Table of contents

The authors have studied the effect of Fermi surface distortions within a two OPW model on the anisotropy and temperature dependence of electron-phonon mass enhancement in zinc. In the calculation the Stark-Falicov nonlocal pseudopotential and anisotropic phonon spectrum have been used. The mass enhancement has been found to be highly anisotropic in the regions where the Fermi surface crosses the Brillouin zone boundary. The temperature dependence agrees with the cyclotron resonance data of Sabo (1970).

Monte Carlo calculations of the pair correlation function of liquid aluminium slightly above the melting point were performed with two pseudopotentials. One of these satisfied the compressibility sum rule and gave much better results in comparison with a neutron scattering g(r).

The results of a path integral variational calculation of the ground state energy of a positron interacting with the phonons of a metal are presented. These results indicate that lowest-order perturbation theory gives a good approximation to the positron-phonon self energy in a metal, implying that vertex corrections are unnecessary.

A transparent description of the 'spin glass' state is presented using a self-consistent internal field distribution. The latter is obtained from an infinite-range Ising model using a Bethe-Peierls-Weiss-like approach. The internal field distribution is delta function like above the freezing temperature and Gaussian below.

The metallurgical and electronic structure of TiFeH_x alloys are probed by ⁵⁷Fe Mossbauer effect studies. The results are qualitatively consistent with and support Reilly and Wiswall's conclusions (1974) on the phase diagram. Volume effects and charge flow are both important in the isomer shift.

The symmetry and strength of the displacement field of hydrogen interstitially dissolved in niobium has been determined by means of Huang diffuse scattering of X-rays (Mo K alpha ₁). The measurements were performed at room temperature in the alpha phase of NbH_x. Detailed measurements of the scattered X-ray intensity close to the (330) reciprocal lattice point yielded the following results: (i) The scattered intensity increased linearly with hydrogen concentration as predicted for isolated point defects; (ii) the displacement field has cubic symmetry, (iii) the double force tensor describing the long ranged displacement field is P_ij= delta _ij(3.37+or-0.1) eV.

The interaction of migrating Al interstitials with impurities has been investigated by damage-rate measurements in dilute Al alloys with Ag, Ge, Mg, and Zn interstitials at irradiation temperatures between 53 and 178K. For comparison the stage II resistivity recovery has been measured during isochronal annealing in the same material. By applying the diffusion theory of the three-dimensionally migrating interstitial to the data the capture radii of impurities for migrating interstitials have been determined. The effective capture radii of all impurities decrease with increasing irradiation temperature. The impurity Mg loses its trapping capability between 100 and 180K whereas the other impurities do trap interstitials even at 178K. The absolute values of the capture radii are considerably larger than expected from elasticity theory.

Using a low frequency torsion pendulum, measurements have been made of the internal friction, Q^-1, of some Pd-H alloys (0.54<or=H/Pd<or=0.7) over the temperature range 4-200K, with particular emphasis on exploring the effect that the transition near 50K has on Q^-1. The authors observed a 'saw tooth' peak in Q^-1 at approximately 50K, which has not previously been reported. From dependence of the character of this peak on the temperature and rate of change of temperature, and the absence of a frequency dependence for its temperature location, the authors infer, from a comparison with some other systems in which similar peaks have been seen, that it denotes the presence of an allotropic transformation at 50K. Relaxation behaviour associated with the 50K peak in Q^-1, gives a relaxation time close to the longer relaxation time observed in spontaneous heat release experiments done in the laboratory with Pd-H near 50K.

A method is presented for obtaining the volume derivatives of the phonon frequencies throughout the Brillouin zone for metals having cubic symmetry. The method is based on the interatomic force constants from the two nearest neighbours and contains one free parameter. The resulting volume derivatives are used to calculate the Gruneisen parameters and the constant-volume temperature derivatives of the phonon frequencies. The calculated values are in good agreement with the recent neutron-scattering measurements for rubidium.

In the course of neutron scattering measurements on sodium near the martensitic transition at 36K, large single crystals were subjected to transformation cycles. On three occasions the original BCC orientation completely disappeared when the partially transformed crystal went through the reverse transition, and large grains (in one case a new excellent single crystal) with different orientations appeared. Analysis of relative orientations before and after a transformation cycle revealed no relationship akin to twinning. Reflections from the HCP phase indicated that it was very poorly developed. No significant changes in phonons in the BCC phase could be detected, neither above the transition point nor below it, apart from a large reduction in intensity in the partially transformed crystal. An explanation of this effect is suggested.

The coherent pseudopotential approximation (CPPA) is discussed in detail, particularly its validity, accuracy and range of applicability. Some extensions of the method are presented and two new types of pseudopotential are proposed.

The enthalpy of formation of binary transitional metal alloys is shown to be due to the electronic d character of their valence states. Predictions about their general trends among the three series are made in a tight-binding scheme, using a simple model for the density of states of the alloys based on an expansion in its moments.

Extends a previous analysis of an effective medium theory (EMA) for liquid or amorphous metals, making use of a one-orbital tight-binding model. Properties of the continuum Green function are discussed including analyticity of the solutions and the effect of non-orthogonality of the wavefunctions. It is shown that the off-diagonal part of the EMA Green function properly excludes overlap of ions, while the lack of such a property for the quasicrystalline approximation (QCA) had led to a paradox in the density of states result, which the authors resolve. Numerical results are given using exponential overlaps and several pair distribution functions, and the results are compared with the Ishida-Yonezawa theory and the QCA. The latter is found to be very unrealistic outside of the long wavelength, low energy region. An EMA calculation using the Percus-Yevick hard sphere pair distribution function gives an interesting structure in the spectral function at a finite wavevector.

The temperature dependence of the amplitude of quantum oscillations in ultrasonic attenuation in white tin has been measured with pulsed longitudinal sound waves of frequencies 390 and 430 MHz in magnetic fields up to 88 kG and at temperatures below 4K. The Fermi surface areas have been extracted from the experimental data using digital Fourier analysis. A procedure has been developed for the accurate determination of effective masses from the field-dependent oscillatory attenuation. Several new masses are reported, particularly from orbits arising from magnetic breakdown. For many of the masses comparison is made with other experimental values obtained from the de Haas-van Alphen and magnetostriction data; there are occasional unexplained disagreements. Comparison of the experimental masses with masses calculated by Knoope (1973) using an APW model for the bandstructure yields a mean enhancement factor of 0.94, slightly higher than values obtained by other methods.

The effects of structure in the density of electron-hole pair excitations is examined by numerical integration of the many-body screening formalism. The results are applied to Ir and Pt.

The temperature dependence of the quadrupole interaction precession frequency of ¹¹¹Cd probes in the intermetallic compounds InBi and In₂Bi has been measured using time-differential perturbed gamma -ray angular correlation. The quadrupole interaction precession frequency of ¹¹¹Cd in InBi displays an approximately linear increase with increasing temperature. Two quadrupole precession frequencies are observed for ¹¹¹Cd probes in the intermetallic compound In₂Bi; a linear decrease with increasing temperature is found for the high-frequency precession and a higher-order temperature dependence is found for the low-frequency precession.

Measurements have been made of the magnetothermopower S of an aluminum single crystal containing 50 PPM of gallium, in transverse magnetic fields H up to 20 kG, at temperatures T between 1.5 and 5.0K. The data were consistent with the equation S(H,T)=A(H)T+B(H)T³, allowing a separation of the electron diffusion component, S_d(H)=A(H)T, from the phonon drag component, S_g(H)=B(H)T³. This separation allowed determination of the quantity Delta S_d=S_d(H to infinity )-S_d(H=0). The average value of Delta S_d for H along the crystallographically equivalent (010) and (001) axes was Delta S_d=(2.23+or-0.25)T*10^-8 VK^-1, in good agreement with the calculation by Opsal and Wagner which included a 45% electron-phonon mass enhancement. For H along the (011) family of axes, a value of Delta S_d=(2.0+or-0.2)T*10^-8 VK^-1 was obtained. This value is slightly lower than the average value of Delta S_d for H along the (101) axes, in agreement with the calculation of Opsal and Wagner.

For pt.I see ibid., vol.6, p.2315 (1976). From a semi-classical analysis of the low temperature electron-diffusion thermopower S_d in a transverse magnetic field H, the authors derive an expression for the difference Delta S_d=S_d(H to infinity )-S_d(H=0) which is independent of the scattering. Evaluating this expression exactly in the one-OPW model a value for Delta S_d which is 50% below the experimental value is obtained. An estimate of four-OPW effects on Delta S_d accounts for only half of this discrepancy. By including a 45% electron-phonon mass enhancement quantitative agreement with the measurements of Thaler and Bass (1976) is obtained.

The solution of the Boltzmann transport equation to calculate the electrical conductivity of very thin metal films is a difficult mathematical problem. However, on physical grounds one expects this conductivity to depend on the total rate of scattering of the electrons, and not on the scattering rate weighted in favour of large-angle scattering as is the case for bulk crystals. This hypothesis has been confirmed by analysis of the results of previous calculations, and by calculations using a new technique that the authors have developed. Thus, it is found that the standard Fuchs-Sondheimer results are applicable to films whose thickness is less than 1/100 of the electron mean free path l_b, provided that one inserts in this theory the value, l_b⁰, appropriate to the total scattering rate. In practice, the use of l_b in place of l_b⁰ usually introduces only a small error.

It is shown that the phase shift and the tight-binding methods for calculating the electron-phonon mass enhancement, give essentially the same functional form for the coupling constant lambda .

Extending some recent work by Shiba (see Prog. Theor. Phys., vol.50, p.50 (1973)), the authors have investigated the influence of transition metal impurities on the amplitude of the Josephson tunneling current, J_s(c,T) (c is impurity concentration and T is temperature). A junction made up of two identical superconducting alloys is considered and T is assumed near the transition temperature T_c.

The measured AC susceptibility of (Pd₉₅Rh₅)Co alloys with between 1.1 and 2.6 at.% Co exhibits a principal maximum which becomes broader, weaker and moves to lower temperatures relative to the zero field maximum, as the externally applied field is increased. This behaviour allows the observation of a second, narrower peak which is identified as the true critical peak. Ferromagnetic ordering temperatures estimated from the positions of these latter peaks agree with those derived from resistivity measurements on the same samples.

The study of magnetic, specific heat and transport properties give experimental evidence for the existence of several magnetic states in the molybdenum-iron system on the molybdenum rich side: at very low iron concentration (c<0.25 at.% Fe) the alloys exhibit a Kondo type behaviour; at intermediate concentrations (0.25<c<3 at.% Fe) the alloys are spin glasses and for higher iron concentrations mictomagnetism takes place (c>3 at.% Fe). It is one of the few systems where such magnetic phases have been observed for magnetic impurities dissolved in a transitional host.

Linear dependence on strain of the orbital magnetic and spin paramagnetic susceptibility tensors for Bloch electrons in metals are derived. The results are applied to a metal with cubic symmetry before deformation. An anisotropic orbital magnetic susceptibility tensor and a paramagnetostriction tensor as well as spin contribution are obtained for a deformed crystal. The orbital and spin contribution to the volume paramagnetostriction are calculated for a single crystal and those contributions to the volume dependence of the total magnetic susceptibility for a polycrystalline sample are also given.

It is shown that in an idealized but realistic model for the band structure of chromium, the single-Q sinusoidal antiferromagnet state must be lower in free energy than other antiferromagnet states including a cubic one. Calculations beyond the idealized model show that the only requirement for this to occur is that on each face of the electron or hole quasi-octahedrons important for the condensate, the root mean square Fermi-velocity average (v_{f perpendicular to} ²)^1/2 perpendicular to the (1,1,1) direction in that octant must be much smaller than (v_f//). Even with the perfect-octahedron model, and with no adjustable parameters, calculated values for the condensate amplitude and the temperature dependence of the vector Q(T) are within 20% of experimental results. The order of the phase transition at T_N and the character of the macroscopic quantum condensate are discussed.

A reinterpretation of nuclear relaxation data on a dilute ferromagnetic Ni_0.98Pd_0.02 alloy is made in the light of multiple scattering theory and the coherent potential approximation. The resultant value of the magnetic moment on the Pd site is higher than previous estimates using rigid band theory.