Table of contents

The data on the structural transition temperature at ambient pressure, the pressure effect and the alloying effect (RAg_1-xIn_x) in CsCl-type compounds RM (R=La, Ce, Pr, Nd; M=Ag, Cd, Tl) are summarised. The R dependence shows common tendencies, which strongly suggests that the d-character electron in the 5d6s band of R plays an important role in the structural transition.

It is shown that a simple hole created in a singlet state of the strongly correlated Hubbard model, with one electron per atom, can substantially lower the kinetic energy of the system by binding to a spin fluctuation to form a magnetic polaron. Calculations are made for the linear chain, and for the square lattice which relates to the CuO₂ layers in the new oxide superconductors. It is also shown that two simple holes can lower kinetic energy by binding to form a pair. The possibility is considered that two magnetic polarons, fermion-like with spin-¹/₂, may also lower kinetic energy by painting, thus leading to a type of superconductivity in which the normal state is not a Fermi liquid and where the transition temperature is determined by the bandwidth alone.

The authors have calculated the electronic structure of the superconducting YBa₂Cu₃O₇ and the non-superconducting YBa₂Cu₃O₆ compounds. They find that on reducing the oxygen content the Cu in the O-Cu-O linear chains becomes insulating. They have also studied some hypothetical YBa₂Cu₃F₂O₆ systems to examine the effect of fluorine on the Cu-O₅ pyramids and the O-Cu-O chains.

Information on the nature of the superconducting state in La_1.85Ba_0.15CuO₄ has been obtained from high-precision differential specific heat measurements on superconducting La_1.85Ba_0.15CuO₄, and non-superconducting La₂CuO₄ and La_1.85Ba_0.15Cu_0.9Ni_0.1O₄ samples between 1.5 and 300 K. From the very small jump of 0.07 mJ mol^-1 K^-2 in C/T at T_c approximately 29 K the authors obtain (dH_c/dT_c)=33 Oe K^-1 and H_c(0)=475 Oe, where H_c is the thermodynamic critical field. A condensation of real-space Bose pairs (e.g. bipolarons) is excluded by the author's results, which indicate that a fraction of around 0.06 of the electrons within kT_c of the Fermi surface are in Cooper pairs. The estimated electron mass enhancement (1+ lambda ) with lambda approximately 0.82 is lower than expected for conventional phonon-mediated superconductivity with T_c approximately 29 K and indicates that an alternative, possibly excitonic, pair interaction may be involved. Phonon spectra are determined that are in agreement with theoretical predictions, and evidence is obtained for strong high-temperature anharmonicity. A striking anomaly at 58 K in the specific heat of the superconducting compound may indicate a soft phonon mode.

Two samples of the compound GdBa₂Cu₃O_7-x, one with x approximately 0 (T_c=93.5 K) and one with x>0.5 (T_c<1.8 K), have been studied by ¹⁵⁵Gd Mossbauer spectroscopy in the temperature range 1.5 K<or=T<or=120 K. A non-zero value of the asymmetry parameter eta in both samples shows the point symmetry at Gd sites to be orthorhombic at low temperatures. The narrow linewidth implies a high degree of perfection of the authors' samples. Significant differences of the frequency spectra of Gd ions between the two samples are derived from the temperature dependence of the absorption cross sections and of the centre shifts.

The coexistence of magnetism and superconductivity in the high-T_c compound GdBa₂Cu₃O_7-x (x approximately 0) and the influence of oxygen deficiency on the magnetic properties in a sample with x>0.5 have been studied by measurements of the specific heat and the bulk magnetisation and by ¹⁵⁵Gd Mossbauer spectroscopy. The magnetic ordering is found to have a pronounced two-dimensional character. The orientation of the ordered Gd moments parallel to the c axis shows the ordering to be due to an anisotropic superexchange interaction. The effect of oxygen vacancies on the magnetic properties is marginal. No indications for magnetic moments at Cu ions was found.

The authors use the one-component-plasma (OCP) model as a reference system instead of the traditional hard-sphere fluid to calculate the liquid-vapour interfacial surface tension of liquid metals within the density functional formalism. The calculated surface tensions of the alkali metals are in excellent agreement with experiment. For the polyvalent metal Al, the result the authors obtain is larger than experimental measurements. They conclude that the OCP system is not suitable to describe the liquid-vapour phase transition in simple metals which have a nominal plasma parameter Gamma -Z²e²/ak_BT larger than the usual freezing value of approximately 178. The calculated interfacial widths in all cases are narrower than the expected experimental values.

The authors have measured inelastic neutron scattering spectra of molten Na_0.5Cs_0.5 and Na_0.85Cs_0.15 alloys using the time-of-flight spectrometer IN6 at the Institut Laue-Langevin. The composition Na_0.85Cs_0.15 corresponds to the maximum value of S_cc(Q to 0) indicating a tendency to phase separation. The width of the total dynamical structure factor S_T(Q, omega ) at low Q is significantly larger for Na_0.85Cs_0.15 than for Na_0.5Cs_0.5. Quasi-dispersion curves derived from peak positions of the total longitudinal current power spectra show a single (overdamped) 'Na-like' mode for Na_0.85Cs_0.15 and both 'Na-like' and 'Cs-like' modes for Na_0.5Cs_0.5. Appropriate dynamical variables in this Q range for these alloys are therefore atom type. No evidence of propagating acoustic modes is found in the (Q, omega ) range studied.

The authors complete a previous study, due to Quelard et al. (1978), where polycrystalline Bi samples were electron irradiated at energies lower than 1.2 MeV and at 20 K, by a study at higher energies (up to 2.5 MeV) of single-crystal Bi and BiTe (93 at.PPM) dilute alloy. The experimental procedure consists in measuring the slope of the resistivity against the electronic flux at various electron energies. The analysis of the data is complicated by the semi-metallic behaviour of Bi: there are two types of charge carriers, and the defects are globally donors. However, the authors confirm the value of 13 eV for the threshold energy given by Quelard et al. From the surprising results obtained for BiTe they can deduce that the mean defect charge depends on irradiation energy and that Frenkel pairs are of donor type whereas point-defect complexes are probably of acceptor type.

The electrical resistivity of the intermetallic compounds LaAg, CeAg and YAg has been measured in the range 10-800 K. The data on YAg are described by the usual Block-Gruneisen formula, but both CeAg and LaAg show higher resistivity values and a different thermal dependence. These anomalies can be ascribed to valence instabilities of the two rare earths, but, unlike most phases with unstable cerium and ytterbium atoms, the corresponding resistivity anomaly does not saturate in the temperature range studied.

Accurate measurements of the electrical resistivity ( rho ) and its temperature derivative (d rho /dT) have been performed in a set of random anisotropy alloys a-Dy_xGd_1-xNi (x=0.1, 0.25, 0.5, 0.75 and 1.0) exhibiting magnetic freezing at temperatures (T_f) ranging from approximately 60 K to approximately 14 K, and a negative d rho /dT in the paramagnetic phase. For the Gd-rich samples, which have a low anisotropy energy D (D/J approximately 10^-2; J=exchange energy), the magnetic resistivity ( rho _m) increases steadily with temperature until saturation above T_f; in contrast, for the Dy-rich samples (large anisotropy, D approximately J) rho _m decreases with increasing temperature near T_f. These results are interpreted qualitatively in terms of a schematic model based on the existence of effective antiferromagnetic pairs located at the boundary of Imry and Ma domains (1975). This interpretation is supported by magnetoresistance measurement performed in the authors' samples. They also analyse the x dependence of rho _m along the series at constant reduced temperatures. An approximately linear dependence of rho _m on x was found both above and below T_f (at least down to T/T_f approximately=0.5, for all the samples except x=0.1). Their d rho /dT results exhibit pronounced anomalies near T_f, which enable the accurate determination of T_f in the samples investigated.

The temperature dependence of the thermoelectric power due to the interference of the inelastic electron-electron interaction and multiple elastic electron scattering on the microscopic concentration fluctuations (the eigenstructural states of the amorphous systems) is calculated. The results are in good agreement with experimental data.

The resistivity of the heavy fermion compound CeInCu₂ has been measured under pressure up to 1.55 GPa (15.5 kbar); the resistivity maximum is swept from 23 K to 80 K with increasing pressure. The compressibility was measured at room temperature, as well as the thermal expansion between 1.2 and 50 K. The Gruneisen parameter evaluated from the shift of the resistivity curve is about 26, the value deduced from the thermal expansion, specific heat and compressibility being 42.

The resistivity of gold films with thicknesses between 18 and 52 nm is measured from 4.2 to 310 K. The effect of the deposition of thin (1.5 mm) gold layers on top of these films will be discussed. During the deposition and the subsequent resistivity measurements the samples are kept under high vacuum conditions inside the evaporation chamber, thereby prohibiting gas adsorption. It is shown that top layers quench-condensed at 30 K lead to deviations from Matthiessen's rule and to an increase in the resistivity which is attributed to an increased surface roughness. Room temperature deposition leads to a similar surface-induced resistivity increase, but no deviation from Matthiessen's rule is observed. The difference between these two situations is attributed to the fact that in the 30 K? film the top layer is amorphous and hence does not contribute to the conductivity. In films with top layers deposited at room temperature the top layer assumes the structure of the base layer and the conduction occurs in the whole film. In both cases the surface scattering increases by only a small amount.

The authors present here a theory for studying the local electronic structure at the surfaces of a disordered binary alloy in the presence of short-range order (SRO) in the system, the alloy being considered as having Bethe lattice topology. This theory offers a way of going beyond the single-site coherent potential approximation (CPA) by allowing the incorporation of the effects of environmental fluctuations. This method leads to an effective medium for this semiinfinite system which can be obtained in a very elegant way by modifying the CPA effective medium. The well-known limits of complete segregation and binary sequence are exactly obtained and, in the random case, the CPA results for the surfaces are reproduced.

The Curie temperature of Fe, Co and Ni as well as of their compounds with Y (including Y₂Fe₁₄B) are calculated by including the effects of spin fluctuations in the long wavelength limit via a renormalisation of Landau coefficients. The numerical values depend on the use of recent calculations of correlation effects and of the band structure. As a result, good agreement is found with the experimental values of T_C of the materials covered.

The temperature dependence of the elastic constants of antiferromagnetic chromium in the poly-Q, single-Q and single-S single-Q states has been measured between temperatures in a little above the Neel temperature T_N and below the spin-flip temperature. The discontinuities in the elastic constants at T_N agree with the predictions of mean-field theory, and their temperature dependence in the transverse spin density wave phase agree with the predictions of a thermodynamic model, if T_N is assumed to depend mainly on volume strain omega and to be linear in omega .

The recent discovery of a periodic distortion of the lattice structure of gamma -Fe precipitates in Cu motivated the authors to re-examine the magnetic structure of these precipitates using a neutron diffraction technique. They find that the spin structure is that of a longitudinal type-1 antiferromagnet as is well known but the spins are within a c plane and modulated in space with a wavevector Q which is the same as that of the displacement wave of the lattice. Several features of the spin structure are discussed.

The dependence of the Mossbauer spectra of sources of ⁵⁷Co in beta -PdH_x and of absorbers of ⁵⁷Fe in beta -PdH_x on hydrogen concentration (0.69<or=x<or=0.98) and temperature (4.2<or=T<or=160K) has been studied. The data reveal a repulsive interaction between the substitutional Fe and Co solutes and the neighbouring hydrogen interstitials. A quantitative evaluation of the Mossbauer spectra yields the configuration energies of Fe and Co surrounded by different numbers of nearest hydrogen neighbours. These energies are found to be independent of hydrogen concentration, but to increase with temperature, such that the average anti-trapping interaction of Fe (Co) with a nearest hydrogen neighbour rises nearly linearly from 29 meV (22 meV) at 90 K at 45 meV (37 meV) at 160 K.

Mossbauer spectra of ¹¹⁹Sn introduced at a concentration of 0.2% into alloys of iron with transition metals are used to derive the interaction potentials between tin atoms and the transition metal atoms at first and second neighbour distances. A semi-empirical theory due to Miedema (1977) and Krolas (1981) is a useful predictor of the interaction at first neighbour distances, and a weak correlation is found between the interaction at second neighbour distances and the atomic volume. The spectra also give some information about the interactions between the transition elements themselves, which are found to be generally strongly repulsive at both first and second neighbour distances.

A positron trapped at a simple metal surface is treated within the jellium model. The positron lifetime and angular correlation (ACAR) spectrum for the surface state are calculated and compared with recent measurements. Results for the lifetime are good but the anisotropy of the calculated ACAR spectra is larger than that observed. It is suggested that surface defect trapping may be responsible for the isotropy of the experimental data. Results obtained for the angular correlation in the Independent Particle Model (IPM) and in the Mixed Density Approximation (MDA) are in close agreement, supporting the use of MDA in future calculations. The anisotropy of the calculated MDA spectra is shown to be insensitive to the selvedge width of the electron density but is affected by an attempt to represent the screening of the positron through a shift in the image plane of the positron potential.