Table of contents

The authors study the compressibility of Cu alloys with Au, Sn, Zn and Ni, according to the recently proposed model by Varotsos and Alexopoulos (1980). It is shown that this model applies to the above alloys as well. Therefore the compressibility of an alloy can be determined for any concentration provided that the compressibility is experimentally known for a single concentration.

A continuum percolation model for heterogeneous mixtures of an ionic conducting solid salt with an insulating second phase is introduced. In this model, the insulating phase is represented by spherical random voids (the Swiss Cheese model) which are allowed to overlap and are randomly distributed in a conducting matrix (the solid salt). A characteristic feature of the model is the existence of a highly conducting spherical shell surrounding the voids, representing the internal interface between the oxide particle and the conducting salt. The relevance of the model for describing the observed effects of insulating particle size on the conductivity of dispersed ionic conductors is critically discussed.

Using recent band-structure results obtained in the local-density scheme, the microscopic longitudinal dielectric function for both the diagonal and the nondiagonal elements is evaluated in the random-phase approximation in the energy range up to 2.5 Ryd. In addition, the influence of exchange and correlation is investigated in a self-consistent way within the time-dependent density-functional approach. The authors have found that the latter may modify the frequency dependence of the real elements of the dielectric matrix by up to 15%, whereas some imaginary elements are changed by up to 40%. In the long-wave limit, the local-field corrections of the macroscopic dielectric function seem to be negligible for most frequencies. For the diagonal elements of the inverse dielectric matrix with non-zero reciprocal lattice vectors, local-field corrections increase the real parts by up to 10% at low frequencies, whereas at higher frequencies their influence is quite small. The imaginary parts are decreased over the whole frequency range by up to 30%. For all spectra, local-field corrections smooth the structures and the inclusion of exchange and correlation decreases the values of the elements of dielectric matrix. In addition, for realistic wavefunctions, the f sum rule turned out to be badly fulfilled. Plausible arguments for this failure are presented.

Positron annihilation in liquid as well as in solid octadecane has been investigated by means of lifetime spectroscopy combined with magnetic quenching experiments. An anomalous magnetic quenching in solid octadecane was detected, which cannot be explained in terms of the usual Zeeman effect on positronium with a modified electron contact density at the positron. A technique to extract the value of the relative contact density without making use of magnetic fields is used. The measurements indicate that the longest lifetime component both in the liquid and solid phases must be ascribed to the decay of a positron-electron bound state.

The authors examine the effect of the different spatial widths of successive subbands on the calculation of the single-particle relaxation and scattering times when up to two subbands are occupied. It is shown that, firstly, the single-particle relaxation time for the second subband is always larger (by a factor of two or more) than that for the ground subband and, secondly, the scattering time for the second subband is sometimes larger and sometimes smaller than that for the ground subband (depending on the electron concentration in the second subband) in qualitative agreement with recent experimental data.

The thin diluted ferromagnetic film with SC symmetry is considered in the third-order Matsudaira approximation. The equations for the description of magnetisation and various correlation functions have been derived and the results of the numerical calculations have been presented. In particular the critical temperature, the critical concentration, the magnetisation and various correlation functions are discussed in detail for various film thicknesses and for several magnetic atom concentrations.

Magnetic properties of UPdAs₂ were studied by both bulk magnetic measurements and by neutron diffraction. The temperature dependence of the reciprocal magnetic susceptibility 1/ chi shows that UPdAs₂ orders antiferromagnetically below 240 K. Above 260 K, 1/ chi exhibits an almost straight-line behaviour with mu _eff=2.88 mu _B and theta _p=51 K. Neutron diffraction experiments confirm the tetragonal space group P4/nmm and yielded refined structural parameters. The magnetic ordering in UPdAs₂ is of AF IA type with magnetic moments aligned along the c direction. The magnetic structure consists of a stacking of ferromagnetic (001) sheets with a ++-- sequence along the four-fold axis. The ordered magnetic moment of uranium at 8.7 K amounts to 1.69(5) mu _B. The temperature variation of the magnetic (101) peak yields the Neel temperature T_N=235 K.

The unoccupied-states potential correction (USPC) in the local spin-density-functional (LSDF) formalism is proposed as a way to take correlation effects into account approximately. The method is based on using different potentials for the electrons in valence and conduction bands. When applied to antiferromagnetic transition-metal oxides (NiO, MnO, FeO, CoO), it solved the long-standing problem of reproducing the experimental values of the band gaps, which are underestimated in normal LSDF calculations. FeO and CoO become insulators, in contrast to the metallic ground state obtained in the usual LSDF calculations. Moreover, the calculated magnetic properties improved considerably due to the introduction of the USPC. It is shown that band-structure calculations using the USPC provide a correct description of available data from spectral measurements (ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, bremsstrahlung isochromate spectroscopy) for these compounds.

Strong enhancement of the gamma , gamma channel in nuclear resonant scattering was observed in a pure nuclear Bragg reflection of an almost perfect single crystal of ⁵⁷FeBO₃. A broadening of the lines in the Mossbauer reflection spectrum up to 30 natural linewidths and reflectivities >or=40% were measured at the exact Bragg position. Furthermore, interference of the hyperfine transitions gave rise to high reflectivity between the lines. In total, the region of high reflectivity extended over more than a hundred natural linewidths. Such a pure nuclear Bragg reflection can be effectively used for broad-band Mossbauer filtering of synchrotron radiation. In addition, rocking curves were measured with high angular resolution. Special line-shapes, characteristic of Bragg scattering at resonance, were observed.

The authors present a detailed analysis of the emission band at 13000 cm^-1 in KZnF₃:Cr³⁺. The spectrum structure at low temperature is interpreted in terms of zero-phonon lines and their replicas which are due to interaction with localised modes of both the defects and phonons of the host lattice.

The Lindemann criterion for flux-lattice melting is extended to the limit of electronically decoupled layers (infinite effective mass perpendicular to the layers). The resulting melting temperature resembles the formula for two-dimensional melting in a film, but with an effective film thickness characteristic of electromagnetic interlayer coupling.

The magnetisation was measured in plastically deformed 34.0 and 36.2 at.% Al-Fe alloys, and superlattice dislocations were observed by electron microscopy. At 34.0 at.% Al content, the spontaneous magnetisation increases considerably as a result of a slight plastic strain and has a broad maximum at about 200 K. At 36.2 at.% Al content, the magnetic susceptibility increases remarkably with plastic deformation and at the same time a spontaneous magnetisation appears and increases rapidly with decreasing temperature. The spontaneous magnetisation is discussed considering the atomic rearrangement due to superlattice dislocations. The change of magnetic properties is difficult to explain by local environment effects only.

The authors have measured the relaxation time in ferroelectric TAAP near the Curie temperature. The dielectric relaxation in this crystal could be described to a good approximation by the Debye equations. The relaxation time exhibits the critical slowing down characteristic of an order-disorder transition.