National Institute Of Materials Physics - Romania

A method for measuring the neutron lifetime is proposed, using simultaneous detection of electrons and upscattered neutrons generated during the ultracold neutron storage.

Results of magnetic measurements performed on xGd2O3.(1-x)[3B2O3.PbO] glasses, in the temperature range 4.2-300 K are reported. For samples with x > 2 mol% Gd2O3, a downward curvature of the reciprocal susceptibilities, at T < 30 K, is observed. At higher temperatures, a Curie-Weiss behaviour is shown. The paramagnetic Curie temperatures for glasses having x > 2 mol% Gd2O3 are negative and increase in absolute magnitude when increasing the Gd2O3 content. The magnetic behaviour of Gd2O3-B2O3-PbO glasses are analyzed in terms of a random distribution of gadolinium ions in glass matrix.

We have observed and identified the optical excitation and emission spectra of Mn2+ ions in the doubly doped CdCl2:Mn:CU system between 77 and 300 K. A comparison is made with the singly doped CdCl2 system. The temperature dependence of the absorption strength is explained by vibrational electronic interaction, and the quantum efficiency is close to unity for low-Mn2+-concentration crystals. In CdCl2 the 1E(g) state of Cu+ lies in the conduction band. By excitation in the main absorption band, 1A1g --> 1E(g), of Cu+ in CdCl2:Mn:Cu crystals an energy transfer to Mn2+ ions takes place through intermediate metastable states (excimer-like species) which form in the lattice near the Cu+ centres. Because these states thermally ionize, the energy transfer rate decreases when the temperature increases and above 100 K the photoionization of Cu+ centres becomes dominant.

The Mossbauer spectrum of ferric oxinate exhibits an asymmetric quadrupole doublet. The modification of the dynamic Mossbauer parameters (to the equal values for each line) following the irradiation, was explained by the internal oxine ligands rotation inside of the octahedral coordination.

The shapes of current versus time curves in ZnS : Mn MISIM structures are studied in dependence on voltage, temperature, and Mn concentration. The experimental curves compare well with those computed on the basis of a tunneling model which takes into account the presence of the internal field due to bulk ionized centres and the filling level of the interface states.

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.