Giorgio Mancini

The structure of lead-silicate glasses: molecular dynamics and EXAFS studies

Molecular dynamics (MD) simulations and extended x-ray absorption fine structure (EXAFS) investigations of the structure of lead-silicate glasses, xPbO(1 − x)SiO2, have been undertaken to elucidate the problem of partially contradicting experimental findings reported in the literature about basic structural units and their interconnection. The MD simulations were performed in a wide range of compositions, x = 0.1–0.9. The atoms were assumed to interact by a two-body Born–Mayer–Huggins interaction potential. The EXAFS measurements were performed for x = 0.3, 0.5 and 0.7, and also for pure crystalline (red) PbO at the L3-edge of Pb. The absorption spectra were analysed within the GNXAS approach. Our EXAFS and MD results are in good agreement, and support some previous suggestions that: (1) the PbO4 groups are the dominant structural units in lead-silicate glasses for any concentration and (2) at lower PbO concentrations the co-existence of the PbO4 and PbO3 groups is possible. The medium-range ordering in the simulated glasses has also been investigated in detail. The connectivity of the SiO4 tetrahedra network breaks at about x = 0.45, whereas the Pb structural units form a continuous (mainly edge-sharing) network even at relatively low PbO concentrations (x > 0.2). The cation–anion ring statistics is also discussed.

EXAFS Experiment on a Copper Liquid Crystalline Coordination Compound

The results on the EXAFS experiments performed on a liquid crystalline copper (II) coordination compound are reported. To the best of our knowledge these are the first results concerning a calamitic liquid crystal. The shell-by-shell data analysis, either in solids or SB mesophase, shows a copper-copper correlation at 3.85 A, a distance which roughly corresponds to the molecular thickness. These results, together with the X-ray measurements previously carried on a monocrystal, suggest a local biaxial smectic phase.

A new program package for investigation of medium-range order in computer-simulated solids

The opportunity to gain detailed information on the representative sets of rings, and chains of atoms is of great importance in the analysis of medium-range order in the computer-simulated structures of solids. In this paper, a new program package (ANELLI) for ring and chain analysis of computer-simulated atomic structures is presented, and examples are given of its usage.

Drift resonance in the quantum Hall effect

Abstract The integral quantum Hall effect (IQHE) has been investigated in GaAsGaAlAs 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.

A redundancy eliminating approach to linearly independent rings selection in the ring perception problem.

Abstract Based on a recently published efficient, exact algorithm to solve the ring perception problem, a new approach is presented to feed the linear independence test on rings to enter a minimal basis with no duplicate information, thus reducing calls to the most demanding procedure in terms of computational order. The efficiency of a perfect hashing algorithm is actually met by a “pre-filtering” method derived from simple considerations.

Critical current in granular superconductor C-Si-W with peak-type re-entrant superconductivity

Some granular disordered superconductors, namely diamond-like carbon-silicon films containing tungsten, show one or more small, clearly detectable peaks in resistivity curves at lower temperatures than that of the superconducting main transition, Tc. Based on the concept of cluster superconductivity and on the percolation theory, we carried out extensive numerical simulations that relate the appearance of the peak-type re-entrant superconductivity to the absence of superconducting percolative paths crossing the entire sample. The numerical determination of such paths allowed us to compute the maximum supercurrent density jc to flow through the sample in the temperature region below Tc.

Magnetic properties of diluted (Zn1-xMnx)3As2 solutions

Abstract The magnetic susceptibility of the single-crystal alloy (Zn1-xMnx)3As2 has been investigated for the composition range 0.001 ≤ X ≤ 0.13 below the percolation threshold. The measurements have been carried out in the temperature range 2–250 K in magnetic fields from 2 up to 3700 Oe. Spin glass transitions have been observed in fields H ≈ 25 Oe at TSG ≤ 5 K. The χ(T) curves obtained for the zero field cooled and field cooled samples differ significantly, both qualitatively and quantitatively, from those relative to the specimens that had been precooled in the absence of magnetic field and successively heated. The observed hysteresis of χ(T) curves is explained by the presence of Mn ions clusters, as well as by a high magnetic viscosity of the SMSC system.

Numerical investigation of non-ohmic hopping conduction in macroscopically inhomogeneous thin layers: strong electron-phonon interaction

The current-field and conductivity-field characteristics of random r hopping as well as r-E hopping systems with a strong electron-phonon coupling have been calculated numerically and discussed by Bottger and co-workers (1985-86). They have assumed a random but macroscopically homogeneous distribution of centres over the sample. However, in most real cases an extremely thin layer can hardly be thought to be macroscopically homogeneous and the local hopping-centre density should depend on the distance from the electrodes. In the present work the authors apply the Bottger-Wegener procedure to random r hopping and r-E hopping systems with macroscopic average density exponentially dependent on the distance from the contact. Only a strong electron-phonon coupling will be considered, i.e. the authors shall consider small-polaron transport in a disordered solid. The influence of the inhomogeneity in the centre distribution on current field and conductivity-field characteristics may be summarized as follows. Firstly, for r-hopping transport in homogeneous layers, Bottger and Wegener observe a decrease in the differential conductivity with increasing field, its local minimum being followed by an exponential increase; the authors confirm the results for homogeneous systems, whereas for inhomogeneous systems they find that, on increasing the degree of the site distribution inhomogeneity the local conductivity minimum is no longer followed by an exponential conductivity increase, but the system becomes ohmic (conductivity saturation), at least up to the fields consistent with the assumption of constant carrier concentration. Secondly, for r-E hopping in homogeneous layers, Bottger et al. observe for not too high an energy spread in the hopping centres that the local conductivity maximum occurring just after the ohmic region is followed by a local minimum, that latter being followed by an exponential conductivity increase for still higher fields.

A Monte Carlo study of r- epsilon -hopping transient currents in thin dielectric layers with a macroscopically inhomogeneous spatial distribution of hopping centres

The authors report the results of the Monte Carlo simulation of the time-of-flight experiment for variable-range hopping transport in thin dielectric layers, which are spatially inhomogeneous on the macroscopic scale (i.e. on a scale comparable with the layer thickness). In particular, the total density of hopping centres (with Gaussian distribution in energy) is assumed to change exponentially as a function of the distance from the contacts. The results of simulations performed for various system dilutions, various widths of the Gaussian distribution in energy, and various degrees of the layer spatial nonuniformity are discussed.

r-hopping transient currents in thin dielectric layers with macroscopically non-homogeneous spatial distribution of hopping centres

The work is a contribution to the understanding of transient currents, which are usually discussed in the context of the classical time-of-flight (TOF) experiment. In particular, r-hopping pulse injection transient currents in dielectric layers with a macroscopically nonhomogeneous distribution of hopping centres have been investigated with the aid of the Monte Carlo simulation, according to a new algorithm. Both the shape of the transient currents and the value of the effective TOF are found to depend strongly on the degree of macroscopic variations in the hopping-centre concentration over the layer thickness. For a smooth change in space centre concentration, the transient currents may be described by simple analytical expressions, suitable for the estimation of the spatial distribution of hopping centres from experimental data.