Table of contents

The memory functions for the intermediate scattering functions and the current-current correlation functions have been determined from data for neutron scattering experiments on liquid lead and bismuth. A connection between these memory functions is derived. The results are compared with theoretical models.

Curium metal was studied by X-ray diffraction up to 52 GPa. Starting with the double-hexagonal close-packed structure (Cm I), a cubic close-packed structure (Cm II) was formed at 23 GPa and another high-pressure phase (Cm III) at 43 GPa. The latter phase can be indexed as an orthorhombic cell of the alpha -uranium-type structure. The volume difference between the cell of Cm III and that of Cm II, together with the formation of a low-symmetry (orthorhombic) crystal structure, suggests that itineracy of the 5f electrons in curium metal starts at 43 GPa. The fact that the delocalisation pressure is higher in curium than in the neighbouring americium, berkelium and californium metals is explained qualitatively on the basis of a comparison of localisation energies.

The authors determine the inelastic diffusion lengths of weakly localising Pd and PdH_x films from measurements of the anomalous magnetoresistance. They observe that these lengths are much shorter in Pd than in PdH_x. This effect cannot be solely accounted for by a variation of the elastic diffusion constant but it implies modifications of the electronic structure. They propose that the electron-electron interactions are enhanced in Pd compared with PdH_x through either s-d scattering events or through scattering on magnetic fluctuations. For the electron-phonon part the authors suggest that the coupling parameter with acoustic phonons is greater in Pd, although superconductivity PdH_x and not in Pd.

The structural properties of equiatomic NiTi have been investigated by single-crystal neutron elastic scattering. The premartensitic phase transition of NiTi is characterised by a second-order displacive transition which is driven by a condensation of the T₂A(110) phonon mode at T_c=75 degrees C. The lattice displacement amplitude behaves like a Landau order parameter with a critical exponent beta =¹/₂. Upon cooling, the martensitic transformation occurs at T_M=35 degrees C by nucleation. The intermediate phase has a distorted CsCl structure of monoclinic symmetry (P2/m), providing 12 equidistributed structural domains.

The crystallisation processes in as-quenched and hydrogenated amorphous alloys of composition Cu_100-xZr_x (x=40 and 50) have been studied using differential thermal analysis and X-ray diffraction measurements. The as-quenched samples with x=40 and 50 yield crystallisation temperatures of 761 and 716K, respectively. X-ray diffraction measurements of fully crystallised samples show that crystallisation products are Cu₁₀Zr₇ from Cu₆₀Zr₄₀, and both Co₁₀Zr₇ and CuZr₂ from Cu₅₀Zr₅₀. Differential thermal analysis scans of electrolytically hydrogenated samples show four features: (1) a small exothermic peak at 460K; (2) a large exothermic peak at approximately 550K; (3) a cluster of exothermic peaks beginning at 710-748K; (4) a large endothermic peak at approximately 1050K. X-ray diffraction measurements show that the features may be identified as (1) relaxation of the amorphous structure, (2) partial crystallisation into ZrH₂ and Cu, (3) complete crystallisation of the remaining amorphous phase into ZrH₂ and Cu and (4) breakdown of the hydride and formation of crystalline products as found in the non-hydrogenated alloys.

Phonon dispersion relations for noble metals has been studied on the basis of the pseudopotential due to George et al. (1978). The cardinal interest for the present work was to study the relative importance of axially symmetric and general tensor forces, between the nearest neighbours and to explore the possibility of a simple pseudopotential for noble metals. It is found that the tensor forces up to first neighbours do play an important role and that a simple pseudopotential can be used as a 'working' potential to study the phonon dispersion curves of noble metals.

The author has recently developed a simple scheme based on the recursion method which allows the relative position of the d bands in an amorphous transition-metal alloy to be determined if the average charge transfer is known. To find the hopping integrals the author uses tabulated values of the overlap parameters (dd sigma , dd pi , and dd delta ) obtained from a fit to the bands of the pure metals. This scheme allows a parameterized Hamiltonian to be built to represent the transition-metal alloys, avoiding the need for a detailed fitting of LCAO bands to the bands of a corresponding crystalline compound. In the procedure the overlap parameters are tabulated for the pure metal and charge transfer is either known or neglected. Therefore the results obtained for the density of states of the alloy are well defined and parameter free. Here the author uses the above procedure to obtain the electronic structure of amorphous ZrCu ( alpha -ZrCu) and alpha -ZrNi alloys for two different compositions. The results are compared with those obtained by other methods and with experiment. The calculations are in good agreement with existing UPS and XPS results for these alloys. The values obtained for the density of states at the Fermi level, N(E_F), for alpha -ZrCu are also compatible with those inferred from experimental data for melt-spun samples.

de Haas-van Alphen frequencies in the (100) and (110) planes of Nb₃Sn have been measured with a typical accuracy of 1%. Several frequency branches have been found between 100 and 1000 T. Effective masses corresponding to some of the orbits at symmetry directions have been estimated from the experiments. A pronounced anisotropy ( approximately 10%) in the upper critical field was observed. The magnitude of the critical field was found to be sample dependent. The DHVA data are interpreted using self-consistent new ASW band-structure calculations which include tetragonal distortion of the unit cell.

The Fermi surfaces of Pb_1-xSn_xTe in the cubic and rhombohedrally distorted phases for both normal and inverted band structures have been investigated using the Shubnikov-de Haas effect. Comparison of the observed Fermi-surface cross-sectional areas with the band structures of Dimmock (1981), for the cubic phase, and of Bangert (1981), for the rhombohedral phase, yield parameters which are in agreement with values established previously for the cubic phase. The complex character of the Fermi surfaces at the points T and L in the rhomobohedral Brillouin zone is found to agree with the predicted splitting of the ellipsoids of the cubic phase into the 'skin' and 'core' of an apple-like composite surface, originating in the lifting of the inversion symmetry of the structure in the structural phase transition. Clear evidence is obtained for the presence of structural domains in the rhombohedral phase. The carrier concentrations deduced from the Fermi-surface volumes are found to agree with those measured from the weak-field Hall coefficients, when account is taken of the anisotropy of the multiple carrier valleys in the band structure.

For pt.I see ibid., vol.15, p.317 (1985). The Shubnikov-de Haas effect has been studied in samples of Pb_1-xSn_xTe which have normal or inverted band configurations at low temperatures. The measurements have been made over a range of temperatures which take the samples through the structural phase transition from the rhombohedral to the cubic rock-salt phase. The changes in Fermi-surface cross-sectional areas with temperature are interpreted in terms of a temperature-dependent band-structure calculation which includes the interaction between electrons and soft transverse optic phonons associated with the phase transition and the temperature-dependent sublattice displacement and shear strain in the rhombohedral phase. The basic band-structure parameters are those deduced from the orientation-dependent study of Pb_1-xSn_xTe at 4.2K in the distorted phase, and the variations with temperature of the energy gap in the alloys is used to yield further parameters. The resulting fit to the variation of the cross-sectional area with temperature is excellent. Measurements of the temperature dependence of the transverse effective mass have also been made in this temperature range and it is found that a temperature-dependent scattering parameter has to be included in the theory to obtain agreement with the observations. This parameter is related to the soft-TO phonon scattering and to the formation of domain walls near the transition.

For pt.II see ibid., vol.15, p.337 (1985). Band-structure parameters derived for Pb_1-xSn_xTe in paper I of this series from a Shubnikov-de Haas study are shown to give good agreement with the carrier concentration dependence observed for the anisotropy and the transverse effective mass in p-type PbTe. The parameters are also capable of yielding the x dependence of the anisotropy of the Fermi surfaces in Pb_1-xSn_xTe in the cubic phase at low temperatures. With regard to the band structures deduced for the rhombohedral phase of Pb_1-xSn_xTe, these are found to be capable of explaining the hitherto unexplained de Hass-van Alphen measurements of SnTe at low carrier concentration. The complexity of the Fermi surface for such low-carrier concentration SnTe is discussed.

The authors study the transport coefficients of a zinc single crystal when a magnetic field is applied in a direction close to the c axis. For fields <or approximately=1.5 T they report a complete set of experiments and theoretical coefficients for a single crystal in the usual rectangular geometry with the emphasis being on the thermal and thermoelectric effects. The agreement between experimental and theory is usually good, though the model calculations are unable to produce sufficient harmonic content in the diagonal coefficients. The experimental data on the electrical resistivities have also been extended to high magnetic fields when the field is both parallel and close to the c axis. Generally speaking, these results are not understood and indicate that we are still ignorant of some of the basic physics appropriate to this situation.

The electronic properties of pure Bi and semimetallic Bi_1-xSb_x alloys (x=0.012, 0.019, 0.033) have been investigated by means of magnetoacoustic attenuation in the DHVA region. Experiments were carried out in the temperature range 1.2-4.2K using longitudinal ultrasonic waves of frequencies up to 100 MHz propagated along the trigonal axis. Magnetic fields between 0.05 and 2.3 T were applied in the yz and xz planes. The Fermi-surface sections and effective masses of electrons were obtained from the angular dependence of the measured periods of ultrasonic quantum oscillations. With increasing antimony concentration the cyclotron masses and the densities of electrons in the alloys decrease, but the shape and the tilt angle of the electron ellipsoids do not change. The electron Fermi surface in the alloys contracts toward its centre, preserving its shape. The temperature and magnetic field dependence of the amplitudes of DHVA-type oscillations were studied and electron cyclotron masses at the Fermi level and the Dingle temperature were obtained. The effects of impurities and dislocations on the lineshape and the Dingle temperature were considered.

The authors present experimental results for the low-temperature specific heat, the magnetic susceptibility and the phonon density of states in the pseudobinary system Nb_1-xV_xN. This system is known to exhibit a pronounced minimum of the superconducting transition temperature around x approximately 0.5. They discuss the possible origins of this minimum. While a significant minimum in the electron-phonon coupling strength is not supported by the authors data from inelastic neutron scattering, the consideration of spin fluctuations offers a consistent description of the experimental findings.

The self-correlation function xi (t) for a Cu-3 at.% Mn alloy has been determined with good statistical accuracy using conventional time-of-flight as well as the more sophisticated backscattering technique. Around the low-frequency (10 Hz) freezing temperature T_f the correlation function xi (t) shows a power-law decay, i.e. xi (t)=at^-v with nu approximately 0.25 above T_f, but nu =0.1+or-0.02 and just below T_f. At lower temperatures (T/T_f approximately=0.75) xi (t) is best expressed as xi (t)=at^-v+b log(t) and yields nu =0.4+or-0.1. At still lower temperatures (T<or approximately=0.5T_f) xi (t) can be expressed as xi (t)=c+at^-v, with a value of nu which is consistent with nu approximately 0.5, predicted by recent mean-field theories as well as computer simulations. An important result of the measurements is that the mean correlation time tau ₁e/ obeys the Vogel-Fulcher law suggesting a glass-like transition with T_g approximately=0.8 T_f(10 Hz).

The intermetallic compound FeSn₂ has been studied by neutron diffraction, magnetisation measurements and Mossbauer spectroscopy. This compound is antiferromagnetic (T_N=378K) and a second transition occurs at T_t=93K. The unit cell of the antiferromagnetic array is the same size as the chemical cell. Between T_N and T_t the magnetic structure is collinear, characterised by ferromagnetic planes (100), coupled antiferromagnetically along the (100) direction; the spin direction lies in the (001) plane near the a axis. AT t_t=93K a spin rotation to the (110) direction is observed. Below T_t, FeSn₂ becomes non-collinear antiferromagnetic and the iron moments form a canted structure along the c axis. The angle between the two spin dynamics is 18 degrees and the moment per iron atom observed is 16.4+or-0.05 mu _B at 5K. This magnetic behavior of FeSn₂ is similar to that observed for FeGe₂. A new description of the non-collinear structure of FeGe₂ is proposed and the moment per iron atom is corrected to mu =1.21 mu _B.

The magnetic specific heat Delta C of Au_1-xFe_x has been determined between 2 and 300K from measurements of the difference in specific heat of pairs of Au_1-xFe_x and Au_1-xN_x alloys. The concentration range investigated, 0.02<or=x<or=0.23, includes the spin-glass, cluster-glass and ferromagnetic regimes. In addition to a linear electronic term at high temperatures for the more dilute alloys, the spin-glass and cluster-glass alloys (x<0.15) are characterised by a single broad maximum in Delta C at a temperature T_G which increases only slowly with Fe concentration from 30 to 40K, and the authors conclude that the mean RKKY and nearest-neighbour ferromagnetic interactions are comparable in magnitude in this concentration region. An additional, sharper maximum in Delta C is observed at the ordering temperature T_c in the ferromagnetic alloys (x>or=0.15). It is shown that the results provide evidence against substantial spatial short-range order but can be explained assuming a random distribution of Fe atoms. No discontinuity in Delta C is observed at the freezing temperature T_F in the spin-glass and cluster-glass alloys, at which a susceptibility cusp has previously been observed, but T_F is found to correlate well with a pronounced maximum in Delta C/T where the entropy is increasing most rapidly with temperature. The magnetic entropy is shown to be consistent with a spin between 1.2 and 1.5 for the Fe atoms.

The Curie temperature of the Dy₂Fe_17-yAl_y series has been measured by means of AC susceptibility. The Al concentration dependence of T_c initially (up to y=3) increases although the magnetic iron atoms are gradually diluted. The present study shows that this effect is not connected with a crystal structure but is due to an increase of the average iron-iron distance caused by Al substitution. The author has shown that the Curie temperature of all heavy-rare-earth-iron compounds of the 2:17 type increases smoothly with increasing volume. A value of 6000K has been evaluated for dT_c/d ln V, resulting in a value of -17 for the magnetic Gruneisen parameter Gamma _m=-d ln T_c/d ln V. As the Curie temperature in these compounds is governed mainly by the iron-iron exchange interaction they have inferred a strong dependence of the J_Fe-Fe coupling on the 3d-3d distance. The Bethe-Neel dependence and a prediction of the magnetic Gruneisen parameter using an itinerant-electron theory of ferromagnets are discussed.

Positron annihilation measurements on single-crystal alpha -Zr show that the Doppler broadening of the annihilation photopeak changes with the crystallographic direction in which the photons are emitted. Conventional peak-height (S) and wing (W) parameters have been measured with the c axis of the specimen either parallel (S/sub ///, W/sub ///) or perpendicular (S_{perpendicular to} , W_{perpendicular to} ) to the cylindrical axis of the coaxial Ge(Li) detector. From 295-600K, S/sub ////S_{perpendicular to} approximately 1.026; from 600-1130K, S/sub ////S_{perpendicular to} decreases linearly to 1.017; at 1179K, S/sub ///=S_{perpendicular to} within the experimental uncertainty ( approximately 0.12%). The low-temperature results are associated with extrinsic behaviour (positron-defect interactions); in the intermediate range intrinsic (or bulk) behaviour dominates; the loss of anisotropy at 1179K is associated with the HCP to BCC transformation. Anisotropy is not apparent in measurements of W. Preliminary results for Ti show similar low-temperature behaviour.

The authors have investigated the structural phase stability of certain brass-type alloys in terms of a third-order pseudopotential theory. The third-order contributions to the structural free energy are found to play important roles in predicting the crystal structures of these alloys.

Small-angle scattering from carbide and gamma-prime precipitates has been observed in a Nimonic PE16 alloy. The size, density and volume fraction of the carbide and gamma-prime precipitates have been evaluated for different aging times and temperatures. The growth kinetics of the gamma-prime precipitates were constant with the theory of coarsening. The chemical composition of the gamma-prime precipitates was found to be Ni₃Al_0.6Ti_0.4.