Table of contents

Powder and foils containing up to 40% alpha"Fe₁₆N₂ have been prepared by annealing nitrogen-martensite. The magnetization of the alpha" phase is deduced from magnetization measurements, coupled with a quantitative phase analysis based on Mossbauer spectra. At room temperature the values are 225-270 J T^-1 kg^-1 corresponding to J_s=2.1-2.5 T. The average iron moment at T approximately O K is 2.3-2.6 mu B, which is in accord with the results of spin-polarized band structure calculations.

We present data, obtained by x-ray diffraction measurements, of the structure factors for pure liquid Si, and for liquid Ni-Si and Cu-Si alloys. We find it possible to analyse these data as well as data for Fe-Si by decomposition of rg(r) where g(r) is the pair distribution function; this analysis provides evidence for Si-Si covalency both in pure liquid Si and in the Si-rich liquid alloys. The degree of covalency in the Si-rich alloys appears to increase with changing species in the order Fe to Ni to Cu.

We report neutron scattering measurements of the dynamic structure factor S(Q, omega ) of liquid ⁴He in the collective excitation regime. The use of the IN6 time-of-flight spectrometer at the Institut Laue-Langevin has enabled us to cover the wavevector region from 0.3 to 2.1 AA^-1 in a continuous manner with an accurately calibrated energy scale and an energy resolution of approximately 0.1 meV; temperatures between 1.24 and 4.95 K were examined at saturated vapour pressure. In the wavevector region investigated, sharp excitations are seen that are unique to superfluid ⁴He; it is the purpose of the present paper to examine their temperature dependence and to discuss measurements of the multiphonon continuum scattering at higher energies. At small wavevectors in the 'phonon' region, the excitations are relatively unaffected by the lambda transition, whereas at larger Q a sharp component disappears at or close to T_lambda . The temperature variation of S(Q, omega ) is much more rapid in the superfluid phase than in the normal phase. In paper II of this series, the results for S(Q, omega ) will be compared with several models for the temperature variation of the ⁴He excitation spectrum.

In this work we present pair distribution functions (PDFs) of amorphous Fe_1-xTb_x (x=0.1-0.5) with a high spatial resolution better than 0.02 nm obtained from electron diffraction experiments. The short-range order of FeTb alloys with up to 32 at.% Tb content is described quantitatively as an arrangement of the known crystalline FeTb phases with various concentrations and structures by superposing PDFs of these structures in the range of the first and second coordination shell up to 0.6 nm. Partial FeFe, FeTb and TbTb radial distribution functions (RDFs) are obtained from this model; coordination numbers and neighbour distances are presented and compared to other authors' results.

A theoretical interpretation of the transition sequences in the (P(CH₃)₄)₂ (CuCl₄)_1-x(CuBr₄)_x solid solution is proposed. Each pure compound (P(CH₃)₄)₂CuCl₄ and (P(CH₃)₄)₂CuBr₄ presents its own transition sequence which contains an incommensurate phase and different locked phases. In the solid solution with x=0.12, phases characteristic of the two pure compounds are observed in the sequence. The interpretation of this phase diagram is based on a coupling of the two order parameters, each of them corresponding to their own phonon branch. This coupling results in a re-entrant symmetry for one of the two order parameters, in agreement with experimental data.

A general theory for the calculation of second-order Jahn-Teller reduction factors for strongly coupled vibronic systems has already been developed. It was based on symmetry arguments and gave results applicable to orbital triplet systems of all symmetries. This paper describes further developments and improvements that have been made in the general theory. As before, symmetry arguments dominate the analysis, which has two distinctive features. Firstly, by using a more fundamental definition of the orbital operators required than that used previously, it is shown how the problems encountered previously in attempting to apply the previous formalism to orbital doublet E(X)e systems are avoided. Secondly, the derivation of general formulae from which symmetry-adapted phonon states may be derived is presented. P is shown that their use in preference to the symmetry-adapted vibronic states used before simplifies the calculation of the oscillator overlaps required. Also, excited-stare energies may be obtained directly as they can be expressed as the sums of various reduced matrix elements among the excited phonon states. As an example, the general method is presented in detail for the strongly coupled E(X)e Jahn-Teller system.

The partial-wave method is used for the calculation of the residual electrical resistivity caused by dislocations and grain boundaries in various metals. The dislocation core is regarded as a resonance scattering line defect containing surplus free volume. Scattering by elastic distortions of the lattice is neglected. An estimation of the effective carrier concentration in transition metals is performed on the basis of Mott's theory. The proposed model for the evolution of the grain-boundary structure with misorientation angle regards the approach and confluence of the line defects forming the boundary, and results in a decrease of the resonance part of the total scattering cross section. The high-angle grain boundaries are assumed to consist of cylindrical voids. Taking into account the values of the steady Burgers' vectors, quite satisfactory agreement with the available experimental data is obtained.

We present a numerical calculation of the tunnelling time of electrons confined in double-barrier structures performed by means of the so-called stabilization method, widely used in quantum chemistry. From the stabilization graphs we find the resonance energy and its width. The method is especially appropriate for treating the case of double-barrier structures (symmetric or non-symmetric) because it allows one to calculate separately the two different tunnelling times (to the left and to the right of the quantum well) contributing to the total lifetime of a resonant level. We use the effective-mass theory. The behaviour of the tunnelling time under applied bias is also investigated and the results are compared with the ones obtained by two alternative approaches, the quasi-classical approximation and the transmission coefficient analysis, respectively. A good agreement between the three methods is obtained for the cases analysed. Finally, the stabilization method as applied here can be employed in the field of scanning tunnelling microscopy of absorbed atoms or molecules where a double-barrier potential also serves as a model for the problem.

A cell has been built to measure the electrical resistivity of single crystals and thin films in a sulphur atmosphere. Resistivity variations during the sulphuration of pyrite samples have been investigated as a function of temperature ( approximately 450-723 K) and of time at constant temperature. The transformation of Fe thin films into pyrite films has been followed through their electrical resistivity changes up to approximately 623 K. Results are briefly discussed by considering the stoichiometric and crystallographic transformations undergone by the films. After these preliminary results some open questions are presented.

A simple expression for the effective dielectric constant of a symmetric two-component composite (aggregate topology) is presented and interpreted in terms of the Bergman spectral representation.

The optical properties and temperature-dependent concentration quenching of Nd³⁺ fluorescence have been investigated in heavy-metal and transition-metal fluoride glasses using steady-state and time-resolved laser spectroscopy. Judd-Ofelt parameters were derived from the absorption spectra and used to calculate the ⁴F_3/2 to ⁴I_11/2 stimulated-emission cross-section and the ⁴F_3/2 radiative lifetime. From the thermal behaviour of lifetimes, a T³ dependence for the non-radiative Nd-Nd relaxation processes has been found in the 15-100 K temperature range for the three families of fluoride glasses investigated, which is in agreement with a two-site non-resonant process.

High-resolution photoemission study of the valence-band electronic structure of A_xC₆₀ (A=K or Cs; x<or approximately=3) close to E_F reveals a phase separation into components exhibiting a dilute solid solution of alkali in fullerite ( alpha -C₆₀) and A₃C₆₀ The alpha -C₆₀ phase is found to be remarkably stable against oxidation. Our analysis shows a strongly correlated electronic system in this phase. The similarity of the present data to previously published results for Li-, Na- and Rb-doped C₆₀ demonstrates the common character of the observed phenomena for this series of alkali-doped fullerites.