Table of contents

The inelastic incoherent neutron scattering spectrum of nominally beta -FeTiH_1.14 shows a broad peak due to optic modes near 100 meV. Its width increases strongly on heating from 10K to 300K. Small side peaks at 70 meV and 85 meV may be due to different phases of FeTiH_x.

The recent data for the temperature dependence of the electrical resistivity of a highly strained sample of potassium are analysed to obtain the electron-electron scattering contribution to the resistivity. The resulting value provides evidence for the existence of the 'anisotropic limit' for electron-electron scattering proposed in the recent theory of Kaveh and Wiser (1980).

The resistivity of amorphous rare-earth alloys in the presence of a magnetic field is studied in the framework of the random axial crystal-field model. The magnetic correlations between the rare-earth ions are consistently taken into account using a two spin cluster approximation. Theoretical curves for both ferromagnetic and antiferromagnetic interactions are presented and discussed together with available experimental data.

The ability of Shaw (1968, 1969, 1970) and Ashcroft (1966, 1967, 1970) pseudopotentials to yield accurate bulk moduli (for solids and liquids) by each of the homogeneous deformation (HD) and long wave (LW) methods is tested. The HD method, though more accurate in principle, is in practice not yet properly applicable; refinement of the present theory of non-local pseudopotentials must be awaited. Ashcroft-Langreth fitting to the observed (zero) pressure may be viewed as a device for circumventing this difficulty and is the most accurate method available; even so, it is only semi-quantitatively successful. The LW method avoids the main uncertainty (the theory of the depletion hole) associated with non-local pseudopotentials and can give semi-quantitative results for Shaw pseudopotentials. It will also give results of rather similar quality for the simpler metals (e.g. Na, Mg, Al) using Ashcroft pseudopotentials but fails for such higher polyvalents as Pb.

A method for determining the electron-phonon interaction function from the temperature dependence of the electrical resistivity for metals, based on a numerical deconvolution of the approximate formula for resistivity, is proposed. The method is tested for model resistivity and then applied for measured electrical resistivity of gold samples with a different residual resistance ratio. By comparison with the known phonon density of states of gold it is shown that the method can yield non-trivial information about phonon states and the electron-phonon interaction. The method, based on relatively simple experimental information, is recommended for those materials where it is difficult to investigate phonon structure and electron-phonon interaction by other means.

An investigation is made of the roles of the structural contributions to the Helmholtz free energy in the order-disorder transition that occurs in certain binary alloys. Detailed calculations of the transient temperature T_c in the CuZn (A2 to or from B2), AuCu₃ (A1 to or from L12) and Mg₃Cd ( epsilon to or from DO₁₉) transitions are presented. In these calculations the static lattice approximation is employed which ignores any effects due to lattice vibrations and only considers configurational disorder.

The screening of a single interstitial hydrogen atom in zirconium is treated using the Anderson model with the host valence electrons represented by the plane wave parts of augmented plane waves in the interstitial regions. The perturbing potential is assumed to be of screened Coulomb form and the screening parameters are determined self-consistently by applying the Friedel sum rule. The perturbed total densities of states are calculated for both tetrahedral and octahedral interstitial sites and show the presence of a virtual bound state at the bottom of the band. The screening parameters for the octahedral and tetrahedral sites are found to be the same, having a value 1.40 au. The tetrahedral site is energetically favoured by the hydrogen.

An interpolation scheme which is applicable to Heusler alloys is developed as an extension of Mueller's method (1966). The topology of the Fermi surface is determined by this interpolation scheme for Cu₂MnAl, Pd₂MnSn and Ni₂MnSn. The shapes of the surfaces of these three alloys are very similar for majority-spin electrons but different for minority-spin electrons. The OPW component and d component are estimated for electrons on the Fermi surfaces. The respective characteristics of three alloys are seen in the proportion of these components of the Fermi surfaces.

The authors have calculated the density of states and conductivity of a series of gold-based liquid alkali metal alloys using a tight-binding model that includes short-range order and charge transfer in a self-consistent way. They take account of the variation of the alkali band widths, electronegativities and Coulomb interactions. The model can account for the diversity of conductivities observed at the equiatomic concentrations which range from metallic values for LiAu to ionic values for CsAu and RbAu. The calculations show that the difference of electronegativity between the alloy components is not the essential factor giving rise to the ionic to metallic transition in this series of liquid alloys, but rather it is the variations in the band widths and the interatomic Coulomb interactions that are the dominant factors.

The effect of electron correlation on the stability of crystal structures in the 3d series is investigated. An approximation to the exact second-order correlation correction is used which, for a small number of electrons or holes in the band, reduces to the weak correlation limit of Kanamori's result (1963). d-d correlation acts to stabilise a structure with a high density of states at the Fermi level. This effect reduces the energy difference between the magnetic and non-magnetic state and will therefore contribute to the BCC-HCP transition of iron at 100K bar.

In a previous paper the pseudo-Green function formalism of Moriarty (1972) was applied successfully to the problem of dilute transitional impurities in a transition-metal or noble-metal host. The author extends this new theoretical formalism to include the effect of the spin-orbit interactions upon the electronic structure of the impurity system. The result is an expression for the change in the density of states upon alloying in which the non-local transition-metal pseudopotentials and the crystal field and spin-orbit interactions appear directly. When applied to the case of dilute AgPt alloys the platinum virtual bound state resonance is shown to be split into a triplet. This splitting is consistent with the accepted values of the spin-orbit parameter of platinum and with the available empirical data.

The authors report measurements of the electrical resistivity of many high-purity aluminium samples in the thickness range 0.1 mu m to 7 mm over the temperature interval 2 to 300K. It is shown that surface scattering of the conduction electrons affects the temperature dependence of the resistivity for samples less than 0.5 mm in thickness up to 70K. Above about 30K the simple Bloch-Gruneisen theory agrees well with the data obtained using the thickest samples, while, for thinner samples, application of a surface scattering theory due to Soffer (1967) leads to satisfactory agreement between theory and experiment. Fitting of this theory allows the determination of the surface roughness parameter defined as the ratio of the root mean height of surface asperities, a, divided by the electron wavelength at the Fermi surface, lambda _e. The authors determine 1<or=a/ lambda _e<or=2.5 which is shown to be in close agreement to other studies on aluminium when these are re-analysed using the theory of Soffer. Further analysis of the data leads to the conclusion that the product of the bulk resistivity with the bulk mean free path is 0.82+or-0.04 f Omega m². They also conclude that the low-temperature resistivity has a contribution of the form AT², attributable to electron-electron scattering with A=10.0 f Omega mK^-2.

Reports the results of an investigation on the quantum oscillations in the thermopower of zinc with a magnetic field of 2-8 T parallel to the c axis. Numerous frequencies in the range 5*10²-10⁵ T have been observed, but of particular interest is the presence of a set of high frequencies that can only be produced by self-interference of the quasiparticles moving in the two-dimensional open-orbit network in the basal plane of zinc; these frequencies correspond to areas of similar size to that of the hexagonal face of Brillouin zone thus demonstrating that phase coherence of the quasiparticles is maintained over large areas in reciprocal space. A comparison of the amplitudes of the dominant oscillatory components in the thermopower and conductivity yields good agreement with theory. All the data were taken in a cylindrical geometry to facilitate the quantitative analysis.

The AC susceptibility and electrical resistivity of five PdMn alloys containing between 3.0 and 5.0at%Mn have been measured down to 1.4K. The AC susceptibility, taken in various collinear biasing fields, exhibits a complicated behaviour; at 3 at%Mn the general features of such data resemble those obtained on more dilute alloys of the ferromagnetic phase of this system, but a detailed comparison leads the authors to suggest that the true ferromagnetic phase terminates in the vicinity of 2.5at%Mn. The 4 and 4.5 at% Mn exhibit a double-peaked structure, while the 5 at%Mn sample behaves in a manner reminiscent of a spin glass. The electrical resistivity data can be understood in a similar manner, with the relatively sharp change in slope of the incremental resistivity of the 3 at% Mn alloy close to the ordering temperature evolving into a featureless behaviour at 5 at% Mn, the latter being consistent with a gradual freezing of impurity spins characteristic of spin glasses.

Inelastic neutron scattering experiments have been used to examine the magnon spectra of Pt₃Cr and Pt₆MnCr in the energy range 0<E<or=4 THz. In this energy range at 4.2K the magnon spectra are isotropic and give values of the magnon dispersion constants, D(Pt₃Cr)=100+or-5 THzA² and D(Pt₆MnCr)=37+or-3 THzA².

The authors have observed for the first time a temperature-independent indirect g shift in the bottlenecked regime. The experimental data may be explained by a recently developed theory of this effect and the parameters obtained by fitting the data reveal that the exchange interactions between the conduction electrons and the Dy and Sm ions are markedly greater than those with the Gd ions. The positive indirect g shift caused by Sm is primarily due to strong exchange fields acting on that ion.

For pt.I see ibid., vol.9, p.1189 (1979). The dynamic screening model describing the relaxation of the conduction electrons after the absorption of an X-ray photon in a simple metal is extended to the physical situation of EXAFS. It is used to calculate the potential of the central atom as a function of the velocity of the photoelectron and to derive the corresponding phaseshifts delta _l. They show a transition between a neutral-like behaviour at low energy and an ionic-like one at high energy. This transition takes place in the whole energy range (above about 1 keV) and not only around the threshold for plasmon emission. The authors deduce from these results the accuracy of the determination of distances that can be expected when using ionic phase-shifts in a metal. This precision is found to depend on two parameters: (i) the distance of the neighbouring atom; and (ii) the energy E_max of the higher part of the EXAFS spectrum. The problem of the transferability of phaseshifts is thus discussed in a more quantitative way. In addition, they obtain the reduction of the amplitude of the EXAFS oscillations due to plasmon losses; this amplitude is greater than the one which is predicted by the mean free path factor e^-2R lambda /, because of the transient processes of the screening mechanism.

Measurements of core and valence level X-ray photoemission spectra are reported for a series of ten Pd_xCu_1-x alloys with varying compositions. The core level binding energy shifts display a distinct dichotomy in behaviour at x approximately=0.4. For x<or=0.4 the Pd shifts are zero within experimental error (+or-0.1 eV), which the authors interpret as resulting from the equality of the Fermi energy of the alloy and of Pd, which implies negligible charge transfer. For x>0.4, the Pd shifts are consistent with a Fermi level different from that of pure Pd. The systematics of the Pd core level lineshapes are shown to be in accordance with this interpretation.