Table of contents

The electronic pair correlation function in beryllium has been measured using X-ray scattering. The results show some structure not present in the free atom calculation. The valence-electron correlation function has been deduced from the experimental data. Assuming that such a correlation function is representative for an electron gas with r_s=2.072, the authors deduce a small but significant correction to the jellium correlation energy at metallic density.

The theory of the entropy of mixing of the compound-forming liquid alloys on the basis of the pseudobinary mixture of hard spheres proposed recently by the present authors is applied successfully to the liquid Na-Pb alloy, for which the experimental entropy data are available across the whole composition range. It is also shown that the usual theory of the entropy of mixing based on the binary hard-sphere mixture fails to describe the behaviour of the entropy of this alloy with the composition.

The screened Coulomb impurity potential with Friedel's extension of the Thomas-Fermi approximation and the square-well potential are considered in the calculation of the Seebeck coefficient in dilute alloys at low temperatures. The electron-phonon enhancement of the Seebeck coefficient is found to be close to the usual mass-enhancement factor.

A general relationship between the characteristic structure in the electrical resistance of a point contact between two metals and the dynamics of the scatterers is used to discuss new experimental results on Kondo systems. The method allows one to measure the energy dependence of the relaxation time of the conduction electrons directly.

Proximity electron tunnelling spectroscopy (PETS) is applied to Ta, yielding results in agreement with those obtained by conventional tunnelling. The inapplicability of a recent 'empirical model' to such clean NS proximity junctions is commented upon.

The pressure dependence of the hyperfine field (HFF) of ¹⁹⁷Au in iron has been measured at room temperature, with the result that ( delta lnH_T/ delta P)_T=-4.6+or-0.2)*10^-4 kbar^-1. This has been used to reduce measurements of the HFF, in the range 4.2K to room temperature, to constant volume. It is found that the temperature dependence of the HFF of Au at constant volume follows an exact T^3/2 law over the whole range of temperature, whereas the HFF of iron deviates from the T^3/2 law above 32K. It is suggested that the temperature dependence of the HFF of Au in iron may be governed by lattice dynamics.

In this critical review, a brief account is presented of the new techniques of 'analytical electron microscopy': that is to say, the chemical and structural analysis of very small amounts of matter. Image formation, diffraction analysis, electron energy loss spectroscopy, and X-ray analysis are all discussed. The discussion is sufficiently detailed to allow the reader to appreciate what can be done with the techniques but it is intended to be easily understood by the non-specialist.

A study of some Mn-Cu alloys in the gamma -phase has been made using neutron and X-ray diffraction at low temperatures. For manganese-rich alloys the variation of magnetic moment with composition was measured and the critical concentration for antiferromagnetic ordering deduced to be x_c=0.72. A measurement of the unit cell dimensions at 4.2K showed the antiferromagnetic order to be accompanied by the tetragonal cell distortion c/a<1, up to the critical concentration. The alternative cell distortions (c/a>1 and orthorhombic), observed with other solutes in manganese, were not detected in these measurements on gamma -Mn-Cu alloys. Using these results a magnetic phase diagram was drawn which could be compared with those diagrams obtained by calculation. The performance of the molecular field model of the martensitic transformation was also investigated for alloys close to the critical concentration. The model was found to be less successful here than when applied to alloys in which antiferromagnetism is well established. The diffuse magnetic neutron scattering from two alloys Mn_0.65Cu_0.35 and Mn_0.60Cu_0.40 was measured and analysed to give information on the spatial variation of magnetic short-range order for specimens close to the critical concentration. This short-range order (SRO) appears to be based on the type 3 antiferromagnetic structure, rather than on the type 1 antiferromagnetic structure, which exists as a long-range ordered state in the manganese-rich alloys. Finally an analysis of the diffuse nuclear neutron scattering from a Mn_0.65Cu_0.35 specimen yielded values of atomic SRO parameters similar to those derived elsewhere, which suggested that the atomic SRO was only weakly developed in the specimens.

A new ultrasonic sing-around system has been used to measure the changes in the elastic moduli of single-crystal specimens of erbium to a much higher precision than has been achieved previously. The variations of the second-order elastic constants C₃₃, C₁₁, C₄₄ and C₆₆ have been measured as a function of temperature over the range 4.2-100K. The variations of the moduli C₁₁ and C₃₃ have also been investigated as a function of magnetic field applied along the c and b crystallographic axes respectively at a series of temperatures in the three magnetically ordered phases of erbium. The results have allowed critical magnetic fields and their dependence upon temperature to be determined.

The specific heat of FCC Fe-Cr-Ni alloys has been determined in an adiabatic calorimeter between 300 and 1200K. The experimental curves reveal two anomalous maxima at 500K and 800-900K with only a weak concentration dependence. The anomalies can be interpreted by thermally excited electron transitions with energy gaps corresponding to absorption bands that have been found in the infra-red absorption spectrum.

A discussion is given of the velocity-dependent forces which arise when two Van der Waals coupled systems move slowly relative to each other. Explicit calculations both for the cases of a molecule interacting with a metal and for two metals interacting with each other indicate that these forces are very small and may safely be neglected in analysing the results of dynamic experiments designed to measure the static Van der Waals force. A result due to Teodorovich (1978) for the case of two metal films in parallel motion is shown to be incorrect.

The consequences of the neglect of backscattering and the Born approximation for the inverse lifetimes associated with impurity scattering in dilute alloys are investigated. The electronic wavefunctions and scattering rates are calculated for clusters in free space with a maximum size of three shells of atoms surrounding the central scatterer. Host cluster electron states are scattered by an impurity cluster, in which the central scatterer is an impurity. The free-space environment is a constant potential equal to the muffin-tin zero of the host potentials. This cluster-in-free-space model has been used before, but now the individual scatterers are described by spherical wells in view of the comparative design of this study. Numerical results obtained for clusters with the BCC structure and with scattering potentials of various depths show that the anisotropy of the lifetimes is adequately represented in the different approximations. However the magnitude of the lifetimes may have been changed drastically from its exact value. The use of the optical theorem for an approximate t matrix is sometimes worse than the Born approximation.

The author has used the local spin density formalism to perform self-consistent calculations of the electronic structure of chromium in the non-magnetic and commensurate antiferromagnetic phases, as a function of the lattice parameter. A change of a few per cent in the atomic radius brings the calculated ground state properties into agreement with experiment. The magnetisation is studied as function of volume in several models, and it is shown that a Stoner picture provides an extremely accurate description of the full calculation provided the sp-d hybridisation is taken into account. It is found that the calculated sublattice magnetisation is extremely sensitive to the exchange-correlation potential used and to the quality of the calculated state density.

The partially filled 4f shell in rare earths gives rise to complex multiplet structure in both photoelectron spectroscopy (PES) and bremsstrahlung isochromat spectroscopy (BIS). The fractional parentage theory previously applied to PES is extended to BIS, and a similarity is predicted between the PES of a 4fⁿ element and the BIS of a 4f^14-n element. Fractional parentage arguments are also applied to the linewidths of the 4f multiplet levels as observed in electron spectroscopy.

For pt.I see ibid., vol.11, p.113 (1980). The 4f and valence states of all metallic rare earths have been studied using X-ray photoelectron spectroscopy (XPS) for the occupied part and bremsstrahlung isochromat spectroscopy (BIS) for the unoccupied part. It is found that the bandwidth increases from Gd to Lu, and that the valence band spectra are in fair agreement with APW calculations. The intensities of the 4f final-state multiplets are well described in terms of the coefficients of fractional parentage. There is a symmetry between the fⁿ XPS and f^14-n BIS spectra. The observed energies of the 4f excitations correspond to transitions to completely screened final states. These values enable one to predict the elements which are liable to interconfiguration fluctuation when their 4f levels are shifted to E_F by the chemical environment or by compression. The large Coulomb correlation energies, U, which prevent the formation of 4f bands in these elements, are directly obtained from the authors' spectra, and are found to be in good agreement with recent calculations. The linewidths and singularity indices of the XPS and BIS 4f lines are determined and discussed.

The electrical resistivity for liquid Ga between 303K and 573K is calculated from the Ziman formula, using structure factors which have been measured with a great accuracy and various pseudopotentials. It is shown that two forms of pseudopotentials provide the authors with a fair agreement between the calculated resistivity and the experimental one measured by Pokorny and Astrom (1976). The part played by the effective mass and the empty-core radius is outlined in a discussion.

The temperature dependence of the electrical resistivity of four samples of aluminium has been measured over the range 1.2-4.2K. The samples were annealed at successively higher temperatures between measurements. It has been found that upon annealing the samples, the electron-phonon scattering term increased by about 60% whereas the electron-electron scattering term decreased by about 20-30%. The effect of the successive annealings has been to reduce the dislocation density in the samples. A theoretical analysis has been presented, based on the idea that a reduced dislocation density tended to give an isotropic electron relaxation time. A calculation has been carried out which yields agreement between theory and experiment for both the increase in the electron-phonon scattering term and the decrease in the electron-electron scattering term.

The authors have measured the electrical resistivity rho (T) of spatially ordered and disordered PtCr alloys containing between 18 and 21 at.% Cr, from 1.4 to 300K. The electrical properties correlate with the magnetic properties as rho (T) in the spatially ordered alloys contains a component which increases with increasing temperature in a manner consistent with spin-disorder scattering from excitations out of a magnetically ordered ground state. The spatially disordered alloys exhibit spin-glass-like features with rho (T) passing through a gentle inflection point at temperatures above that of the susceptibility maximum (T₀). However the low-temperature behaviour of the spatially disordered alloys is anomalous in that no maximum appears in the incremental resistivity. Various causes for the lack of such an anomaly are discussed tentatively.

A theory is presented for the resistivity of U, Np and Pu metals. The acoustic phonon contribution is calculated, taking the temperature dependence of the sound velocity in the f band into account, and subtracted from the experimental curves. The remaining contribution to the resistivity can be explained in terms of the magnetic scattering of conduction electrons from the f electrons, with values of the exchange constant much bigger than kT. These large values suggest an explanation to the constant susceptibilities as a function of temperature in terms of f spin fluctuations.

This paper reports some new results on the thermoelectric (TEP) behaviour of gamma -Ce and beta -La (both FCC) at high pressures. The TEP behaviour of gamma -Ce with temperature shows a large anomaly when compared with that of beta -La. These studies support the fact that in gamma -Ce, a 4f virtual bound state (VBS) lies in close proximity to the Fermi level.

The boundary conditions appropriate to the hydrodynamic model of metals are derived. Under appropriate circumstances one of these boundary conditions reduces to the Forstmann and Stenschke boundary condition or the Boardman et al. boundary condition: it need not reduce to either. The number of boundary conditions applicable at metal-vacuum and metal-metal interfaces is shown to be the same in apparent contradiction to several simple analyses. Explanations are given for this apparent discrepancy.

Recently reported neutron diffraction studies have shown that TbBe₁₃ exhibits a magnetic screw-type structure below 16.5+or-1K. New magnetic measurements have been performed on a polycrystalline sample of TbBe₁₃ to obtain accurate information on the crystal-field level scheme and the exchange interaction. A mean field model which takes full account of the crystalline electric field is described. The best fit to experimental data, including the neutron diffraction results, is obtained for the crystal-field parameters x=-0.8 and W=0.05K which yield a 10K overall splitting and a non-magnetic Gamma ₃ doublet ground state at only 0.7K below a Gamma ₅⁽¹⁾ triplet. Thus TbBe₁₃ is confirmed as an induced moment system. The role played by the anisotropy energy is evidenced by the thermal dependence of the magnetisation and the modification of the screw structure by external magnetic fields is analysed briefly within the framework of a simple two-layer interaction model.

Magnetovolume experiments on non-stoichiometric Ti(Fe_0.5Co_0.5) compounds show that the ferromagnetic behaviour of the stoichiometric compound must be ascribed to the presence of Fe,Co antistructure atoms in a highly susceptible matrix. The effect of pressure on the matrix susceptibility is analysed in terms of the ratio of the effective (I) and bare (I_b) interactions between the itinerant electrons.

The bulk spin susceptibilities chi of a Feeders-Martin itinerant antiferromagnet, in the presence of both non-magnetic and magnetic impurities, are calculated for both the Hartree-Fock and the random phase approximations using the Abrikosov-Gor'kov method. Also the coupling of the spin density wave and the impurity moments is shown to lead to dramatic modifications in the impurity moment susceptibility chi _imp.

Presents evidence that the dominant excitations in CuMn, AgMn and AuMn spin glasses at low temperature lead to a T^3/2 behaviour in the low-temperature heat capacity and magnetisation, similar to that observed for the resistivity. This is contrary to the linear temperature dependence previously believed to describe the heat capacity of a spin glass. This evidence is obtained from a closer analysis of published specific heat and very low-temperature magnetisation measurements. The specific heat and magnetisation are correlated through a simple spin-wave model and are in accord, through the model, with independent measurements of the strength of the RKKY interaction in these alloys. The authors' own magnetisation measurements on the very dilute PdMn system also show an approximate T^3/2 behaviour at very low temperatures.

Zero-temperature computer simulations are reported for the Sherrington-Kirkpatrick random Ising model of a spin glass, including an external field. Results are presented for the internal field distribution P(H), and for the ground state energy and magnetisation as functions of field. P(H) has a linear rise from H=0 for all external fields. The zero-temperature susceptibility chi (0) is close to unity when equilibrium states are examined, in agreement with Parisi's replica symmetry breaking theory and in conflict with linear response theory. The linear response result chi (0)=0 can be obtained by searching for metastable local energy minima close to the zero-field ground state in configuration space.

The ⁵⁹Co nuclear magnetic resonance was observed in Gd₂(Co_1-xFe_x)₁₇ with the Th₂Zn₁₇-type structure of x=0, 0.03, 0.1, 0.2 and 0.7 using the spin-echo method at 4.2K. The composition dependence of the observed spectra corresponds to the variation of the easy direction of magnetisation. The resonance, which was observed at 78 MHz when x=0 and which was attributed to the 6c sites, disappeared at about x=0.1. The dependence of the relative intensity of the spectral lines on x is compatible with the preferential occupancy of particular crystallographic sites by Fe. In Gd₂Co₁₇, quadrupole splittings were observed by the steady-state method at 77K.

The magnetic part of the nuclear spin relaxation rate, R_M, has been measured at constant T in liquid Tl_1-cPb_c, Tl_1-cBi_c and Tl_1-cAu_c (c<0.6), for the Bi nuclear spin. The 182 mu s isomer ^207mBi, produced and aligned by the Tl ( alpha , 2n) reaction, and the TDPAD technique were used. Together with data from the literature, R_M is established for Tl-Pb and Tl-Bi over the whole concentration range. For each alloy, the temperature dependence was measured at one intermediate concentration (c=0.5 or 0.4). The results agree with the free-electron model, revealing that structural and impurity effects on R_M are small.

⁵⁷Fe and ¹⁹Sn Mossbauer investigations of the antiferromagnetic compound FeSn are reported from 77K to well above its magnetic transition temperature. The observation of a single nuclear Zeeman pattern from 1(a) Sn sites implies that the spin ordering in the basal plane is collinear. The ⁵⁷Fe Mossbauer studies show that, whereas in the paramagnetic state the observed spectrum is a well-defined single quadrupole doublet, the spectrum in the magnetic state is quite complex due to the presence of a system of three different local principal electric-field gradient (EFG) axes for the crystallographically equivalent Fe atoms which are characterised by significantly different anisotropic nuclear hyperfine fields. It is found that the magnitudes of the observed hyperfine fields can be explained satisfactorily in terms of an isotropic contribution h_iso and enhanced dipolar fields originating from the external magnetic moments with a single enhancement factor of about 8.2, for the three nuclear Zeeman patterns simultaneously. The value of h_iso has been estimated as a function of temperature. On the basis of the above model, the direction of the nuclear hyperfine field should deviate from the direction of magnetisation due to the presence of an appreciable component of the enhanced dipolar fields in a direction perpendicular to the direction of magnetisation.

The authors have performed optical measurements by multi-angle reflectance between 2 and 30 eV on gadolinium and dysprosium films condensed on glass substrates, in a vacuum of 2*10^-10 Torr. Optical constants have been determined with the help of the multi-angle method and the Kramers-Kronig analysis. The optical conductivity, sigma , optical constants, epsilon ₁ and epsilon ₂, N_eff and energy loss functions have been deduced. The authors have compared their results between 2 and 18 eV with those of Erskine and Flynn (1976) and have shown that the shake-up structure for the 5p thresholds comes from a 5p and a 4f open shell coupling.