Table of contents

From measurements of the difference in specific heat of palladium and copper the temperature dependence of the electronic heat coefficient gamma of palladium is determined between 1.5 and 280K. At room temperature gamma is close to the bare band value and at lower temperatures the temperature dependence of the electron-phonon enhancement of gamma is observed for the first time. Strong evidence is also found for an electron-paramagnon enhancement of gamma at low temperatures.

Recent very precise measurements of the Hall coefficient have demonstrated a small but significant temperature dependence for a number of metallic glasses. The authors show that these results, which are contrary to the prediction of the Boltzmann theory, are in good agreement with the theory of electron-electron interaction in highly disordered conductors.

Positron lifetimes in tin have been measured in the temperature range 80-500K. For the first time the transitions both from the beta to the alpha phase and back to the beta phase could be detected by the technique of positron annihilation. Moreover, it was even possible to observe the state and the progress of each transition as a function of time.

The symmetry and strength of the displacement field around N atoms interstitially dissolved in Nb single crystals is determined from Huang diffuse scattering of X-rays. The measurements were performed at various temperatures from -180 to 150 degrees C. The force dipole tensor describing the long-range displacement field has tetragonal symmetry. It shows no temperature dependence within the present accuracy.

For pt.I see ibid., vol.14, p.2457 (1984). The diffuse X-ray scattering from the displacement field around interstitially dissolved N in Nb single crystals was studied at large scattering vectors. For the case of isolated point defects the so-called Stokes-Wilson scattering was observed for the first time. The authors demonstrate that Huang scattering, Stokes-Wilson scattering and the asymmetry of the scattering intensity distribution can be described by the same formalism. From this new description the displacements of the nearest-neighbour Nb atoms and the lattice location of the interstitial N atom are determined.

The atomic structure of a collection of charged hard spheres is studied as a function of ionic charge and electronic screening using a pair potential of the Yukawa form. By varying the ionic charge, Q, and the screening parameter, lambda , independently, the authors have determined the effect of each separately. For a temperature typical of liquid metal alloys (T approximately=900K) they find that a structure similar to that of an ionic liquid occurs for Q>or approximately=0.6e if lambda =0 and for Q=0.8e if lambda <or approximately=0.5. They also show that under certain extreme conditions it is possible for the structure of a system to become more like that of an ionic liquid as the electronic screening increases.

Computer simulations are performed on amorphous one-component systems with an isotropic smooth pair interaction and periodic boundary conditions. It is found that after careful static relaxation vacancies, vacancy clusters, voids and slightly compressed regions of cylindrical and spherical symmetry, but not planar symmetry, are stable. The author concludes that these defects can exist at zero temperature in real amorphous systems since realistic (anisotropic, oscillating, three-body) interactions tend to increase the stability beyond that of the author's simple model. The effect of various potentials and of positive and negative pressure on the stability of defects is investigated. Methods for definition and sensitive detection of density and pressure variations are presented.

The anelastic relaxation spectrum of the Nb-5 at.% Ti system has been investigated for various deuterium concentrations, c_D, between 0.07 and 1.76 at.% in the temperature range 1.2-300K. Three relaxation processes located around 170, 100 and 50K have been observed at a vibration frequency of 20 kHz, and a marked frequency-dependent rise of dissipation is measured below 5K. The maxima increase in height and shift in temperature with increasing deuterium content, but the shifts of the high- and low-temperature peaks are of opposite sign. The peak at the lower temperature grows with the square of c_D and becomes the predominant process at higher concentrations. The relaxation processes are attributed to the stress-induced reorientation of titanium-deuterium complexes. These observations are interpreted under the assumption that interaction with Ti substitutionals shifts the energies of the trapping sites occupied by deuterium atoms randomly.

Phonon dispersion in the equiatomic intermetallic compound Ti₅₀Ni₄₇Fe₃ has been studied by inelastic neutron scattering in view of the pre-martensitic phase transition which transforms the high-temperature CsCl structure into an intermediate phase. The TA₂ transverse acoustical phonon branch along (110), measured at room temperature (RT), 83, 150, 250 and 350 degrees C, shows a minimum around q=(¹/₃, ¹/₃, 0) 2 pi /a which decreases dramatically as the temperature is decreased from 350 degrees C to RT. A new superlattice peak, close to (⁵/₃, ⁴/₃, 0), appears at 20 degrees C. The general features resemble low-dimensional systems where Fermi-surface instabilities and the formation of charge density waves have been observed. The new superstructure of the intermediate phase, which initially appears to be incommensurate, becomes commensurable at lower temperatures.

The effect of the interatomic interaction on the band distortion in a ferromagnetic state is investigated within the Hartree-Fock approximation, with particular attention given to nickel. New ordering parameters for the band disortion caused by the interatomic interactions are introduced. It is found that spin-up and spin-down bands are distorted in a different manner, while the intra-atomic interaction does not cause this band distortion even in the many-band case. For exchange interactions of short range it is shown that the present treatment is equivalent to the band calculation. The ordering equations in the one-band model are solved numerically, with the result that spin polarisation is always accompanied by band distortion.

For pt.I see ibid., vol.14, p.2525 (1984). The importance of interatomic interactions in the problem of electron correlation for photoemission is proposed through examinations in the T-matrix approximation. It is found that, for the band narrowing due to the correlation effect, the contribution from the interatomic interaction is comparable with that from the intra-atomic one, while the satellite binding energy is essentially determined only by the strength of the intra-atomic interaction. From the above result it is concluded that the interatomic interaction improves the consistency of the band narrowing and satellite binding energy, which the T-matrix approximation calculation in the Hubbard model cannot. It is also suggested from the calculation for the intensity curve of valence-band XPS that the interatomic interaction may be the main origin of the observed asymmetric distortion of the main curve.

Electronic energy-band-structure calculations have been performed for NdH₃ using the composite-wave variational of the APW method. From the band-structure data the density of states (DOS) and the joint density of states (JDOS) have been calculated for this hydride. The DOS and band-structure results show that, in comparison with the features of NdH₂, the electronic structure is relatively unaffected by the presence of octahedral hydrogen except for the formation of a new band on the low-energy side. This new band is separated by an indirect gap of 0.09 Ryd from the d-band states. Thus the authors' results can account for the semiconducting behaviour of NdH₃. Comparison of their optical results, calculated from JDOS data, is not possible at present, since no such experimental study has yet been made on this hydride.

The DHVA effect in single crystals of TiBe₂ has been investigated in magnetic fields between 10 and 35 T using a pulsed-field system. Two sheets of Fermi surface are found, an electron sheet around Gamma and an X-centred hole sheet. Both are split into a majority-spin ( to ) and a minority-spin ( down arrow ) surface. In the field region studied Delta F=F_to -F_{down arrow} amounts to about 200 T or 10% of the average frequency values. With the field direction parallel to (110) the field dependence of the splitting of the Gamma orbit is studied in detail. It is found that the difference in spin-up and spin-down Fermi surface areas is directly proportional to the magnetisation, within the experimental error. This is in agreement with the Stoner model. Departures from this model due to spin fluctuations are expected below 10 T. The effective masses of the Gamma and X orbits show large enhancements of about lambda =3 and 1.5 respectively.

Presents a formalism for calculating the two-photon momentum densities rho _{2 gamma} (p) for substitutional random alloys with the KKR CPA band-theory scheme and apply it to Cu₆₀Ni₄₀ and Cu₆₀Pd₄₀ random alloys. In particular they study rho _{2 gamma} (p) along the directions Gamma X, Gamma K and Gamma L in Cu₆₀Ni₄₀ and along Gamma X in Cu₆₀Pd₄₀ and compare it with the appropriate electron momentum distribution. They conclude that the extent to which the positron wavefunction influences the results of positron annihilation experiments is slight and it can now be studied in quantitative detail.

The author has calculated the residual resistivity of an Al-Zn alloy in the first stages of aging at 20 degrees C, determining the influence of numerous basic parameters (shape, density and concentration of Guinier-Preston (GP) zones, pseudopotentials and decomposition kinetics) on the resistivity anomaly. By considering the GP zones to be truly spherical and evolving temporally with a diameter proportional to the one-third power of the aging time the authors achieved agreement between experiment and theory.

For pt.II see ibid., vol.14, p.681 (1984). All-electron local density functional theory and the full-potential linearised augmented plane-wave method for thin films are used to calculate the electronic structures and cohesive energies of hexagonal close-packed paramagnetic monolayers of the 3d transition metals Sc to Cu. The energy band structures and densities of states show an increasing narrowing of the d band and a dehybridisation between the p- and d-like states. The density of states of Sc is found to be remarkably high at the Fermi energy. The work functions of the monolayers increase from Sc to Ni and drop for Cu, thus showing the same trend as the experimental work functions of clean metal surfaces other than Mn. The cohesive energies of the monolayers exhibit the same systematic trend as do those of the bulk metals, i.e. small values are found in the middle of the 3d series due to large spin-polarisation effects in the free atoms.

The authors have computed the electron density of states of solid solutions of vanadium-based transition-metal alloys V₉₀X₁₀ using the tight-binding recursion method for degenerate d bands in order to calculate the alloy superconducting transition temperature with the McMillan formula. As observed experimentally, for X on the left-hand side of V in the Periodic Table one obtains an increase of the critical temperature T_c while for X on the right-hand side of V T_c decreases. A detailed comparison with experiment indicates that one cannot neglect the variation of the electron-phonon interactions when the bandwidths of the two constituents are different. Another important conclusion is that for alloys in the split-band limit, such as VAu, VPd and VPt, agreement with the experimental data can be obtained only by assuming that these alloys have a short-range order favouring clusters of pure vanadium.

Superconducting transition temperatures and vacancy fractions were measured in the A15 intermetallic compound V₃Ga after quenching from various high temperatures. The vacancy concentrations were found to be smaller than 10^-3, consistent with Miedema's model for vacancy formation in intermetallic compounds. The reduction of T_c by thermal disorder could be described well by the Bragg-Williams approximation and a theoretical relation for T_c as a function of atomic order, obtained by Fahnle (1982) from the linear-chain model of Labbe and Friedel (1966).

The specific heats of two samples of PdD_X (D/Pd=0.85, D/Pd=0.83) have been followed through the region of their superconducting transition temperatures. In support of the indications from the magnetisation measurements reported in paper I of this series, the specific heat measurements show two closely spaced peaks corresponding to two superconducting transition temperatures-one for a transformed phase and one for the original untransformed phase. As was discussed in I, these could be ordered and disordered beta phases according to the phase diagram proposed by Bond and Ross (1982). A discussion is given of the effect of such a transformation on obtaining unambiguous experimental values of T_c(X) for comparison with values derived from theories of superconductivity in PdH_X(D_X).

The critical behaviour of a ferromagnetic superconductor is studied using a field-theoretical renormalisation group method. The authors show that, within the epsilon expansion, the coupled superconductor-magnet system exhibits no real stable fixed points, indicating that the system undergoes a first-order superconducting-ferromagnetic transition due to the fluctuating internal induction field. Recent experiments on the rare-earth ternary compounds (Er_1-xHo_x)Rh₄B₄ and HoMo₆S₈ are compared with the predictions of the theory.

The connection between electronic structure and ferromagnetism is studied for the Fe-Co alloy system by means of self-consistent local spin-density functional calculations. The local environment of these BCC-substitutional alloys is modelled by different ordered compounds: besides BCC Co and Fe these are Co₃Fe and Fe₃Co in the Fe₃Al structure and FeCo in the CsCl and NaTl structures. FeCo in the CsCl structure is studied in great detail giving the ferromagnetic band structure, decomposed state densities, Wigner delay times and the spatial form of the spin density. The role of the nearest-neighbour coordination in determining the value of the magnetic moments is discussed for all the compounds studied. The calculated and measured magnetic moments are compared and trends are analysed and explained.

The local density of states at Ni and Y sites and the total density of states are calculated for the intermetallic compounds Y₂Ni₁₇, YNi₅, Y₂Ni₇ and YNi₃ in the recursion method. It is shown that the occurrence of ferromagnetism in Y₂Ni₁₇, Y₂Ni₇ and YNi₃ is due to a peak in the density of states near the Fermi level. The temperature dependence of the spin paramagnetic susceptibility is calculated and the results are compared with the observed ones. The metamagnetism in Y₂Ni₁₆ and some local magnetic properties on Ni and Y atoms are discussed.

Parastriction and third-order paramagnetic susceptibility results in DySb are presented here for both tetragonal and trigonal symmetries. They allow a temperature analysis of all the magnetic and magnetoelastic properties of this compound in its paramagnetic phase. Quadrupolar interactions originating from the magnetoelasticity and the pair coupling appear to be insufficient and additional interactions seem to exist in DySb: the presence of a two-ion magnetoelastic coupling is quite possible, strongly influencing in particular the third-order magnetic susceptibility for the tetragonal symmetry. In addition, fluctuations appear to be active in the neighbourhood of the Neel temperature. The first-order character of the antiferromagnetic ordering agrees with the positive third-order susceptibility as expected in a Landau expansion of the free energy, and results from a complex balance between various interactions.

The spin-wave excitation spectrum of ferromagnetic DyAl₂ has been measured at 5K by neutron inelastic scattering methods. The scattering at selected momentum transfers has also been measured at elevated temperatures. The results are used to obtain crystal-field terms and exchange interactions. Strong magnon-phonon coupling effects occur. At elevated temperatures the spin waves exhibit considerable lifetime effects well below T_c and an excited-state spin wave is observed.

In continuation of the authors' earlier suggestion that the anomalous behaviour in spin-glass alloys just beyond the percolation concentration may be explained by the transition to a mixed phase (a randomly canted ferromagnetic phase), they study the effect of this transition on the susceptibility and small-angle neutron scattering cross section within a similar formalism. The authors show that the results are in agreement with experimental data.

Two improvements are presented to a previous molecular-field description of intermetallic compounds exhibiting spin-glass behaviour. Firstly, allowing for the possibility of antiferromagnetism permits the authors to obtain phase diagrams for systems like Eu_1-xGd_xS and, secondly, the role of impurity-induced destruction of the magnetic moments is discussed. Phase diagrams over the entire concentration range are then obtained for systems like Y(Fe_1-xAl_x)₂.

Amorphous Gd_xAg_100-x alloys have been prepared by melt-spinning within the compositional range 28<or=x<or=40. All the alloys exhibit a transition to an ordered phase at low temperatures. The transition temperatures vary linearly with Gd content and point to a critical concentration at about 24% Gd. The nature of the ordered state is incomplete ferromagnetism in which full spin alignment is frustrated by the presence of antiferromagnetic interactions. The frustration increases in severity with decreasing Gd concentration and for x<or=30 the alloys display some of the features normally associated with spin glasses.

The optical reflectivity of Al below 3 eV has been investigated in the 300 kbar pressure range (22% volume change) at T=300K. The dominant optical interband absorption at h(cross) omega =1.5 eV associated with the splitting between parallel bands shifts to higher proton energy at an initial rate of 4.35+or-0.3 meV kbar^-1. The volume dependence of the corresponding pseudopotential Fourier coefficients is dU(200)/d ln V=-1.58 eV. The second parallel band absorption feature at energy 2U(111) ( approximately=0.4 eV at 1 bar) exhibits a similar pressure shift and becomes clearly observable in high-pressure reflection spectra. The optical response of Al under pressure basically follow the predictions derived from an empty-core pseudopotential in combination with the Ashcroft-Sturm expression for optical interband absorption in polyvalent nearly-free-electron metals, but with a significantly larger width of the low-energy absorption band at energy 2U(111). X-ray diffraction studies support the stability of the FCC phase up to 300 kbar.

Presents Auger electron spectra of Si, Ti and compounds of Ca, Sc, Ti and Cr. From a combination with photoelectron spectra the correlation energy U for the various two-hole states was deduced. For Sc, Ti, TiSe₂ and CrSe₂ the correlation energy U between two holes in the valence band is negative. The authors suggest that the negative U is caused by a dynamic bipolaron effect. They found that Auger decay involving valence states is strong for TiSe₂ and CrSe₂, due to a large admixture of 3d states in the valence band. Other features in the spectra are discussed in terms of multiplet splitting and satellites.

The presence of stacking faults in metastable cobalt green-compacts obtained by compaction of reduced cobalt oxide powder was evaluated at different pressures up to 660 MPa. The influence of stacking faults, present and induced by pressing, on the transformation from alpha -(FCC) phase to epsilon -(HCP) phase is discussed. Coercivity was also measured, and internal stresses were calculated using both X-ray and magnetometric measurements.