Publications

The effect of a periodic distribution of easy directions on the surface macroscopic properties of nematic samples are considered. Our analysis shows that the effective surface energy of the sample contains an interference term coming from the two different easy axis. This allows to introduce an effective uniform easy direction, depending on the two easy directions and containing an interference term too, as suggested a few years ago by some researchers.

A texture model suitable for use in Rietveld-refinement programs is proposed. It is based on the series expansion of the pole distribution function in symmetrized harmonics. The model was tested on a textured plate sample of Al2O3.

This Comment refers to a recent publication on the ac hopping conduction of the Fibonacci chain [Phys. Rev. B 43, 1183 (1991)]. The renormalization procedure proposed by its authors gives a correct formula for the conductivity [Eq. (15)]. Nevertheless, their conclusion regarding the vanishing of the dc conductivity disagrees with the low-frequency asymptotics previously derived by us for the same model, and also disagrees with the general result given by the resistance-network analogy of the Miller-Abrahams rate equations. Their result assumes the factorization of the frequency in Eq. (15), which we prove is not true. We show also that their numerical curves, which apparently support their conclusion, are strongly influenced by errors in the low-frequency range.

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 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.