Microstructural defects in nanocrystalline iron probed by x-ray-absorption spectroscopy

An EXAFS (extended x-ray-absorption fine structure) study of nanocrystalline iron prepared by high-energy ball milling is presented. Accurate data analysis has been performed using a recently developed ab initio multiple-scattering method (GNXAS). The crystalline iron EXAFS spectrum, taken as reference, has been analyzed taking proper account of the important multiple-scattering signal. Results of the data analysis are shown to be in good agreement with known structural values. The dramatic decrease of the first-neighbor coordination number found in nanocrystalline iron, as a function of the milling time, is shown to be related to the presence of a large defect density. The corresponding decrease of the coordination numbers of the second and further neighbors, as well as the significant decrease of the multiplicities of the three-atom configurations, confirms this interpretation. A simple model which takes into account the presence of atomic defects is shown to lead to marked reduction of coordination numbers and three-atom multiplicities in agreement with the experimental data. Selected models without lattice defects are shown to be unable to give a quantitative explanation of the intensity reduction of the EXAFS signal found in milled nanocrystalline iron. An in situ annealing of the sample (up to about 1100 °C) milled for the longer time (32 h) is shown to reduce considerably the density of the defects, as expected, although no complete recovering of the crystalline order is found. The transition from α iron to γ iron has been directly observed for such nanocrystalline material

Emission Limited Current in Low-Mobility Solids

Results of computer-calculated solutions of a set of equations describing emission-limited currents (ELC) in low-mobility solids are presented. The continuity of current equation with electric field-dependent mobility is used to calculate the field dependence of ELC under various injection conditions. Its shape and steepness are found to change strongly with decreasing scattering length of injected carriers, approaching the standard description of the one-dimensional Onsager model with field-independent mobility. The recombination of the charge carriers at the contact is shown to reduce the current without remarkable change of its functional dependence on practically accessible electric fields

Reentrant superconductivity mechanisms in amorphous carbon-silicon films containing tungsten

A model-based on Josephson junctions array and Coulomb blockage-is presented that is capable to explain reentrant resistive peaks in diamondlike carbon-silicon films containing tungsten. Such systems show a main transition together with one or two reentrant or quasireentrant ones: computer simulations for one reentrant resistive peak have been performed that confirm with clear evidence the experimental data

Xanes and Exafs of Yba2cu3o-approximately-7 - Multiple-scattering Data-analysis and Evidence For A Structural Distortion At Tc

The polarized copper K-edge x-ray absorption near edge structure (XANES) of YBa2Cu3O7 has been calculated by using the full multiple scattering approach. The Cu K-edge EXAFS of YBa2Cu3O≈7 as a function of temperature is also analyzed. From this analysis we obtain evidence for a slight structural modification at Tc which can be associated with the superconductive transition

Vanadium Site Structure in V2O5 Gel by Polarized EXAFS and XANES

The local structure at the V site is investigated by polarized X-ray absorption spectroscopy in vanadium pentoxide gel V2O51.6H2O. Polarized absorption spectra allows to separate experimentally the contribution of neighbour atoms in different directions. EXAFS analysis were performed using spherical wave aproximation (SWA). When polarization vector E is in the direction of the axis of the pyramid, EXAFS analysis gives only the short double bond lenght at 1.58 Å, but XANES indicates the aditionalpresence of the oxygen at 2.7Å

Structure of Oriented V2O5 Gel Studied by Polarized X-Ray-Adsorption Spectroscopy at theVanadium K Edge

The local structure around the vanadium site in the oriented V2O5⋅1.6 H2O dehydrated xerogel phase is investigated by polarized x-ray-absorption spectroscopy (XAS). A large dychroism is found both in the extended x-ray-absorption fine structure (EXAFS) and in x-ray-absorption near-edge structure (XANES). A joint analysis of EXAFS and XANES data is reported. EXAFS analysis has been performed using a spherical-wave propagator and XANES analysis by full multiple-scattering calculation. A detailed discussion about the origin of all the different features in the experimental XAS spectra and a method to extract small contributions in the EXAFS part of the spectrum is presented. Moreover, the local structure around the vanadium atom in the V2O5 hydrogel phase is derived

Drift Resonance in the Quantum Hall Effect

The integral quantumHalleffect (IQHE) has been investigated in GaAsGaAlAs heterostructures in the temperature range 0.1 ⩽ T ⩽ 4.2 with magnetic fields B ⩽ 8.5T. The source-drain conductivity has been measured in alternating current regime at frequencies from 0 up to 1 GHz. Distortion of the Hall-plateaux has been observed. The phenomenon is connected to a drift of electrons and holes cyclotron orbits in magnetic field along closed equipotential trajectories on the relief of a random potential