National Institute Of Materials Physics - Romania

The localization of electrons in a one-dimensional tight-binding model with diagonal aperiodicity given by the Rudin-Shapiro sequence is studied in the frame of the transfer-matrix formalism. It is proved that the trace map of the model possesses a polynomial invariant. It is also proved that an infinite sequence of values of the on-site potential exists such that for each of these the energy E = 0 lies in the spectrum of the periodic approximants corresponding to the even generations of the chain. Accurate numerical computations show that the states associated to the center of the spectrum are weaker than exponentially localized even for rather small amplitudes of the on-site potential. Scaling laws that govern the spatial decay and self-similarity of these states are derived for various values of the potential strength. It is also shown that the decrease of the potential amplitude qualitatively changes the self-similarity of the wave functions on increasing length scales.

The static dielectric susceptibility of a two-dimensional electron plasma in a strong perpendicular magnetic field is obtained, including the electron-electron interactions with use of a Green's-function formalism at finite temperature. Below a critical temperature, the susceptibility ceases to be negative definite, having poles related to a second-order transition into charge-density-wave states. Below yet another critical temperature, the negative sign is restored for some wavelengths and for some filling factors that correspond to unstable homogeneous states. The dielectric constant, as well as the thermodynamic density of states, can take on negative values.

The low-field intragranular irreversibility line of BiSr2Ca2Cu3O10 high-temperature superconductors was obtained from transport critical-current measurements performed on polycrystalline bulk sintered samples with a pronounced granular character. For such samples, the intra- and intergranular critical-current densities are very different, the latter being largely mediated by tunneling. The method is based on the high sensitivity of the intergranular critical current to the local magnetic field at the intergrain contacts. The value of this field is strongly influenced by the grain magnetization and therefore depends on cooling conditions. The irreversibility temperature was determined as the temperature above which the voltage-current characteristics measured with the sample cooled in applied-field and in zero-field-cooling conditions become identical. For the investigated samples, an exponential decrease of the irreversibility field with increasing temperature was observed. A description of the irreversibility line in terms of thermally assisted flux flow is given.

Samples with different Si/Al ratios and ionic exchange degrees were studied. Test molecules were pyridine, trimethylpyridine, methylquinoline and carbon monoxide. There were evidenced Bronsted and Lewis sites, as well as Ag+ species. The role of these sites in methanol conversion was discussed.

The linear relations of the neutron and X-ray optics in crystal powder diffractometers are extended with second-order terms. An expression for the angular deviation from the Bragg-peak position is derived which reduces to that of Wilson [Mathematical Theory of X-ray Powder Diffractometers (1963). Amsterdam: Philips Technical Library] for the case of characteristic X-rays in parafocusing geometry. A technique of computing second-order aberration effects is described. Line-shift formulae are given for conventional and curved-crystal diffractometers.

The quadratic-response function of the particle density to a static harmonic potential is calculated for a totally occupied Landau level. Then, with use of a variational approach, it is shown that the linear component of the density response is dominant for any wave vector q of the external potential and for a large range of amplitudes, including those which remove the gap up to the next Landau level. The contribution of the nonlinear components does not exceed a few percent if q < l(B)-1 and may be statisfactorily described by the quadratic component if q < 2l(B)-1, where l(B) = (HBARc/eB)1/2 is the magnetic length.

This paper describes the growth of CdF2 single crystals using the Bridgman method. Unwanted vapour reactions are avoided by using an argon atmosphere and glassy carbon crucibles. The installation for the growth of CdF2 single crystals is described in detail. Optical absorption of CdF2 crystals is shown for wavelengths of 200 to 800 nm. Transmission spectrum in the 2.5-10-mu-m range shows a low level of impurities which do not affect the optical qualities.

The steady-state conductance G of thick-film SnO2 chemoresistive gas sensors is measured for different CO concentrations c in air, in the 0-300 ppm range, at 0 and 100% relative humidity. The conductances G0 at 0 and G at 120 ppm CO in normal air are measured as a function of time in non-steady-state conditions, during the first two hours of stabilization, after keeping the sensors unheated (out of operation) for seven days. The time dependence of the conductance at sudden changes of the CO concentration between c = 0 and c not-equal 0 in normal air is also measured. Experimental results indicate that the ratio (G2-G0(2))/G0 depends almost linearly on c down to very low values (about 1 ppm), it is fairly independent of humidity in the steady state, and of time during the stabilization stage. This can offer a possibility for using SnO2 sensors not only for the detection, but also for the quantitative measurement, of small concentrations of a combustible gas in air.

A theory of correlated hopping diffusion on lattices with site-exclusion interaction is presented. We develop a variational procedure to estimate the diffusion constant D. We derive a method of obtaining several sequences of decreasing upper bounds for D. The method is extensively discussed for the particular sequence that starts with the mean-field approximant D(MF). The first term of a second sequence D(MF) approximately is obtained by a suitable renormalization of the jump rates and is closer to D than D(MF). This modified mean-field approximation enables us to formulate an improved version of the Miller-Abrahams equivalence to a resistor network that includes correlation effects.

The dimensionality of the dissipation process in the Bi2Sr2Ca2Cu3O10 high-T(c) superconducting compound was analyzed. This material shows two-dimensional superconductivity at high temperatures, almost up to T(c). Critical-current-anisotropy studies performed on ceramic samples with preferential crystallite orientation revealed the absence of strong intrinsic pinning. The dissipation is caused only by the magnetic field component perpendicular to the CuO2 planes.