atmospheric Radiation Measurement Program (Interagency Agreement No. DE-AI02-05ER63985

Magnetic and hyperfine interaction in RFe4Al8 (R = Ce,Sc) compounds

Magnetic properties of ScFe4Al8 and CeFe4Al8 compounds have been studied by magnetization and Mössbauer effect measurements. Magnetic transition temperatures estimated from Mössbauer spectra (B = 0) 170 K for CeFe4Al8 and 225 K for ScFe4Al8 are not confirmed by magnetization measurements. Contrary, the pronounced maxima at Tmax = 130 and 125 K in DC magnetization curves (B = 1 kOe) were found for ScFe4Al8 and CeFe4Al8, respectively. Thermomagnetic, the so-called zero field (ZFC) and field cooling (FC) experiments show temperature-dependent irreversibilities below the "freezing" temperature, Tf, which diminishes with application of external magnetic field. The Mössbauer studies show the coexistence of magnetically (sextet) and non-magnetically (quadrupole doublet) split patterns in the wide temperature range far away from Tmax. All these observations indicate that the systems studied are either a spin-glass or are the mixture of AF and spin-glass state. © 2001 Elsevier Science B.V