Table of contents

The hydration of the silver(I) ion in an acidic 3.7 mol l^-1 solution of silver(I) perchlorate in heavy water has been investigated by neutron diffraction using isotopic substitution. The average Ag-O and Ag-D distances within the primary hydration shell are found to be 2.41(2) and 2.97(4) AA, respectively, which corresponds to a tilt angle of 45(4) degrees . The hydration number as estimated from the peak sizes is 4.1(3).

Calculated volumes of formation for substitutional impurities in alkali halides are shown to be approximately consistent with the empirical rule that for miscible crystals the average volume per ion pair scales linearly between the two single-component extremes (Vegard's rule). The small deviations found from the rule are found to be in good agreement with available X-ray measurements.

The authors have performed a self-consistent numerical calculation to obtain the complete phonon spectrum of the Su-Schrieffer-Heeger Hamiltonian (1979, 1980, 1983), and found a rich structure containing solitary modes, solitary-modulated modes, staggered modes, staggered-modulated modes, surface modes, surface-modulated modes, and ordinary extended modes. The spectrum shows a soft-mode phenomenon around k=0.

An analysis of recent experimental results on image-potential-induced surface states is presented. Plasmon dispersion and single-particle effects in the medium response are included as well as the bound-state nature of the electron creating its own image potential. The formalism is accurate enough to explain well the existing experimental binding energy data. The author predicts a peak width narrower than the experimental one. This should be confirmed with increasing experimental resolution.

The authors investigate the random anisotropy axis model (RAM) with additional cubic anisotropy in mean-field theory in the limit of infinite anisotropy. The model has in zero magnetic field and p dimensions a 2^p-fold degenerate ferromagnetic ground state with a maximum number of possible order parameters. In finite fields the high-temperature phase becomes unstable along a critical line H_c(T). For low fields this line has the same field-temperature dependence as the de Almeida-Thouless line in spin glasses.

A new gauge order parameter is presented for a most general family of spin glasses. The new order parameter is identical to the Edwards-Anderson order parameter in Ising systems, but differs from it for any other model. The relation of the new order parameter to the ideas of sensitivity to boundary conditions and its tendency to order are discussed. Some explicit examples are given.

The NMR spectrum of holmium has been studied, at 1.3K, in ferromagnetic holmium hydroxide (T_c=2.54K). The measured hyperfine parameters are shown to pertain to the ionic ground state, not to a thermal average over ground and excited states. The dipolar splitting is in excellent agreement with crystal-field parameters derived from optical spectroscopy. The ratio of nuclear to electronic antishielding factors for Ho³⁺ derived from the quadrupolar splitting is gamma _N/ gamma _E=152+or-10.

The author derives for the first time a first-principles theory of the Knight shift (K) in solids including the effects of periodic potential, spin-orbit interaction and indirect exchange interactions. It is shown that K can be expressed as a sum of contributions due to itinerant and localised electrons. The importance of the theory in possible applications is discussed.

Two effective dielectric constant theories of composite solids are re-examined in the light of recent experimental work by Kaiser et al. (1985). Correlation effects between fields scattered by particles are discussed. The theories correspond to a single-cluster approximation in disordered alloys. The author concludes that disagreement between theory and experiment can be resolved if one goes beyond the single-site approximation and considers larger clusters. Another previously neglected effect is that of the depolarisation field on the sample surface. He shows results implying the relevance of such an effect by contrasting the plate and sphere geometry.

Energy loss of fully stripped slow point charges in an interacting degenerate electron gas is calculated on the basis of the dielectric theory. The influence of small temperature (T) is examined. It is shown that the correction in the energy loss is proportional with T² in the T to 0 limit.

Chiolite Na₅Al₃F₁₄ exhibits a structural phase transition at T_c=150K; this transition is studied by means of Raman scattering, X-ray diffraction and calorimetric measurements. The experimental data are consistent with a ferrodistortional-ferroic transition, from a tetragonal room-temperature phase P4/mnc (Z=2) to a monoclinic low-temperature phase P2₁/n (Z=2). The Raman spectra show the existence of a soft mode, Raman active in both phases and corresponding to E_g symmetry in the tetragonal phase; it is assigned to a rotational motion of the AlF₆ octahedra within the (Al₃F₁₄)_n^5n- complex layers. The behaviour of this soft mode is interpreted as the result of a coupling with the strain of the same symmetry (pseudo-proper ferroelastic transition).

For pt.I see ibid., vol.18, no.36, p.6555 (1985). A model, based on a Landau development of the free energy, is presented to account for the mechanism of the isotranslation phase transition from P4/mnc space group to P2₁/n, occurring in chiolite Na₅Al₃F₁₄ at T_c=150K. Coupling between the E_g optical soft mode and the strain of the same symmetry is considered (pseudo-proper ferroelastic transition) and expressions are derived for the behaviour of the soft-mode frequencies and for that of the elastic constants c₄₄ and c₅₅. The model is consistent with the experimental results obtained for the optical soft-mode (in paper I); furthermore, there is evidence of a changeover in the transition regime at T₀ approximately=120K. It is interpreted as the result of a predominant contribution in the order parameter due to the optical mode below T₀, whereas in the vicinity of T_c, the predominant contribution is due to the strain.

Certain types of real-space renormalisation groups (which essentially approximate Bravais lattices through hierarchical ones) do not preserve standard thermodynamic convexity properties. The author points out that this serious defect is not intrinsic to any real-space renormalisation. It can be avoided if form invariance (under uniform translation of the energy scale) of the equation connecting the Bravais lattice (which he intends to study) to the hierarchical one (which approximates it) is demanded. In addition to this he analyses expressions for the critical exponents nu and alpha corresponding to hierarchical lattices; these are consistent with recent analysis by Melrose (1983) of the fractal intrinsic dimensionality, which he considers to be correct in order for hyperscaling to hold for hierarchical lattices.

Electron spin resonance (ESR) and DC conductivity measurements show that the free and bound polarons model can be applied to WO₃-doped V₂O₅. At low temperature, small polarons are trapped in the electrostatic field due to the W⁶⁺ impurity and conductivity only arises from the long-range hopping of bound polarons between localised defects. At high temperature, the small polaron is freed from its trap and a thermally activated hopping along vanadium ions is observed. ESR experiments show, however, that two bound polaron states occur: the first one, at very low temperature, when the electron is trapped on a single vanadium site adjacent to the W⁶⁺ ion; the second one, at higher temperature, when the unpaired electron still remains in the electrostatic field of the W⁶⁺ ion, but is allowed to jump among several non-equivalent sites surrounding the impurity.

The authors present a model for the electronic structure of a mixed-valent 'f impurity' in semiconducting rare-earth compounds. The host semiconductor 'spd' states are treated in a realistic band model whereas two ionic configurations 4fⁿ and 4f^n-1 in their fundamental multiplets are considered: 4fⁿ is singlet and 4f^n-1 is (2J+1)-fold degenerate. These two configurations are mixed through f-d hybridisation V_fd which they treat in a variational method. Especially, they study local valence instabilities by considering the competition between the promotion energy Delta of an f electron into the d-band states and the available band energy due to the interaction U_df between the f hole and the d-band electrons. Their results essentially show an intermediate valence for reasonable values of Delta , U_df and V_fd.

Electron spin resonance (ESR) studies of the iron Fe³⁺ photo-excitation and photo-quenching processes are presented. A theoretical fit to the data enables the authors to estimate the position of the Fe²⁺ energy level within the ZnS forbidden gap to be approximately 2.1 eV below the edge of the conduction band (CB). The magnitudes of the two complementary transitions: valence band (VB) to iron to CB are determined and discussed within the framework of recent theoretical approach to the photo-ionisation processes of transition metal impurities. The ratio sigma _OV^max>or=10* sigma _OC^max of the maximum cross sections is found with sigma _OV^max estimated to be 8+or-1*10^-18 cm².

The weak-localisation correction to the Boltzmann conductivity is discussed for a two-dimensional system emerging from the electric quantum limit and eventually behaving in a three-dimensional manner. The form of the correction for a system with arbitrary subband wavefunctions is found when two sub-bands with well separated energy minima are occupied. This produces a correction similar to the two-dimensional result. A criterion for the breakdown of this regime is derived. Within the framework of an idealised model the weak localisation is calculated across the whole two- to three-dimensional range. The calculation shows how the form of the correction changes from ln (L/l) in two dimensions to (1/l-1/L) in three dimensions.

A Boltzmann equation calculation of the thermopower in narrow-channel systems that exhibit sub-band structure predicts sharp peaks in and changes in sign of this quantity. This is due to large changes in the density of states of the system at energies corresponding to sub-band minima. Structure in the electrical conductivity due to the same property of these systems is found to be significantly smoothed by lifetime-broadening effects such that it could be rendered almost experimentally unobservable. The authors calculate the effects of lifetime broadening on the structure in the thermopower. Since this structure is more marked than in the electrical conductivity the lifetime broadening masks it less easily. Significant structure is predicted to remain for systems that have a k_Fl value of three in the electric quantum limit.

The theory of free-carrier absorption is developed for quasi-one-dimensional semiconducting structures (ultra-thin wires) for the case where the carriers are scattered by acoustic phonons via the deformation potential coupling and the radiation field is polarised along the length of the wire. The absorption is found to be an oscillatory function of the area of the cross section of the wire and is enhanced over its bulk value in going to wires of decreasing cross section.

The authors calculate the first quantum correction to the ground state energy and to the spin wave spectrum for a Heisenberg antiferromagnet in a magnetic field. A consistent treatment of quantum corrections, obtained through an expansion in 1/S, is required to obtain a self-energy that satisfies the Goldstone theorem. They find that in the Heisenberg antiferromagnetic linear chain the two-sublattice ground state becomes unstable for any S because of the quantum fluctuations in presence of an applied magnetic field. This finding is in accord with rigorous results for S=1/2.

A continuum interface model is constructed to study the low-temperature properties of an Ising domain wall in a weak random pinning potential, which preserves the symmetry of the order parameter locally. The statistical width of the domain wall and its surface tension are computed by three methods: simple energy accounting, dimensional arguments, and approximate renormalisation-group calculations. All three methods yield a positive interface tension for d>1 as for pure thermal disorder. Quenched randomness, however, roughens the interface for d<or=5. For the model under consideration, the author does not find a depinning transition or a qualitative change of the roughening at non-zero temperatures, respectively, in contrast to other authors. Depinning can however be achieved by external perturbations such as electric fields or external stresses.

The relaxation rates Gamma ₁ and Gamma ₂ of hydrogen in amorphous Ni₃₅Ti₆₅ have been observed in the temperature range from 100K to 450K at the external fields 0.2 T, 0.8 T and 2.1 T. The temperature and field dependences of the Gamma ₁ relaxation rate indicates that the diffusion of hydrogen cannot be described by one thermally activated process but with a gaussian distribution of activation energies with a maximum at Q=0.35 eV, and half-width Delta Q₁2/=0.16 eV. The pre-exponential factor tau ₀=7*10^-13 s was found to be of the same order as for crystalline materials in the adiabatic limit.

A response function describing processes with three electromagnetic waves (polaritons) in crystals has been defined. This function has been calculated by a specific method for the case of polariton Fermi resonance, i.e. when one of the polaritons interacts with the two-phonon states of a non-degenerate polar vibration. Some features of the three-polariton interaction in the two-phonon band have been investigated, like modulation and broadening of the spectrum, anti-resonance and wave-vector mismatch. The information obtained from nonlinear optical spectra about polariton and two-phonon states has been analysed.

The electronic structures of scandium nitride layers have been investigated by X-ray and ultraviolet photoelectron spectroscopies. The spectrum of stoichiometric ScN in the valence region is found to be consistent with the results of band-structure calculations, except for the width of the forbidden band of which evidence is only obtained by the experiments. Attempts to synthesise a non-stoichiometric ScN_x compound failed. When layers deficient in nitrogen were grown, analysis of the electron core levels demonstrated that they were composed of mixtures of metallic scandium and stoichiometric ScN. This result is discussed in parallel with the titanium nitride case in terms of a vacancy stabilisation mechanism for transition-metal nitrides emphasising the role of the metal-metal interactions that form around a nitrogen vacancy.

Observation of frequency crossings as sharp minima in magnetothermal resistivity provides spectroscopic information about paramagnetic centres in crystals. The authors present calculations of the angular dependence of the crossings between the six transition frequencies of S=3/2 and ²E centres in trigonal sites.

Measurements have been made of frequency crossing signals in the magnetothermal resistivity of Al₂O₃. These occur when the frequencies of two resonant scattering centres become equal and they provide spectroscopic information on those impurity or defect centres present which interact with phonons. Nine samples were examined after various treatments and the angular dependences of the signals are compared with those for S=1, S=3/2 and ²E centres and with specific centres with S>3/2. A number of tentative assignments have been made.