Table of contents

Measurements of the linear thermal expansion coefficient of rubidium chloride are reported between 30 and 260 K. The results are believed to be accurate to within 1% down to 200 K, falling to 2% at 30 K. The Gruneisen parameter gamma = beta VK_T/C_v has been shown to fall monotonically from a high temperature limiting value of 1.46. The quasiharmonic approximation has been applied to these and complementary heat capacity and elasticity data to calculate the moments of the vibrational frequency distribution and their volume dependence, interpolation being employed at the lowest temperatures. The results of these calculations conform to the pattern of behaviour which has emerged from earlier comparative studies of sodium and potassium halides.

The authors have demonstrated that a paramagnetic resonance spectrum can be observed using strain modulation of the paramagnetic centre by ultrasonic waves. The technique is useful for simplifying a complicated spectrum and for determining the effects of strain on the paramagnetic resonance parameters.

A general and compact scheme is presented to calculate the self-energy of an electron in disordered alloys. The effect of atomic correlation is taken into account in a systematic way.

For Pt. I see ibid., vol.4, L118 (1971). A selfconsistent method is presented to calculate the self-energy of an electron in amorphous solids, glassy solids and liquids within the framework of the tight binding approximation.

For Pt. II see ibid. vol.4, L121 (1971). An approximate selfconsistent expression for the self-energy of an electron in a liquid metal is obtained by generalizing the concept of the coherent-potential approximation.

The authors report the observation of stimulated Stokes scattering of 5.3 mu m pump radiation by spin flip excitations in InSb, using a 10.6 mu m CO₂ laser, together with a tellurium frequency doubler, as the 5.3 mu m source. The effects of resonant enhancement on the 5.3 mu m scattering are discussed and comparison made with 10.6 mu m results. The observation of atmospheric water vapour absorption lines near 5.5 mu m demonstrates the application of the spin flip source to high resolution spectroscopy.

It is shown that the proposals of Harvey and Kiefte (see abstr. A13386 of 1971) concerning spin Hamiltonian hyperfine operators are erroneous.

Marcelja's treatment of the resistivity of antiferromagnets above the Neel temperature is shown to be incorrect. (See abstr. A33573 of 1971).

At least three new bands have been observed in the near infrared region of the absorption spectra of fluorites of different origin after heavy gamma irradiation in the range 10⁶ to 10⁹ R. Studies were also made with a sample of synthetically pure CaF₂ crystal and the results indicate that the rare earth impurities abundantly present in the fluorites play an important role in the production of these new bands. The possibility of assigning these bands to some irradiation produced colloidal centres in the fluorites is discussed.

The effect on the transmitted radiation line shape of the localization of Wannier excitons to a given layer in thin layer structure crystals is considered, with particular reference to GaSe.

The long time (low energy) response of extended systems to a suddenly switched localized spin perturbation is studied in three special situations: the Kondo problem, the X-Y model and the ferromagnet.

The mobility of an ion in a Fermi liquid is investigated by using the Van Hove scattering function of the ion. The assumptions underlying this approach are investigated. Fermions scatter elastically off the negative ion in pure ³He, due to a Mossbauer effect. An analogous effect is not found for ionic mobilities in ³He-⁴He mixtures at any concentration or temperature. The scattering model of Bowley and Lekner is discussed for the positive and negative ions, and the mobilities are analysed in terms of the Van Hove scattering function in all temperature and concentration regions. The pressure dependence of the positive ion mobility is analysed using the Bowley Lekner scattering potential with good agreement with experiment.

Neutron scattering techniques have been used to measure the temperature dependence of phonon frequencies in a free-standing crystal of ³⁶Ar. Dispersion curves were obtained for phonons of transverse polarization with wavevectors in the (100) direction at 4, 40 and 77 K. Comparison with lattice dynamical models indicates that most of the observed frequency shifts are caused by the change in volume of the crystal.

The specific heats of ferrous bromide and ferrous iodide are reported in the approximate temperature range 6 to 295 K. The results are believed to be accurate to within 2 per cent at 295 K and 5 per cent at 6 K. The specific heats displayed anomalies at 11.4+or-0.1 K and 8.9+or-0.1 K respectively, and these have been associated with transitions to a state having ferromagnetic coupling between Fe⁺⁺ ions within layers, and antiferromagnetic coupling between adjacent layers. The approach to and departure from the peak in each curve of deduced magnetic heat capacity against temperature is logarithmic, and the entropy changes are consistent with values calculated assuming a spin ⁴/₂ for the Fe⁺⁺ ions, that is 13.1+or-0.6 J mol^-1 K^-1 for both solids.

A quantitive theory of the superexchange interaction between Ir⁴⁺ ions in ammonium hexachloroplatinate and related compounds, based on Judd's formulation, is presented. The overlap model of superexchange proposed by Bradbury and Newman is extended in order to predict magnitudes of the various parameters; the high symmetry of the pair system makes such an analysis particularly illuminating. In the simplest approximation the model predicts purely isotropic exchange for nearest pairs of Ir⁴⁺ ions; this is consistent with the experimental observation that the anisotropic parameters are only a few percent of the isotropic parameter. The anisotropy is explicable in terms of obvious refinements of the model, For next nearest ion pairs, the interaction is predicted to be strictly isotropic, as observed. The relative magnitudes of isotropic and anisotropic parameters and of the nearest and next nearest ion pair isotropic parameters, and the dependence of the parameters on bond length are compared with experiment.

Flash evaporated, ferroelectric, barium titanate films have been prepared in the thickness range 0.04-2.0 mu m. They consist of oriental tetragonal crystallites having dimensions comparable with film thickness. Thickness dependent permittivity and other dielectric properties are interpreted in terms of Schottky barriers at the film-electrode interface. Switching is observed in films as thin as 0.08 mu m and loops and current transients are surprisingly similar to those from bulk ceramics. Switchable spontaneous polarization values are as high as 20% of the bulk single crystal value.

A formula is given for the deviation from the small signal value of the gain in the local dc field in the presence of a group of many waves with random phases. For very large fields the gain correction is positive and very small for all frequencies. Numerical calculations for smaller fields in semiconducting cadmium sulphide show that the gain correction changes sign and becomes larger in magnitude for fields below 20 to 30 times the synchronous value and frequencies close to and above the small signal peak gain frequency; below the latter frequency, the low-field gain correction has a positive peak and the peak gain frequency is consequently reduced in the presence of incoherent flux.

A simple model Hamiltonian is proposed for a metal-insulator- metal tunneling junction, which permits the direct calculation of the tunneling current without introducing any effective Hamiltonian. The model rests on the use of localized functions; this procedure avoids the difficult matching problem at the boundary between the barrier and the electrodes. Moreover the use of Kjeldysh's perturbation theory for nonequilibrium system allows an explicit calculation of the current to all orders in the applied bias. It is found that the transfer coefficient appearing in the expression for the current is energy dependent. This model can be systematically extended to include many body effects.

Scott and Jeffries (1962) used a simple form of the orbit-lattice interaction, H_OL= epsilon Sigma _l,m upsilon _l^m, in order to explain relaxation rates in rare earth salts. This simple form is a modification, by averaging the strain epsilon , of the form H_OL= Sigma _l,m upsilon _l^m epsilon _l^m, used by Orbach (1961). If the Scott and Jeffries modification is used to calculate effects involving H_OL in second order one obtains terms which cancel identically. Such complete cancellation should not occur, and does not in calculations using the complete form of H_OL. The origin of the difficulty is traced by calculating the spin-lattice relaxation rate for the Raman process. It is related to the orthogonality of terms involving different normal modes. Some other aspects of this orthogonality are discussed; in particular it is shown that the usual assumption, that one need consider second order matrix elements involving only the lowest excited state, may be misleading. It is shown that all of these orthogonality effects may be included by retaining the more general expression for H_OL used by Orbach, and treating the epsilon _l^m as incoherent. This requires a postponement of the averaging of the strain epsilon until a late stage in the calculation.