Table of contents

It is shown that positron annihilation lifetime spectra obtained previously for annealed Cd contain a long-lifetime component for T<150K ( tau ₂ approximately=270 ps and I₂<or approximately=7%). It is concluded that the low-temperature effects observed in Cd arise from positron trapping and that the fraction of positrons trapped decreases with increasing temperature (dI₂/dT<0).

Theoretical calculations of the positron-vacancy binding energies in the BCC refractory metals Nb, Ta and W have been carried out using the authors' previously-developed supercell method (1977). The results indicate that the positron is sufficiently tightly bound to the vacancy in these metals that significant thermal detrapping is highly unlikely. Positron annihilation characteristics in the Bloch and vacancy-trapped states of the positron were also calculated for W. The general shapes of the core-electron contributions to the angular correlation curves in the two states were found to be different.

Molecular-dynamics calculations are reported for a system simulating a liquid Na-K alloy. A propagating sound mode is observed, but its detection by neutron-scattering techniques is likely to be difficult because the decay of density fluctuations is dominated by interdiffusion. No evidence has been found of any propagating concentration fluctuation.

High-resolution X-ray photoemission spectroscopy (XPS) measurements of a variety of metallic glasses are presented. The metallic glasses were obtained by melt spinning or splat cooling. The results are used to give an insight into the structure of the electronic states in metallic glasses.

For original paper see B.K. Rao et al., ibid., vol.9, p.L51 (1979). Dynamical exchange effects in the dielectric function of jellium are described by a frequency-dependent local field function G(q, omega ), which the authors earlier derived by the dynamical exchange decoupling method. In the present note, it is analytically deduced that this expression for G(q, omega ) is logarithmically divergent at omega =h(cross) mod q²/2+or-qk_m mod m. This logarithmic singularity is due to the discontinuity of the equilibrium distribution function at the Fermi level. In a recent comment, Rao et al. argued that G(q, omega ) does not exhibit a singular behaviour. This statement is shown to be incorrect.

For original paper see ibid., vol.9, p.L51 (1979). For comment see F. Brosens et al., ibid., vol.10, p.L27 (1980). Using the regularisation procedure for treating singular integrals it is shown analytically that the local field factor G(k, omega ) does not have any logarithmic singularity at omega = mod (k²/2)+or-k mod . This refutes the claim by Brosens et al. made in the preceding Letter and justifies the conclusions of the authors' earlier work.

It is shown that the anisotropy of electron-dislocation scattering leads to a sample dependence for the electron-electron scattering contribution to the electrical resistivity. Including this anisotropy accounts quantitatively for the marked sample dependence recently observed for the low-temperature electrical resistivity of potassium.

It is suggested that the fine structure of the electronic energy spectrum arising from electron-phonon interactions persists in an amorphous metal, causing the enhancement of the thermopower observed experimentally for amorphous CuSn alloys at low temperatures.

It is known that, in alkali metals, the large rate at which deviations from Matthiessen's rule are induced as a function of residual resistivity is comparable to that for polyvalent metals. It is suggested that this has a natural explanation if the existence of charge density waves is included in the analysis.

The optical reflectivity of the liquid alloy Au₈₁Si₁₉ was measured in the energy range from 0.5 to 6 eV. The overall reflectivity behaviour is in good qualitative agreement with the one recently published for amorphous Au₈₁Si₁₉ prepared by getter sputtering in argon. However, the reflectivity values in the liquid state are higher than in the amorphous state and give rise to more reasonable values for the inverse relaxation time. The calculated plasma frequency and the electronic relaxation time in the Drude range are consistent with measurements of the Hall coefficient and the electrical resistivity in the liquid state.

The electronic structure of a non-metal vacancy in cubic NbC and VN is investigated using the SCF-cluster approximation. Self-consistent charge densities are obtained for the solid with perfect order as well as with a vacancy in an unrelaxed lattice. Partial local densities of states and electron-phonon parameters are evaluated and found to be influenced by the presence of a vacancy even at more distant atoms. The superconducting transition temperature as a function of the vacancy concentration is calculated and agrees well with experimental values.

A theoretical study of the Fermi surface on aluminium and its strain dependence, based on a local pseudopotential model is presented. New values for the relevant pseudopotential matrix elements in the absence of strain obtained from a fit to recent high precision de Haas-van Alphen measurements are given for different sizes ((4*4), (8*8), (16*16)) of the secular matrix. The Fermi energy and its derivatives are computed by summing over occupied states. The strains are separated into volume-conserving shears (tetragonal or angular) and volume changes. Eight orthogonalised plane waves (OPWS) are required in the secular matrix in order to describe the effect of a general homogeneous deformation on the Fermi surface. The strain response of ten carefully extremalised orbits has been calculated, with the slopes of the form factor at the first two reciprocal lattice vectors as only free parameters.

The characteristics of the lattice on either side of the metal-semimetal transition in Ni_{1- delta} S have been determined using a panoply of experimental methods-specific heat, Mossbauer absorption inelastic neutron scattering, ultrasonic and X-ray diffraction. These different methods provide a coherent picture of the changes in lattice dynamics and thermodynamic properties at the first-order phase transition. Most significant is the softening of the acoustic modes, and in particular the decrease in the shear constant C₄₄ in the high-temperature metallic phase. The transition as a function of temperature is mainly driven by the associated lattice entropy and not directly by electronic or magnetic entropy resulting from the change in electronic structure. Nevertheless, the change in electronic structure is probably indirectly responsible for the change in C₄₄ in the metallic phase through modification of the population regions of the Fermi surface under shear stress.

The theory is given of the non-radiative surface plasmon-polariton modes of metal cylinders, modelled by a spatially dispersive free electron gas within a Bloch hydrodynamic formulation. Dispersion equations for homogeneous and inhomogeneous cylinders are derived for nth cylindrical harmonic, and the dispersion properties are investigated as a function of n, cylindrical radius and surface structure. The position, and splitting with n, of the surface modes is found to be considerably different from those based on a model that neglects spatial dispersion. This is particularly true for very small cylinder radii.

The residual electrical resistivity of Ni-Cu alloys containing 37, 41 and 58 at% Cu was measured after isothermal annealing at temperatures between 623K and 750K. Equilibrium resistivity values were determined between 373K and 750K after sufficient isothermal annealing or 3 MeV electron irradiation. The equilibrium resistivity was found to decrease approximately linearly with increasing reciprocal temperature and to be proportional to the Warren-Cowley short-range-order parameter alpha ₁ as derived from neutron scattering experiments in the literature. The resistivity kinetics of the cluster process not being significantly influenced by vacancy annihilation can be described by a simple power law with an exponent gamma =1.44+or-0.3. From the temperature dependent atomic jump rates an effective activation energy for the clustering of 2.1+or-0.3 eV was obtained.

By means of the KKR method for the Green function of any system of non-overlapping muffin-tin potentials, the authors investigate the electronic structure of liquid transition metals. The density of states (DOS) curve of liquid Ni is calculated in several single-site approximations. The effective-medium approximation (EMA), proposed by Roth (1974), gives a reasonable DOS curve, which is surprisingly close to that of solid Ni with FCC structure. Using that DOS, the susceptibilities and entropies of liquid transition metals are analysed and compared with experiment. The electrical resistivities of liquid transition metals are also calculated, and reasonable agreement with experiment is obtained.

A recently published relativistic model pseudopotential has been generalised to derive (i) a reliable magnetic model pseudopotential and (ii) an effective Hamiltonian for calculating the electronic properties of a simple metal in a weak applied magnetic field. This effective Hamiltonian has been applied to obtain an expression, which includes both the magnetic and the spin-orbit interactions, that can be used to calculate the conduction-electron magnetic susceptibilities of solid alkali metals and liquid simple metals. The applications indicate that as the temperature increases the s component of the conduction-electron wavefunction in a metal such as Pb increases, so that the effect of the spin-orbit interaction, and thereby its contribution to the electronic magnetic susceptibility, decreases very strongly.

The DC magnetic susceptibility of W(Fe) alloys with Fe concentrations between 0.1 and 2.5 at.% Fe was measured with zero field and field cooling in the temperature range 1.6 to 295K. The samples were used as received after melting or after additional heat treatments. W(Fe) alloys show spin glass properties. The heat treatment causes a more random distribution of the Fe ions. This affects the properties of the alloys with higher Fe concentrations. The results are in agreement with a phenomenological cluster model. The clusters are characterized by an anisotropy energy which is explained in terms of dipole and RKKY interactions. Similarities and differences to properties of other spin glasses are discussed.

The thermal expansion and Zr(Fe_1-xCo_x)₂ have been measured. The results indicate that the magnetovolume coupling between spontaneous volume magnetostriction and spontaneous magnetisation is very large and is almost independent of the concentration of Co in the ferromagnetic region (i.e. for x=0.0-0.4). Furthermore, an extraordinarily large negative thermal expansion at low temperatures and a large forced volume magnetostriction at 4.2K have been observed in the ferromagnetic compound with x=0.4, both of which are comparable in magnitude to those observed in Fe-Ni Invar alloys. Fairly large forced volume magnetostriction, which does not vary linearly with the applied fields, has been observed in mictomagnetic compounds with x=0.5 and 0.6.

The NMR spectra of ⁵⁹Co and ¹¹B in the intermetallic borides (Co_1-xMn_x)B (0<or=x<or=0.20) and (Co_1-xFe_x)B (0<or=x<or=0.40) at 4.2K have been measured. The data indicate that the magnetic transitions to the ferromagnetic state, which occur at concentrations of 6% manganese and 10% iron respectively, are produced in an inhomogeneous way. In the (Co-Mn)B system, this includes manganese pair effects. The magnetic wipe-out radii are 4.4and 2.7 AA for Mn and Fe impurities respectively. Also studied is the contribution of Mn and Fe impurities to the spin-lattice relaxation of ⁵⁹Co in these alloys. The contribution of the impurities to the relaxation of the longitudinal nuclear magnetisation of ⁵⁹Co is characterised by a dependence on time that is described by exp(-(t/ tau _li)¹2/): this indicates that the relaxation of the ⁵⁹Co nuclei arising from the presence of the impurities is diffusionless.

For pt.I see ibid., vol.10, p.141 (1980). The internal fields in ⁵⁹Co, ⁵⁵Mn and ¹¹B in the ferromagnetic alloys (Co_1-xMn_x)B and (Co_1-xFe_x)B at 4.2K have been measured by use of the spin-echo technique. Well resolved satellite structures of the boron resonance were observed for the two systems. This shows that the B hyperfine field is mainly determined by the interactions with the nearest-magnetic-neighbour atoms. The measurements indicate that, in the (Co-Mn)B system, cobalt atoms that have three or more manganese neighbours carry a ferromagnetic moment of about 0.2 mu _B, while the (Co-Fe)B system only iron atoms carry a ferromagnetic moment. This result can be explained by the fact that the magnetic moment of iron in FeB is not large enough to induce a magnetic moment on cobalt.

The multiplet splitting of the X-ray photoemission (ESCA) Fe 3s core levels of the ferromagnetic iron borides, FeB and Fe₂B is discussed. It is shown that the observed multiplet splitting is larger than that expected from calibration with the magnetic moments of highly ionic compounds. This is interpreted by the operation of minority spin correlation in the metallic iron borides, which affects the high spin states and thereby increases the experimental splitting. This effect is due to the itinerant d electrons, and it is suggested that these electrons must be considered in order to understand ESCA multiplet splitting in many metals and alloys.