Table of contents

The method consists of calculating the Fourier coefficients of the atom-plane interaction. It is shown how to calculate these coefficients when starting from a pair potential given numerically in real space. The Fourier expansion converges very quickly so that it provides a very convenient method for the study of planar defects.

Equilibrium measurements of the temperature dependence of the peak coincidence rate from positrons annihilating in Cd show that there are two regions of linear slope at temperatures below the vacancy trapping region. The electron-positron momentum distribution measured in the region of intermediate slope exhibits smearing consistent with positron localization in a self-trapped state. In this state a decrease of 8% in the core annihilation rate is also observed.

The linear response of a metal to an impurity embedded near the surface is treated within the static Hartree approximation. Using a model potential, an analytic expression for the asymptotic form of the displaced charge density oscillations is obtained, allowing the local approximation of Brown et al. (1974-75) to be evaluated.

The effects of 3d transition metal substitution for Ga in the equiatomic alloy CoGa have been studied. The occurrence of ferromagnetism beyond a critical concentration appears to be related to the electron concentration rather than a change in crystallographic ordering.

Ferromagnetic and paramagnetic resonance absorption of microwaves and antiresonance have been studied in a Gd_0.61Y_0.39 crystal. The similarities and dissimilarities between this crystal on the one hand, and Ni, Tb and Gd_0.25Tb_0.75 crystals on the other as regards variation of the applied field for resonance and linewidth with frequency and temperature are pointed out.

The magnetoreflection spectrum of antimony has been measured for circularly polarized radiation. The lowest energy peak for the 0.149 eV gap is observed to split into four components, two for each polarization. These show considerable deviations from the Cohen-Lax two-band model, and values of m_c=0.240 m₀, g_c=99 for the conduction band, and m_v=0.0164 m₀, g_v=76 for the valence band, have been inferred from a generalization of the Maltz selection rules.

The changes of the elastic shear moduli C₄₄ and C'=(C₁₁-C₁₂)/2 have been measured after electron irradiation of aluminium at 4.5K. Aside from dislocation pinning, a diaelastic effect which is proportional to the radiation-induced defect density has been observed. From these data the diaelastic polarizabilities under an external shear strain of the radiation-induced interstitials can be extracted. The results are very similar to those of Cu. The observed polarizabilities of alpha =-23 and alpha '=-13 are unusually large (for point defects) and highly anisotropic i.e. alpha / alpha ' approximately=2. These findings can be understood quantitatively on the basis of a model calculation which takes into account the (100) split configuration of the interstitial and the effect of the highly compressed atomic arrangement within the interstitial core region on the interatomic force constants. Thermal annealing studies indicate that di-interstitials present at the end of stage I have an even higher diaelastic polarizability than single interstitials and that the diaelastic polarizability decreases strongly when the interstitials form dislocation loops.

An extensive investigation of quantum oscillations in ultrasonic attenuation in white tin has been carried out with pulsed longitudinal sound waves of frequencies 390 and 430 MHz in magnetic fields up to 88 kG. The data were recorded digitally and analysed by means of the Fourier transform on a computer. Comparison of nearly forty of the observed frequency branches with previous data collected by de Haas-van Alphen and RF size effect produced, in general, excellent agreement. Several new branches confirm the observation of Matthey et al. (1974) that Craven's model of the Fermi surface of white tin needs modification in the fourth and sixth zones. Nine frequency branches (detected when the magnetic field was nearly parallel to (001)) have been assigned to magnetic breakdown orbits. In addition, four breakdown orbits were observed with the field direction near (110), two of which constitute the first observation of tunnelling at the ends of the third zone dumbbell. A comparison of the theoretical and measured oscillation amplitudes of the breakdown orbits in the (110) plane yields the breakdown fields H₀^A approximately 50 kG at the dumbbell waist and H_O^B approximately 420 kG at the ends.

Ultrasonic quantum oscillations have been studied in lead single crystals using 70-110 MHz longitudinal waves at about 1.3K in magnetic fields up to over 7 T. The results are interpreted in terms of the Fermi surface; they confirm the presently accepted third-zone surface and they show the presence of electron pockets in the fourth zone, previously believed to be empty. The fourth-zone occupancy is shown to be about 4*10^-3 electrons per atom.

The recently derived pseudopotential of Rasolt and Taylor (1970, 1975) has been used to carry out a computer simulation of the properties of solid Na near its melting point. The authors have calculated the elastic constants, the thermodynamic Gruneisen parameter, the pressure, the specific heat and the quantity (dp/dT)_v at both 293K and 361K. The calculation was performed for molar volumes that correspond roughly to those observed experimentally at zero pressure. A system composed of 250 particles is used, in which each atom is allowed to interact with its nearest 112 neighbours; periodic boundary conditions have been introduced to allow for the simulation of an infinite system. The overall agreement of the calculations with experimental data is good when one makes allowance for possible contributions from the electron gas terms. However, the shear constant c₄₄, which contains no such contributing terms, is definitely smaller than the experimental values of Fritsch, Geipel and Prasetyo (1973). This may indicate an inadequacy of the pseudopotential.

The semiclassical theory of ultrasound absorption in metals, based on the Boltzmann equation, has been generalized to the case of Rayleigh waves. The choice of the expression for the dissipated energy is discussed in detail. The result of the phenomenological theory of Rayleigh wave absorption in a viscous solid is derived from this semiclassical theory in the limit that the electron mean-free-path is short compared to the sound wavelength.

In Kondo systems, de Haas-van Alphen experiments determine a spin-splitting term in the conduction-electron self-energy. It is shown that for all temperatures and magnetic fields, and in the presence of normal potential scattering from the magnetic impurity, spin-splitting of Landau levels is given exactly with Re Sigma _spindown(0)-Re Sigma _spinup(0)=cJ(S_z)_K Sigma _sigma (0) is the conduction-electron self energy at the Fermi level and (S_z)_K is the expectation value for the impurity spin including Kondo effects. This equation also applies for both the large-U (Kondo) and small-U limits of the Anderson model, with (S_z)_K to (S_z)_A for the small-U case. (S_z)_K,A does not obey the field and temperature dependence of the Brillouin function, but instead follows the Langevin function. This is so because the Friedel-Anderson impurity d orbitals are populated successively by many electrons during the impurity-spin relaxation time.

The authors determine the electronic contribution to the energy of formation Delta E in completely disordered dilute alloys from an expansion in successive powers of the impurity concentration; a similar analysis can be used for the magnetic (spin fluctuation) contribution to the energy of formation. Numerical results are presented and discussed, for alloys of transition impurities in noble (copper) or transition (first series) metals, using a realistic band structure and aluminium-based alloys. For transitional alloys a simple sum rule is used which relates Delta E to the host density of states.

The electrical resistivity of a series of aluminum-based zinc solid solution alloys containing 0.05 to 1.88 at.% Zn, has been measured as a function of the temperature in the region 4.2-350K. From the resistance data, the deviations Delta (c,T) from Matthiessen's rule have been evaluated. In the temperature range 6.5-75K, the temperature dependent part of the resistivity varies as Tⁿ, with 1<n<3.8.

Previous radio frequency size effect measurements in potassium have suggested that (1/l_ep)_eff, the effective inverse electron-phonon mean free path, depends upon the mode of RF excitation employed in the experiment. The authors report new measurements, made using both bilateral and unilateral excitation, which show that (1/l_ep)_eff is not affected by the mode of excitation. This result is supported by simple physical arguments and confirmed by a numerical calculation. It was impossible, however, to resolve the discrepancy between the value of (1/l_ep)_eff measured in this work and that measured by Tsoi and Gantmakher (1969) and Blaney and Parsons (1970) in earlier work.

Using the Nagaoka formulation, the author has evaluated the current-current correlation function to calculate analytically the frequency-dependent electrical conductivity of dilute magnetic alloys at low temperatures. Because of the approximations involved in the calculations the results are valid for frequencies, omega << omega _max approximately 10¹³ Hz with concentration of impurities, c approximately 10^-4 or less. In contrast with the numerical calculations of Murata and Wilkins (1968), the real part of the conductivity does not show any peak with respect to the frequency. The real and imaginary parts of the conductivities can be written as a universal function of c, in agreement with the calculations of Murata and Wilkins.

Small-angle neutron scattering measurements have been made on Au-Fe alloys as a function of temperature in the concentration range between 10 and 23 at.% Fe using cold neutrons of wavelength 6.7 AA. These reveal strong critical scattering peaks in alloys with 17 at.% and higher concentrations of Fe, whereas in alloys with lower Fe content the present measurements have failed to detect a similar behaviour. In these latter alloys, especially in the region around 15 at.% Fe, the neutron scattering results confirm the formation, with decreasing temperature, of large regions of correlated spins as inferred from the strong superparamagnetic behaviour observed in earlier magnetization measurements. The authors attribute the critical scattering in alloys containing 17 at.% Fe to the onset of ferromagnetism. The Curie temperatures thus obtained are discussed with reference to other measurements on this alloy system.

Measurements of the magnetic excitation spectrum of ferromagnetic Pd₃Fe (T_c=499K) have been made at 293, 373, 423, 473 and 573K by neutron inelastic scattering techniques. Constant energy and constant momentum transfer modes of operation were employed with reduced wavevectors between 0.11 and 0.40 in the ( zeta 00) direction. At 293K a single branch of the dispersion relation for magnetic excitations is observed which is characterized by an exchange stiffness of 52.7+or-2.7 THz AA². The intrinsic linewidth is a strong function of wave vector and varies from 0.0+or-0.5 THz at zeta =0.13 to 2.5+or-0.5 THz at zeta =0.30. When the temperature is raised the excitation frequencies fall and the linewidths increase. Between 293 and 473K at zeta =0.3 the frequency falls by a factor of 1.8 and the linewidth increases by the same factor, but a broad inelastic response is still observed. At 473K at wavevectors less than zeta =0.13, the magnetic scattering has the form of a peak of appreciable width centred on zero frequency.

Mossbauer spectra on ⁵⁷Fe and ¹¹⁹Sn in perovskite carbides with formula M₃M'C (where M is Fe or Mn, and M', Al, Ga, Ge, Zn, Sn) have been observed. All those compounds are characterized by a large quadrupole coupling amounting to 1.3 mm s^-1 and by low internal magnetic fields, around 70 kOe, at M atom. The effect of carburation on the hyperfine parameters and the different contributions to the internal magnetic field are thoroughly discussed. Only a model of itinerant magnetism can account for the various observed magnetic anomalies: first-order transitions, unresolved magnetic Mossbauer spectra, deviation from unity of the ratio of the magnetic moment deduced from the Curie constant to the magnetic moment at saturation, deviation from the linear moment-field relation.

The photoassisted field emission is described as a tunnelling process through a barrier including the surface potential wall of the free electron metal and the external DC electric field potential. The corresponding transition matrix elements are calculated with application to sodium. The functional dependence of the electron self-energy on the longitudinal dielectric function is derived for finite temperatures. Also discussed is how the boundary conditions for the incoming light beam of arbitrary polarization depend on the transversal and longitudinal dielectric functions. Finally, a transformation formula is presented, which introduces a finite relaxation time parameter in a proper way so that the Thomas-Fermi limit and the Drude approximation take their appropriate values.