Evolution of magnetotelluric, total magnetic field, and VLF field parameters in Central Italy: relations to local seismic activity

Magnetotelluric data were collected at Collemeluccio (41.72°N, 14.37°E) in Central Italy from summer 1991 to spring 1998. Analyzed by means of tensor decomposition on the geoelectric potential and robust estimation on the geomagnetic field, this set of data allowed the investigation of the electric properties at different time-periods. The variation of some indicators, related to the phenomenon of electromagnetic induction, is presented here in its time evolution and compared to local and regional seismic activity. Tectonomagnetic field observations from absolute magnetic field level in Central Italy were also made on data simultaneously recorded at four magnetometer stations, using L'Aquila Geomagnetic Observatory as a reference for differentiation. Recent results gathered from a system of two VLF search coil wide-band antennas, installed in the L'Aquila Observatory, are also discussed in relation to local seismic activity

Evolution of magnetotelluric, total magnetic field, and VLF field parameters in Central Italy: relations to local seismic activity

Magnetotelluric data were collected at Collemeluccio (41.72°N, 14.37°E) in Central Italy from summer 1991 to spring 1998. Analyzed by means of tensor decomposition on the geoelectric potential and robust estimation on the geomagnetic field, this set of data allowed the investigation of the electric properties at different time-periods. The variation of some indicators, related to the phenomenon of electromagnetic induction, is presented here in its time evolution and compared to local and regional seismic activity. Tectonomagnetic field observations from absolute magnetic field level in Central Italy were also made on data simultaneously recorded at four magnetometer stations, using L'Aquila Geomagnetic Observatory as a reference for differentiation. Recent results gathered from a system of two VLF search coil wide-band antennas, installed in the L'Aquila Observatory, are also discussed in relation to local seismic activity

Rocksalt ZnMgO alloys for ultraviolet applications: Origin of band-gap fluctuations and direct-indirect transitions

Rocksalt Zn x Mg 1−x O alloys are theoretically and experimentally investigated for near-and deep-UV optoelectronics with a tunable band gap of 4.2-7.8 eV. Regarding the key question about the composition x, at which there is a transition between the direct and indirect gaps, we performed ab initio calculations for various Zn concentrations and all possible atomic arrangements in eight-to 64-atom supercells. We show that, depending on the detailed Zn distribution (clustered, random, or uniform distribution), the alloy band gap can vary by as much as 1.27 eV. The band gap is indirect for clustered and random Zn arrangements in the supercell. For uniform Zn arrangements, the gap is also indirect, except for x < 0.5 and atom uniform arrangements excluding Zn-O-Zn nearest neighbor bridges, for which the direct gap can be lowered below the indirect gap by about 0.1 eV. The mechanisms of band-gap fluctuation, Zn clustering, and direct-indirect band-gap transitions are analyzed and explained in terms of atomic contributions to band structures by projecting Bloch functions onto localized Wannier functions. Simultaneously, cathodoluminescence measurements were performed on a set of Zn x Mg 1−x O multiquantum wells grown by molecular beam epitaxy on MgO substrates. We observed strong and broad emission bands, redshifting with increasing Zn concentration but featuring no clear-cut evidence for any direct to indirect band-gap crossover. We argue that these alloys are well suited for deep-UV optoelectronics, thanks to the rare combination of strong exciton binding energy, coupling to phonons, and carrier localization, which is favored by the marked flattening of the top valence bands by both short-range and long-range Zn-Zn interactions

Epitaxial Layers Versus Bulk Single Crystals of GaN. Temperature Studies of Lattice Parameters and Energy Gap

Gallium nitride epitaxial layer grown by molecular beam epitaxy and bulk crystal grown at high pressure were examined by using X-ray diffraction methods, and by optical absorption at a wide temperature range. The free electron concentration was 6 × 10$\text{}^{17}$ cm$\text{}^{-3}$ for the layer and about 5 × 10$\text{}^{19}$ cm$\text{}^{-3}$ for the bulk crystal. The experiments revealed a different position of the absorption edge and its temperature dependence for these two kinds of samples. The structural examinations proved a significantly higher crystallographic quality of the bulk sample. However, the lattice constants of the samples were nearly the same. This indicated that a rather different electron concentration was responsible for the different optical properties via Burstein-Moss effect