Table of contents

The dimensionality crossover in selective site percolation on a triangular lattice is discussed in terms of a renormalisation group. A preferred cell to preferred site transformation keeping the original lattice symmetry gives the exact critical line with the flow direction expected from the universality hypothesis. In the chain limits, the critical exponents of both the correlation length and the dimensionality crossover are reproduced exactly for any scaling factor.

In a recent paper Powell et al. (1982) compare CS₂ lattice frequencies measured at 150K with lattice dynamical model calculations appropriate to 0K and 95K. Here it is shown that the comparison is greatly improved by using 150K lattice parameters in the calculations.

The mutual friction parameters B and B' are calculated near the lambda point. They can be expressed in terms of the complex kinetic coefficient Gamma associated with the order parameter and diverge as (T_lambda -T)^-13/. Therefore the critical behaviour of Gamma can be known from data of B and B'. The helical mode of a single vortex is also investigated near the lambda point.

The Valenta model (1957) of a thin ferromagnetic film in the critical region near the Curie temperature is considered. It is shown how to determine the spatial distribution of spin correlation time when the spatial distribution of magnetic susceptibility is known. It is also shown how to determine spatial distributions of magnetic susceptibility and the mean exchange integral when the spatial distribution of spin correlation time is known.

The Valenta model (1957) of a thin ferromagnetic film in the critical region where the decay of fluctuations in magnetisation is described by a diffusion and damping process is considered. For the diffusion coefficient of fluctuations the self-consistent procedure is applied, which allows the authors to obtain the spin-lattice relaxation time. Critical indices of spin-lattice relaxation time, the broadening of the Mossbauer line, and diffusion and damping coefficients of fluctuations are obtained. Some general conclusions about the behaviour of all critical indices in different models of thin films are also given.

The spin dynamics of a quasi-one-dimensional ferromagnetic Ising system (CH₃)₃NH.CoCl₃.2H₂O (T_N=4.135K) is studied by measuring the proton spin-lattice relaxation rate at 3-300K in externally applied fields of 2.5-33 kOe parallel to the Ising axis. The overall behaviour of the temperature and field dependence including a rate maximum at a temperature well above T_N is found to be governed by the q to O and omega to 0 component of the susceptibility.

The Auger linewidths for the L_IL_II,IIIV processes in Al, Mg and Na have been calculated by a Green function method, and found to be in good agreement with the experimental evidence. Surface effects are analysed and shown to be negligible.

The properties of electron-irradiation-induced defects in VSe₂ are studied by electronic transport, magnetic susceptibility and lattice parameter measurements. A well defined magnetic defect exists in the irradiated samples and it is identified as a V interstitial between the VSe₂ sandwiches in analogy with the previous interpretations on the nonstoichiometric V_1+xSe₂. Lattice parameter measurement suggest a strong volume relaxation due to vacancies in V layers. Transport properties show a deviation from Matthiessen's rule which seems to stem from the interaction of the defects with the charge-density waves. The results are compared with observations made on other metallic layered dichalcogenides in which the irradiation defects can be considered similar with respect to local crystallography. It turns out that the effects of the defects are strongest in the material with the strongest inherent charge-density waves. Substitutional disorder often has more subtle effects: only substitution by group VIIIB metals is comparable with the irradiation-induced defects.

The adiabatic elastic constants C₁₁, C₁₂ and C₄₄ of ⁷LiH and ⁷LiD have been obtained from measurements of ultrasonic wave transit times through single crystals over the range 4.2-300K. The temperature variation follows quite closely the approximation C_ij(T)=C_ij(0)(1-D epsilon ). The anisotropy factor A=C₄₄/(C₁₁-C₁₂) and the deviation ( Delta =C₁₂-C₄₄) from the Cauchy relation have been used to make comparisons with the alkali halides. It has been found that the Gruneisen parameter gamma is approximately constant at 1.2 for both ⁷LiH and ⁷LiD from 50 to 270K. There are signs of anomalous behaviour of the elastic constants in the region of 11K, where a lambda -type heat capacity anomaly has been reported.

The velocity autocorrelation function in a simple molten salt is expressed in terms of both the longitudinal and transverse components of the momentum density fluctuations. Charge-ordering effects tend to suppress fluctuations in the total number density with the result that there is no significant coupling (via the longitudinal current component) to well defined collective density excitations. Hence, unlike the one-component plasma where the mass and charge density fluctuations coincide, the velocity autocorrelation function is not oscillatory, and gives results in quite good agreement with computer simulation data. The authors extend the method to investigate momentum transfer, and by taking charge ordering into account are able to discuss the charge current autocorrelation function. A simple physical explanation is given for the deviations from the Nernst-Einstein relation between the self-diffusion coefficients and the conductivity.

For polar or covaLent semiconductors, it is shown that there is an exact relationship between the equivalent-orbital (or sp³ hybrid) model of Leman (1962) and Weaire and Thorpe (1971) and the bond-orbital model. This provides a way of considering ordered or topologically disordered materials with both metallic and covalent character in terms of bonds. The connection with the transformations of Weaire and Thorpe is discussed with regard to an example, the Bethe lattice.

Quasi-self-consistent relativistic energy band structures are reported for the stoichiometric actinide monocarbides (ThC, PaC, UC and NpC). Three computational procedures used in this work (muffin-tin partitioning of space, the X_alpha approximation and self consistency) are examined critically for PaC. The computed band structures, although superficially similar, show marked differences-the major trends across the series are the dramatic increase in f-electron participation in the bonding, the change in character of the Fermi energy from d to f at PaC and the increasing localisation and concomitant narrowing of the f-electron bands. It is concluded that at ThC 5f electron participation in the bonding is minimal, but that this contribution is important for PaC and the later members of the actinides.

Numerical investigations are presented for the vibronic coupling of a degenerate E electronic state to a degenerate e_g vibrational mode in the case when both the linear and quadratic Jahn-Teller couplings are included. To understand the effects of the quadratic Jahn-Teller coupling, the calculations of eigenvalues and Ham's reduction factors are made for various values of the linear and quadratic terms. The Renner effect on the system has been investigated and new classification of eigenstates is proposed. The same calculations have been extended for vibronic system of a degenerate T electronic state coupled to a degenerate e_g vibrational mode.

The author has investigated percolation properties of unfrustrated plaquettes in the two-dimensional Ising model with nearest-neighbour bonds +or-J distributed at random. The effects of plaquette correlations are considered using a two-parameter position-space renormalisation group (PSRG) approach, and it is found (within the framework of this approximate PSRG) that this correlated site percolation problem is in the same universality class as random site percolation. The percolation threshold is then found from large-cell PSRG methods to be indistinguishable from that for randomly placed sites.

The nature of those S^z=0 excitations of an anisotropic Heisenberg chain that have been found to be typical in the low-energy excited states is considered. A simple estimation is used to take into account the effect of complex phase shifts on the long-distance decay properties of the wavefunction. It is established that pairs of complex wavenumbers that do not form two-strings correspond to pairs of free spins, while quartets described two independent pairs of bound spins.

For pt.II see ibid., vol.15, p.999 (1982). An introduction to the experimental and theoretical research of spin dynamics in the critical region of a 3D (weakly anisotropic) Heisenberg ferromagnet is presented. Nuclear spin lattice relaxation times (T₁) which have been measured in Rb₂CuBr.2H₂ORb₂CuBr₄.2H₂ for T>T_c, B=0, are described with a phenomenological model for the spin correlation functions. The temperature dependence as well as the absolute values of T₁ are described very well by this (hydrodynamic) model. The small anisotropy in the exchange interaction is found to determine strongly the dynamic behaviour near T_c.

For pt.III see ibid., vol.15, p.6403 (1982). Experimental results of nuclear longitudinal and transverse relaxation rates (T₁^-1 and T₂^-1 respectively) of ¹H and ⁷⁹Br(I) in Rb₂CuBr₄.2H₂O in the critical region are presented. This paper is the second of two about nuclear relaxation rates in the critical region of this 3D (weakly anisotropic) Heisenberg ferromagnet. The first treats the relaxation experiments in the paramagnetic region T>T_c, B=0. This paper deals with the T<T_c, B=0 ferromagnetic state and the paramagnetic region T=T_c, B>0. The discussed hydrodynamic spin-wave theory with a 'non-Ornstein-Zernike' expression for chi _q/sup /// gives for both regions a very good description of the temperature and field dependences of the relaxation rates. The small anisotropy in the exchange interaction is found to determine strongly the dynamic behaviour near the transition point.

For pt.IV see ibid., vol.15, p.6411 (1982). An extension of the Resibois-De Leener model (1966) for spin correlation functions which includes the influence of external magnetic fields and finite temperatures is introduced. It is suggested here that, apart from the exchange constants J_ij, the important parameter in this description is still the magnetisation rho (B, T). With this model, using the earlier experimentally determined spin-diffusion constant, a satisfactory description is obtained for experimental data on spectral densities of the spin autocorrelation functions in the paramagnetic state far from any spontaneous ordering effects. At low temperatures (large values of p) a satisfactory description of the experimental spectral densities of parallel autocorrelation functions is found within the spin-wave model of Vaks and co-workers (1968). Furthermore, the origin for the transverse nuclear spin relaxation time T₂ of ¹H in Rb₂CuBr₄.2H₂O is also discussed. At low temperatures/high fields the dipolar interaction between like proton spins in the dominating interaction determining T₂(¹H). At high temperatures/low fields the influence of electron spin fluctuations is seen on the transverse relaxation times as well as the nuclear dipolar interaction.

Results of magnetic measurements carried out on single crystals of TmFe_1-xMn_xO₃ (x=0, 0.12, 0.21 and 0.33) at temperatures in the region 2-700K and neutron diffraction investigations performed on powder samples at temperatures 80-650K are presented. The compounds are weak ferromagnets at room temperature. For compounds with a lower amount of manganese (x=0.12 and x=0.21) the spin reorientation phase transition G_xF_z-G_zF_x is observed as was already reported for pure TmFeO₃(x=0). For the compound with x=0.33 a phase transitions sequence unusual for the pure orthoferrites is established. On the temperature dependence of the spontaneous magnetisation along the crystal a axis a compensation point was observed in the case of all compounds investigated. A qualitative explanation of the experimental results is given on the basis of recent theoretical considerations about the influence of Mn³⁺ doping ions on rare earth orthoferrite properties.

Water and oxygen-free Mn_1-nZn_nF₂ and Mn_1-nCo_nF₂ have been prepared by a simplified method through the whole concentration range. Magnetic susceptibility measurements of the powdered samples have been used to determine the Neel temperatures in association with measurements of sublattice magnetisation using neutron diffraction.

The energy dependence of the total yield resulting from the incidence of low-energy electrons in the energy range of 0 to 30 eV above the vacuum level on various graphite surfaces: (A) the basal plane of single-crystal graphite, (B) the basal plane of highly oriented pyrolytic graphite and (C) polycrystalline graphite, is presented. The authors have also explored (D) vapour-deposited carbon film on polycrystalline graphite surfaces. The spectra are discussed partly in terms of the onset of inelastic electron-electron scattering accompanied by interband transitions and partly by excitation of collective modes. A peak at 14.2 eV is ascribed to the Q_1u⁺ to Q_2g^- transition while features at 26 and 21.2 eV have been ascribed to the excitation of volume plasmons and surface plasmons, respectively.

For a molecule placed near the surface of a polarisable sphere, the exact solutions of the Maxwell equations are given for both elastic and inelastic scatterings, which refine in several ways the electrostatic results obtained so far. Based on the solutions, the criterion for the occurrence of enhanced Raman scattering is presented and the energy balance and the image dipole effect are discussed. Also, the enhancement factors are calculated for a metallic sphere of Ag, Au and Cu using experimental data of the dielectric constants and for a model dielectric sphere with a positive dielectric constant. Among other things, the possibility of the enhanced Raman scattering due to the retardation effect involved in a dielectric sphere is emphasised.

The growth mode and electronic structure of palladium adsorbed on the (100) and (111) surfaces of silver at room temperature have been studied using LEED, Auger electron spectroscopy, angle-resolved photoemission and work function measurements. For both surfaces and overlayer grows in a layer-by-layer mode for approximately three monolayers after which it begins to relax to the bulk palladium structure. At submonolayer coverages a p(1*1) lattice gas occurs on the (100) surface whereas flat monolayer platelets are formed immediately on the (111) surface. Electron energy-parallel momentum (E-k/sub ///) dispersion relations were measured for palladium monolayers supported on both surfaces. For the monolayer. The difference in position and width of the resonant bound palladium 4d state on an isolated palladium atom supported on the (100) surface in comparison with an impurity atom in the bulk is consistent with the change in coordination number.