Table of contents

High field (up to 50 kG) nuclear spin lattice relaxation data taken on ⁶³Cu in CuMn alloys allow an unambiguous determination of the mechanism responsible for the impurity contribution to T₁, the direct dipole-dipole interaction between Mn electron spins and ⁶³Cu nuclear spins. The analysis of the data allows a determination of the temperature dependence of the electron spin relaxation time tau ₁ which fits a Korringa like relationship tau ₁ varies as T^-1. A discussion of these results compared to direct EPR measurements is then attempted.

The observation of a positive g shift in a polycrystalline Mg-0.5 at %Gd alloy has been used (see Can. J. Phys. vol.49, 352, 1971) to explain the origin of a logarithmic increase (with increasing temperature) of the incremental resistivity of a MgGd alloy of comparable concentration, in terms of the Kondo effect with a positive 'J_eff'. The combination of a wave vector dependent effective exchange coupling in conduction with a significant potential phase shift means that this interpretation is not necessarily correct.

The work function of stainless steel surfaces in a clean vacuum system at 10^-9 Torr was reduced by as much as 1.6 eV by transverse electrostatic fields of the order of 1 V cm^-1. Preliminary results are presented.

Identities are presented which express the individual Brugger constants for an arbitrary contribution to the free energy in terms of the linear combinations of Brugger constants that are calculated by the method of homogeneous deformation is given. Using Cauchy relations and the new relations between elastic constants of different orders derived by Cousins (1971), the electrostatic contributions to the 17 first, second and third order elastic constants of hexagonal metals are found to involve the calculation of only five quantities: the equilibrium electrostatic energy density and four shear constants. Results are tabulated for a range of axial ratios.

Measurements have been obtained of the diffuse scattering of X-rays from an aluminium crystal over the temperature range 100-500 K. It is found that the scattering from transverse modes is as expected from the known phonon frequencies; neither anharmonicity nor atomic deformation with displacement from equilibrium position modifies this scattering. The scattering from longitudinal modes at medium values of the scattering vector is adequately represented by including the lowest order anharmonic process, but at high scattering vectors the next order anharmonic term must be included. At very low scattering vectors an additional contribution is found in the total intensity from longitudinal scattering vectors an additional contribution is found in the total intensity from longitudinal modes. The extra scattering is not associated with anharmonic behaviour although it is temperature dependent. It may be due to atomic deformation or electron-phonon interaction.

The crystallographic features of twins terminating inside the grains and of associated emissary slip have been correlated using transmission electron microscopy. It is observed that when the slip patterns are simple, the Burgers vector of slip dislocations is always parallel to the direction of the twinning shear; this conclusion is consistent with the suggestions of Hull (1961, 1964) and Sleeswyk (1962). It is proposed that the emissary sub-structures evolve to relax the stresses at the tips of the twins which have stopped growing inside the crystal, and their evolution could be rationalized in terms of Hull's coalescence model. It is suggested that the coalescence reaction is more likely to occur than the dissociation reaction proposed by Sleeswyk, because the former is energetically less unfavourable.

The phonon limited mean free path has been measured in cadmium using the limiting point method. The measurements are on the lens part of the Fermi surface and extend from (0001) to (1010) in the (1120) plane. The mean free path varies approximately as T⁵ except near (1010) where the temperature dependence is T⁶ or even T⁷. A model has been proposed, which accounts for the experimental results over much of the (1120) plane. This model takes account of elastic anisotropy and distinguishes between electrons rendered ineffective by a single phonon scattering event and those rendered ineffective by multiple scattering events. The model should be applicable to any metal over a wide temperature range.

Semiempirical calculations of Fermi surface dimensions have been carried out for gold using the Green function nonrelativistic method and a Gaspar-type potential. The exchange energy coefficient and the exchange and coulombic energy screening parameters have been varied in accordance with a suggested new optimization procedure for noble metals. The results are disappointing and are unlikely to lead to a band structure more useful than that obtained previously by Ballinger and Marshall (1969). The results are discussed in relation to the many recent band structure calculations for this metal.

The authors have used the differential reflectance technique to study the changes in the optical properties of aluminium on alloying with small concentrations of manganese and copper. The measurements indicate that these impurities drastically modify the band structure of the pure metal. At concentrations of only a few atomic percent, the impurities so strongly scatter the aluminium conduction electrons that structure in the energy bands from Brillouin Zone boundaries is washed out, and the electrons in these alloys tend towards a free-electron response, though with short relaxation times. The data support Friedel's model of a rather wide impurity band of states, spanning the Fermi level in the manganese alloys but lying 5 eV below the Fermi level in the copper alloys.

Oscillations below the helicon edge, with harmonic structure that might be due to the four fold symmetry of the Fermi surface, have been reported in the literature for both In and Al. These oscillations are discussed in terms of Gantmakher-Kaner oscillations, of Doppler shifted cyclotron resonance modes and of a minimum in delta A/ delta k_z. The difficulty of interpreting these oscillations is emphasized.

A new Green function technique to solve the Schrodinger equation in two media joined on a surface is applied to a square well potential in a free-electron gas. The method gives both the bound states and the change in charge density and density of states.

A single antiphase boundary in CuAu II constitutes a two-dimensional Ising system and it is suggested that the phase change from CuAu I to CuAu II corresponds to the free energy of a single boundary passing through zero as the Ising temperature is approached.

Measurements of the resistivities of liquid Mg, Ca, Sr and Ba are described. The values obtained at the melting point are, respectively, 26.1, 33.0, 84.8 and 30.6 mu Omega cm. Resistivities calculated with the aid of three model pseudopotentials are in the range 7 to 21 mu Omega cm. The discrepancies between the observed and calculated resistivities are discussed in terms of recent contributions to pseudopotential theory.

The electrical resistivity and thermoelectric power of a number of PdPt alloys across the complete range of composition, have been measured as a function of temperature from 1 K to 100 K. The most distinctive feature of the measurements is that the low temperature characteristic thermopowers at the two ends of the alloy series have opposite signs. The concentration dependence of the residual resistivity is roughly parabolic in form, although the rate of increase of resistivity per atomic percent solute is twice as great at the palladium rich end. Both this, and the changing sign of the diffusion thermopower, is remarkably similar to the behaviour predicted for the NaK system on the basis of a pseudoatom model. Near the middle of the composition range there is a 30% decrease in ideal resistivity, which one can correlate with a very shallow resistance minimum in one of the concentrated alloys, and explain on the basis of a temperature dependent modification of the residual resistivity.

Heavily cold worked Nb-60 at% Ti wires have been studied. The wires had previously been heat treated and characterized by electron microscopy (Neal et al 1971). The critical supercurrents have been measured as a function of applied magnetic field at temperatures between 4.2 K and the transition temperature, and interpreted in terms of the pinning force per unit volume on the vortex lattice. Empirically the pinning force is given by F_p=f(b)B_c2ⁿ (2<or approximately=n<or approximately=2.3) where b is the reduced field B/B_c2 and f(b) depends on the particular heat treatment but not on the temperature at which the critical current is measured. Present pinning theories are reviewed but, as none account satisfactorily for the results, a simple model based on pinning by the cell walls of the defect structure has been developed. It depends upon the Ginsburg-Landau parameter being enhanced in the cell walls and fits the data for the material given the optimum heat treatment very well. It is suggested that the other heat treatments produced cell walls with a different internal structure such that the basic pinning interaction and hence f(b) and n are different.

The nature of quasiparticle decay rates caused by electron-electron scattering in type I superconductors is investigated, and found to be of comparable magnitude to phonon decay in some metals, for example aluminium. Its effect on the tunnelling density of states is also examined.

The static dielectric screening function is studied in fcc Pd, using the selfconsistent Hartree approach of Ehrenreich and Cohen (1959). The integral equation for the inverse susceptibility is solved in a number of approximations in the manner applied to Ni by Hayashi and Shimizu (1969). The results of a single d band model calculation and a combined s and d band calculation with and without core polarization effects are included along the principle symmetry directions (100), (110) and (111). In addition the N process and Thomas-Fermi results are given. The models are then compared to a long wave limit fitted dielectric function calculated from the elastic constants C₁₁, C₁₂, and C₄₄ taken from sound velocity measurements. The agreement of the 2-band result including core polarization with the long wave fitted dielectric function is extremely good and suggests that a single point-ion description of phonon effects may be useful in this metal.

The authors have observed the pattern of magnetic field penetration on the top surface of short cylinders of indium and tin when the field is applied parallel to the cylinder axis. They have then computed numerically the normal superconducting state phase boundary, the variation of the average field distribution in the cylinder and the field distribution outside it. Together with the experimental results the computations explain the coarse structure of the field penetration. They also show that the fine intermediate state structure in the penetration annulus can be described by Landau's unbranched model of the intermediate state, suitably modified. Finally, the magnetic moment and the average magnetic induction in the cylinders have been derived.

Magnetic measurements from 1.5 to 300 K on the complete series of Cu₃Au-type R Pd₃ phases have been made. LaPd₃ and LuPd₃ are diamagnetic at room temperature. CePd₃ shows Pauli-type paramagnetism indicative of a nonmagnetic virtual bound state. PrPd₃ and NdPd₃ are affected at low temperatures by the presence of the crystal field. The magnetic properties of SmPd₃ and EuPd₃ are determined by the energy levels of the lowest multiplet. GdPd₃ and (possibly) TbPd₃ become antiferromagnetic at 7.5 and 2.5 K respectively. DyPd₃-ErPd₃ obey the Curie law. The magnetic properties of these phases are discussed in relation to crystal field effects and a possible band structure.

The magnetic and structural properties of the neodymium- dysprosium alloy system have been measured over the entire composition range. The crystal structures change from hcp to Sm type to dhcp arrangement with increasing neodymium concentration. Magnetization and initial susceptibility measurements suggest that the dhcp phase does not order magnetically. In the Sm phase antiferromagnetic behaviour is observed while the hcp alloys show a step ordering process. The Neel points follow a 2/3 power law variation with a reduced de Gennes function G given by the difference of the de Gennes functions for the two components of the alloy.

The pressure dependence of the magnetization delta zeta / delta p is calculated in the itinerant electron model of ferromagnetism. It is assumed that the relative population of s and d bands, the width of the d band W and the effective interaction I_eff vary with pressure. Under these assumptions, a general expression for delta zeta / delta p is obtained which is exact within the Stoner model. The two important parameters which govern the pressure dependence of the magnetization in this model are the s-d transfer under pressure dn/dp and the pressure dependence of the effective interaction delta I_eff/ delta W. The temperature dependence of delta zeta / delta p is evaluated numerically for a single parabolic band and the values of parameters appropriate to nickel. The calculated temperature dependence of delta zeta / delta p is compared with the experimental data of Tange and Tokunaga (1969). Good quantitative agreement is obtained in the whole range of temperatures if the following values of the parameters dn/dp and delta I_eff/ delta W are chosen: dn/dp=-1.62*10^-7 bar^-1, delta I_eff/ delta W=8.68*10^-2. From the pressure dependence of I_eff the correlation effects in nickel are estimated. It is found that the ratio I_eff/I₀=0.4+or-0.1, which implies an immediate correlation effect.

For Pt. I see abstr. A1732 of 1972. The electric field gradients at nearest and in some cases next nearest host Cu nuclei to various impurities in dilute Cu alloy single crystals have been determined by the quadrupole effects in nuclear magnetic resonance. The impurities are In, Sn, Sb and Ge. The field gradients at nearest neighbour sites are far from rotational symmetric around the direction towards the impurity and in the case of CuGe it does not even have the main component in that direction. These results question the validity of the theoretical models assuming spherical symmetry for the charge density around the impurity, at least in host metals like copper.

Photoelectron energy spectra for photon energies up to 21.2 eV have been obtained from lead surfaces prepared by evaporation in the ultrahigh vacuum measuring chamber. The results, which cannot be explained on a simple density of states model, are discussed with reference to previously reported band calculations. Attempts were made to reduce the work function by a deposit of caesium but this altered the spectra drastically, probably due to a reaction between lead and caesium.

In continuation of earlier work (1971) the L absorption spectra of ⁷²Hf to ⁷⁹Au in pure metallic form have been investigated with a one metre curved mica crystal spectrograph of transmission type. Except at the L₁₁ absorption edge of ⁷⁸Pt, a white line is detected both at the L₁₁ and L₁₁₁ edges of ⁷²Hf to ⁷⁸Pt. The peak position and width of the white line are reported. As one goes from ⁷²Hf to ⁷⁸Pt, it is noticed that the width of white line at the L₁₁₁ edge decreases with the filling of the 5d band and vanishes completely for ⁷⁹Au. An attempt is made to correlate the existence, intensity and distance of the white line from the main edge with the theoretically determined density of states.

The optical properties of AuMn alloy systems have been measured by a polarimetric technique through an energy range 0.8 eV to 4 eV. The Mn impurities are found to increase the classical collision frequency of the intraband absorption. The onset of direct interband transitions shifts slightly to higher energies with increasing Mn concentration in approximate agreement with Friedel's theory. Extra absorption bands are found to be present in the alloys compared to the pure noble metals, and these are interpreted as arising from spin split virtual levels. The total spin splitting of the impurity levels is about 4.8 eV for AgMn and about 4.6 eV for AuMn.