Table of contents

A criterion is established for localisation of a Boltzmann electron gas implying ranges of stable and unstable states with respect to thermal excitation results as a consequence of two competitive effects: the localising influences of electron self-interaction and of delocalisation due to the sufficient thermal excitation.

The problem of exciton effects on soft X-ray absorption edges in metals is examined within the integral equation formalism. A new contribution to the absorption edge enhancement for first-class transitions is predicted.

The existence of transverse electric fields on account of a multivalued redistribution of electrons between valleys situated symmetrically to a heating electric field is investigated for n-Si at liquid neon temperature. The transverse voltage appears when the heating field exceeds a certain value of about 8 kV m^-1, at least in a part of the sample, and saturates for higher fields applied. It can be switched by magnetic fields of only some 10^-3 T, characteristic for a domain formation in the sample with respect to the transverse field.

A simple model which illustrates the frustration effects found in amorphous Ising antiferromagnets is introduced. It is shown that the phase diagram of this model may be obtained from the phase diagram of a suitably constructed spin glass reference model, and the implications of this are discussed. The phase diagram of a model with an annealed distribution of even and odd membered rings of spins is explicitly obtained. The difficulties introduced by a non-zero external magnetic field are briefly discussed.

The Sherrington-Kirkpatrick model, generalised to quantum spins, is discussed within an exact replica formalism and reduced to a (time-dependent) single-site problem. The existence of a phase transition is established for all values of the spin S.

The activation volume for the RIV relaxation, that is for the reorientation of a dimer, has been determined for calcium fluoride crystals doped with europium, gadolinium, terbium or dysprosium. The results range from 1.05+or-0.15 cm³ mol^-1 for europium to 0.3+or-0.15 cm³ mol^-1 for dysprosium. These values are appreciably less than the activation volumes for the motion of bound interstitials or bound or free vacancies. In contrast, the activation enthalpies for the reorientation of the dimer are rather greater than those for interstitials or vacancies. These results, together with the results of a computer simulation, show that a 2-2-2 cluster cannot be responsible for the relaxation. It is pointed out that a gettered 2-2-2 cluster cannot be ruled out and, in fact, contains several of the characteristics which are consistent with the experimental results of the present work.

It is now known that noise with an approximately 1/f power spectrum is found in all types of electrical conductor (and possibly in non-electrical systems). Various aspects of the phenomenon, and theories which have been proposed from time to time, are reviewed in the light of published experimental evidence.

Formation and annihilation of Li colloids and hydrogen bubbles in LiH have been studied with optical absorption ESR and thermoluminescence. The optical absorption measurements of irradiated LiH gave S- and colloid bands as well as a broad band. The theoretical calculation using Mie theory shows that the observed S- and colloid bands can be ascribed to large and small Li colloids with radii of 220 and 50 AA, respectively. Extended calculation of light scattering by bubbles or voids indicates that the observed broad band is due to hydrogen bubbles with radii less than 600 AA. The colloids and bubbles are annealed out above 80 degrees C; accompanying thermoluminescence follows second-order kinetics. The possible models of the annihilation of colloids and bubbles in LiH are presented.

Inelastic neutron scattering measurements on a single crystal of NaNO₃ have yielded the temperature behaviour of the lattice vibrations propagating along the threefold axis. Frequency, shifts and anomalous linewidths are observed near the order-disorder transition temperature (T_c=276C). In general, the observed behaviour is similar to that of the ammonium halide lattice modes.

For pt.I see ibid., vol.13, p.1635 (1980). The use of the recursion method is generalised to mixed diatomic crystals of the type AB_xC_1-x, in the framework of the 'augmented space' introduced by Mookerjee (1973). From the form of the first coefficients of the continued fractions one obtains very exact criteria for predicting whether the mixed crystal is of one- or two-mode type. Phonon spectral densities are calculated for two typical cases: ZnS_xSe_1-x and ZnTe_xSe_1-x.

The local polaron problem in which a local electronic resonance in a wide conduction band is coupled to a local phonon mode is considered. Earlier work is extended by a more complete treatment of the case where a finite Fermi level is present which is important for intermediate valence materials. The authors demonstrate by variational and perturbational methods that there can be a renormalisation of the level width by a factor exp(-n), where n is the number of phonons in the polaron cloud. They discuss the conditions for this renormalisation to occur and in particular establish the importance of the closeness of the resonance to the Fermi level. The physical basis for the conditions and the relationship to the problem where only a single electron is present, where renormalisation only occurs if the level lies at the bottom of the band, are brought out. Renormalisation due to electron-hole pairs is also discussed. The authors suggest that in a lattice of resonances, renormalisation should lead to suppression of intersite scattering. The implications for intermediate valence systems are discussed.

The Landau-Ginsburg free-energy functional (effective Hamiltonian) is derived for compressible Ising systems where the Ising spins are coupled with quantum phonon fields and a transverse field is present. In the zero transverse field case, the phonon dynamical modes also disappear from the effective Hamiltonian, which then compares with those studied by Sak and De Moura et al. in the non-quantum compressible Ising systems. In the presence of dynamics in the spin system (compressible transverse Ising system), the phonon modes get effectively coupled in the Hamiltonian and while at non-zero temperatures this does not affect the critical behaviour (which remains the same as in the non-quantum case), the presence of these dynamical modes effectively increases the lattice dimensionality by unity at zero-temperature transitions (T_c=0) where one observes the critical behaviour corresponding to a d+1 dimensional non-quantum compressible Ising model. The critical behaviour of a cooperative Jahn-Teller phase transition when T_c to 0 is also discussed.

The critical behaviour of a dilute Ising spin system on a general anisotropic elastic solid is studied using the replica trick and epsilon expansion up to first order. The dilute Ising 'Khmel'nitzkii' epsilon ^1/2 fixed point is shown to be stable (for epsilon <<1) with respect to isotropic as well as anisotropic compressibility.

The angular and energy dependence of the dynamical diffraction of gamma rays was studied experimentally in the symmetric Bragg case. Mossbauer diffraction spectra were measured with high angular resolution in pure nuclear Bragg reflection of a nearly perfect ⁵⁷FeBO₃ single crystal. In the spectrum taken at the exact Bragg position, very broad hyperfine resonance peaks were observed. In the spectra just above and below the Bragg angle the reflection peaks were much smaller and were shifted to energy positions above and below resonance respectively. The results were interpreted by means of the dynamical theory and compared with computer calculations. The measurements represent the first direct observation of the enhancement of nuclear resonance scattering in the case of a thick crystal reflection. Furthermore the angular dependence of the resonance scattering by a perfect single crystal has been measured for the first time.

Raman scattering from phonons, magnons and low lying electronic excitations of Co²⁺ in CsCoBr₃ have been measured between 8 and 100K. Above T_N¹=28.3K, CsCoBr₃ is a one-dimensional antiferromagnet and for this phase eight of the eleven possible single-ion transitions within the ⁴T_1g(⁴F) ground term have been assigned. The observed energies can be accurately fitted using a 3d⁷ crystal-field model which includes the intrachain exchange within the molecular field approximation. Scattering from one-magnon and two-magnon bands has been identified. Below T_N¹ the main spin wave peak develops structure associated with the different modes of Ising chains coupled in a ferrimagnetic triangular lattice. Near T_N¹, the temperature dependence of the scattering intensity associated with the E_1g phonon and all excitons is dominated by a spin-dependent mechanism involving the interchain pair polarisability. Analogous effects are predicted to occur for compounds having a similar structure.

The Raman spectra of single crystals of Nd(OH)₃, Gd(OH)₃, Tb(OH)₃ and of Y(OH)₃ have been investigated at helium temperatures and partially in external magnetic fields B<or=12 T. The Raman active optical frequencies have been assigned and the frequencies of the k=0 modes determined and correlated with the modes in the isomorphous rare-earth trichlorides. The interaction between the optical phonons and the 4f electrons becomes apparent in the magnetic phonon splitting in the zero-field frequency shift and, in favourable circumstances, in a phonon-electron anticrossing which depends on the magnetic field. In Tb(OH)₃ all these effects can be observed simultaneously. In the same crystal the magnetic phonon splitting occurs spontaneously below its ferromagnetic Curie temperature. All effects can be treated theoretically using a Jahn-Teller type interaction Hamiltonian bilinear in the electron and phonon variables. The numerical values of the coupling constants have been found to agree for the different interaction effects. There are indications of a systematic variation of these numbers with the type of rare-earth ion and the phonon mode considered. The contribution of optical phonons to the effect of interionic energy transfer in Tb(OH)₃ by virtual phonon exchange is considered and compared with the aforementioned results.

The energy transfer between single Cr³⁺ ions and N₂ pairs in ruby is interpreted by energy diffusion on single ions. It is shown that the superexchange is responsible for this diffusion. The probability of transfer between single ions and the diffusion coefficient are evaluated. Experimental results on the transfers from fourth next-neighbour pairs to single ions and to third next-neighbour pairs are given. It appears that the diffusion is weak on the fourth NN pairs. The temperature dependence of the transfer probabilities are determined and interpreted.

The general theory of nuclear magnetic relaxation in homogeneous adsorbed systems is developed in a way which takes account of the specific features of two-dimensional motion. It is shown that the reduced dimensionality has the effect of decreasing the relaxation times from their naively expected values by as much as a few orders of magnitude, and of causing T₁ and T₂ to be unequal even in the presence of fast atomic motion. The theory is in close agreement with observed relaxation times of adsorbed helium-3.