Table of contents

The critical behaviour of an m-component spin system with helical ordering is studied using the renormalization group method to order epsilon ² (where epsilon =4-d). For m=1 and 2 the system is equivalent to a 2m-vector model. For m=3 a first-order transition is expected. The effect of the commensurability of the helical structure with the lattice has been considered and is shown in certain situations to change the order of the transition.

Intensity autocorrelation measurements of the diffusion constant of lysozyme molecules in aqueous solutions as a function of applied magnetic field reveal no evidence of the cooperative behaviour proposed by Ahmed, Calderwood, Frohlich and Smith (1975, 1976) to explain their observation of a large peak in the diamagnetic susceptibility of lysozyme at an applied field of 600 Oe. Careful susceptibility measurements using a SQUID system show that the diamagnetic susceptibility of lysozyme and lysozyme solutions is constant up to a maximum field of 720 Oe.

Using far-infrared absorption spectroscopy, the variation of the frequency of the zone-centre magnon in Fe_1-cMg_cCl₂ has been measured for the complete range of c. The magnon intensity does not vanish at the percolation limit. Instead it gradually becomes an electronic transition of Fe²⁺ in MgCl₂ as c approaches 1. A mean-field model describes the frequency variation of the absorption quite well, but no simple model gives a satisfactory description of the lineshape. The measurements provide evidence for the existence of a local magnetic mode predicted by an ATA calculation for c=0.1.

The so-called 'chain method' of calculating RHEED intensities is investigated. Convergence of the various expansions is tested and the range of feasibility for the full multiple scattering treatment is found to be up to 5 keV incident energy at present. This covers the energy range proposed for determination of atomic coordinates at surfaces, and the sensitivity of RHEED to atomic positions is investigated.

The defect structures of lightly doped alkaline-earth fluorides are examined using theoretical methods. Calculations are reported on cluster structures and reorientation mechanisms. Simple models are proposed which account for the principal results of spin resonance, relaxation and ITC studies of these crystals. The interpretations of the results of the latter experiments are based on novel interstitial migration mechanisms, which are related to structural features of more heavily doped crystals.

For pt.I see ibid., vol.9, no.10, p.1845 (1976). Models for interstitial aggregation in doped fluorides are discussed. Two problems are considered: first the nature of the interstitial-interstitial interaction, where it is shown that earlier treatments based solely on the ionic model must be modified to allow for a large covalent term in the interaction between interstitials. The proposals are supported by a molecular orbital study of F^--F^- interactions. Problems in the interpretation of diffraction data on heavily doped crystals are also examined. The author is able to rationalize the experimental observations by extensions of earlier cluster models and by further models based on cluster interactions. Lattice energy calculations confirm the proposals.

Many suggestions have been made to improve the original model of exciton decay which leads to the displacement of halogen ions in the alkali lattice. Several of the models involve thermally-activated movement of the crowdion defect (the H centre). In this paper the models and experiments are reviewed. Many of the problems are resolved by considering the possibility of exciton diffusion before the decay of the excitons by luminescence or a non-radiative process involving a replacement collision sequence. In this model, the collision sequence need only require a few replacements to achieve defect stability. The model also covers the production of H_A centres in impure alkali halides and the temperature dependence of the process.

The yield of electrons captured into continuum states of 200-300 keV protons is measured for conditions of both (111) channelling and random impact on single-crystal gold foils. The measured yields under channelling conditions are approximately 80% of those under random impact. To discuss this theoretically it is shown that the channelled protons capture only from the 5d electrons whereas the random beam captures from 5d, 5p and 5s. Using atomic Hartree-Fock wavefunctions throughout reasonable agreement with experiment is obtained, particularly under channelling conditions.

Assuming that the interaction between metallic pseudo-ions is of the form introduced by Evans (1974, 1975), an approximate expression for the excess free energy of the system is derived using the thermodynamic perturbation theory of Weeks, Chandler and Anderson (1971). This excess free energy is then minimized with respect to a parameter which specifies the ion density profile, and the surface tension is given directly. From consideration of the dependence of the interionic forces on the electron density it is predicted that the ions should take up a very steep density profile at the liquid metal surface. The values of the surface tension calculated for liquid Na, K and Al from a simplified version of the theory are in reasonable agreement with experiment.

The desorption process of a gas atom from a general potential is examined numerically within one- and three-dimensional systems. The formal extension to three dimensions of a previously reported one-dimensional theory is presented and numerical results are compared with experiments. Simple expressions for the delay time result from the physical processes involved in the desorption step being strongly dependent upon the functional form of the potential near the bottom of the well. Results obtained when comparisons with physical systems are made, yield interaction potentials compatible with realistic values of both adatom surface bond lengths and well widths.

A first-principles approach for calculating the dielectric function of the uniform electron gas is presented. Developing the Green functions for the response of the system by an equations-of-motion and a perturbation theory for the mass operator, it is shown that the theory can be pushed beyond the existing results without the loss of algebraic tractability. The analysis leads to successive approximations to the mass operator and to the dynamic 'load field correction' whilst making contact with the existing theories in a transparent manner. As the method uses no decoupling approximation but depends upon an algebraic development of the Dyson equation, comparison with diagrammatic theories becomes very easy.

Magnetophonon resistance oscillations have been observed in n-PbS at 77K. These oscillations arise from scattering of electrons by longitudinal optical phonons between Landau levels when the optical phonon energy h(cross) omega _LO becomes a multiple of the cyclotron energy h(cross) omega _c. After corrections for the phase shift arising from the variation in amplitude of the resonance peaks and for the polaron effect, the cyclotron mass is found to be m_c*=0.083 m₀.

The temperature dependences of the mobility and thermoelectric power are calculated for pure mercury telluride. The scattering of electrons by charged point centres and optical phonons is taken into account. The variational principle is used in the calculation because the relaxation time cannot be introduced for optical-phonon scattering. It is shown that the conduction-valence interband processes in optical-phonon scattering, possible in this zero-gap semiconductor, give an important contribution to the total scattering. It is demonstrated that the electron mobility in pure HgTe is determined by charged-point-centre and optical-phonon scattering and that the experimentally observed minimum of the conductivity between 30K and 50K can be explained by the interband resonant contribution to optical scattering.

Earlier thermal conductivity measurements of Mg:Al₂O₃ showed the presence of strong resonant phonon scattering, although positive identification of the scattering centre was not possible. This paper describes further thermal conductivity measurements of Al₂O₃ specimens doped with Mg, Cr and Fe in various combinations in an attempt to identify this scattering centre. The results are shown to be consistent with the theoretical predictions for Cr⁵⁺, and thermally detected EPR measurements on the samples indicate that Cr⁵⁺ is indeed present in the samples exhibiting phonon scattering. The thermal conductivity analysis indicates that three transitions within the low-lying energy-level system of Cr⁵⁺ are important.

For pt.I see ibid., vol.9, no.10, p.1955 (1976). A strong EPR absorption, excited by the radiofrequency electric field, is found in magnesium-doped Al₂O₃ when the concentration of magnesium exceeds that of all other impurities. The strength of this EPR absorption correlates with a strong resonant phonon absorption reported by Brown. From the detailed behaviour of the absorption and in particular its strong similarity to the known behaviour of the 3d¹ ions Ti³⁺ and V⁴⁺ in Al₂O₃, it is deduced to be due to the isoelectronic 3d¹ ion Cr⁵⁺ in the Al³⁺ site. A cluster model of the Jahn-Teller effect is used both to identify the ion and to deduce that the Jahn-Teller reduction factor is gamma =0.025. There is strong evidence that the Cr⁵⁺ site axes do not coincide with the crystallographic C₃ axis but are each inclined to it at an angle of 0.3 degrees in the bc plane, an effect probably due to the details of charge compensation.

The electron paramagnetic resonance spectrum of Gd³⁺ in LaCl₃.7H₂O, with 1/500 dilution of Gd/La ions, is studied with an X-band spectrometer at 295K, 79K, 4.63K, and 1.59K. The low-temperature data is used to determine the signs of parameters. In particular, the following values of the g tensor and the zero-field splitting parameters b₂⁰ and b₂² are found: at 295K g_zz=1.994+or-0.005, g_xx=1.996+or-0.005, g_yy=2.001+or-0.005, b₂⁰=-1.177+or-0.015 GHz, b₂²=0.659+or-0.015 GHz; at 1.59K, g_zz=2.000+or-0.020, g_xx=1.992+or-0.020, b₂⁰=-0.918+or-0.060 GHz, b₂²=1.023+or-0.060 GHz. The values of all other parameters are also determined. Estimates are made of the extent of admixture of the excited ⁶P_7/2 state with the ground state ⁸S_7/2.

The chemical shifts of nuclear magnetic resonance in Hg_1-xCd_xTe alloys have been measured for the three nuclei at various compositions. A calculation of the band structure allows one to predict the observed shifts in the pure compounds HgTe and CdTe. In alloys the band structure is no longer a useful tool since the shifts are related to the local electronic structure. The shifts of mercury, cadmium and tellurium behave very differently as a function of cadmium content: mercury shift changes strongly, cadmium shift is weakly affected while the tellurium shift is practically constant. It is shown that the variation of the mercury shift occurs when second neighbours of mercury are substituted by cadmium atoms. On the other hand, the tellurium atoms are already surrounded by p-electrons; as the number of these electrons does not change, there is only an anisotropic chemical shift of tellurium.

X-ray photoelectron spectra show changes in the valence bands accompanying the temperature-dependent semiconductor-metal transition in La₂NiO₄ and the localized-itinerant electron transitions in LaCoO₃ and YCoO₃. In La_1-xSr_xCoO₃ the valence band changes with the variable x in a manner reflecting the known changes in electronic properties.