Table of contents

The authors have investigated by neutron spectroscopy the effect of tensile elastic stresses on H diffusion in V (T=297 K, (H)/(V)=0.015 ( alpha -phase); sigma _max=6.1*10⁷ Pa). They could establish no stress-induced diffusional changes and, in particular, no superdiffusion. Their results make significant stress-induced variations of the interstitial H sites unlikely since this should also noticeably influence H diffusion.

The evolution of the specific heat jump at the magnetic-field-induced metal-spin-density-wave transition in quasi-1D conductors is shown to exhibit an oscillatory behaviour with the magnetic field with a discontinuity at the transitions between two spin-density-wave subphases. Whereas the specific heat jump is smaller than the BCS value in low fields, it becomes larger than this value in high field, and then equal to it in infinite field. The results obtained are in qualitative agreement with experiments on Bechgaard salts.

The effects of localisation and Coulomb interaction on electrons in disordered systems are considered. The final results show that the non-linear sigma model for localisation is no longer valid if one introduces the Coulomb effect at the same time. These two effects lead to a new type of model.

The authors investigate the validity of the constant photoconductivity method for obtaining the sub-band-gap absorption coefficient, and hence the density of states, in amorphous semiconductors. They find that in hydrogenated amorphous silicon the sub-band-gap photoconductivity is dominated by transitions from the valence band tail to the conduction band, while the density of recombination centres remains constant in conditions of constant photocurrent, the photoconductivity depends on a capture cross section which may show energy (and thus wavelength) dependence. The behaviour of the photoconductivity under varying illumination intensity is also determined.

In the Y-Ba-Cu-O system, oxides with nominal cation composition (Y_1-xBa_x)₂CuO₄ and x ranging from 0.2 to 0.5 have been prepared and examined for the occurrence of superconductivity. Four-probe DC resistivity and AC susceptibility measurements reveal that all these compositions exhibit superconductivity with superconducting onset temperatures in the range 90-95 K. However, the transition width and the temperature at which zero resistance is observed depend on x or the Ba/Y ratio.

The authors report the results of resistance measurements for Ba_xY_yCu₅O_3(3-z) for x=2.9, y=2.2; x=3.0, y=2.0 and x=3.1, y=1.8. The x=3.0, y=2.0 sample shows a superconducting transition at 107.6 K with a width of less than 2 K and the resistance is zero at 94.8 K. The sample shows zero resistance at liquid nitrogen temperature under 1 A measurement current. The main phase of this system is a hexagonal ABO₃ structure.

The authors propose a new fractal lattice-a multi-layer Sierpinski gasket with infinite ramification. The critical behaviour of an Ising model of this is investigated. In particular, the crossover behaviour is shown in a renormalisation group flow diagram.

The temperature dependence of the principal refractive indices of quartz is investigated at temperatures between 300 and 1000 K, with special emphasis on their behaviour at the alpha - beta transition. In the alpha -phase the temperature variation is analysed within the framework of the Landau theory of first-order phase transitions. The existence of pinned states in the incommensurate regime is evident from hysteresis studies near the alpha - beta transition.

The migration of point defects is usually treated using Vineyard's theory, but recent work has shown that this is inadequate for some important materials. The authors discuss techniques based on molecular dynamics simulation which do not rely on the approximations of this theory. They have made calculations for a rigid-ion model of cobalt oxide that demonstrate the practical effectiveness of the molecular dynamics techniques. Their results indicate that for this particular model the Vineyard theory is qualitatively reliable.

It is shown that electronic-structure calculations by multiple-scattering theory are not restricted to the case of muffin-tin potentials but can be extended to the general case. The exact nature of the multiple-scattering equations is shown and their convergence behaviour with respect to the angular momentum cut-off is discussed. The numerical solutions are as easy to obtain (as for muffin-tin potentials) apart from the single-site scattering problem which is computationally more difficult.

A numerical investigation of collisions of kink-anti-kink (KK) pairs in an easy-plane classical antiferromagnetic chain is presented here, for the case of an applied field within the easy plane. An approximate kink profile as obtained from an earlier ansatz was used as the initial condition for numerical integration on a discrete lattice. As a function of the applied magnetic field and kink ansatz parameter theta _A, which measures the tilt of the spins out of the easy plane, the authors have found distinct parameter regimes resulting in transmission, reflection, and annihilation of the KK pair. Results for both the in-plane (XY) and out-of-plane (YZ) kinks are summarised, and comparison is made to KK scattering in the easy-plane ferromagnet. Similarly to the case of the easy-plane ferromagnetic CsNiF₃, it is found that these results imply difficulties in the interpretation of experiments on tetramethylammonium manganese trichloride (TMMC) in terms of classical soliton theory. For example, for fields from approximately 20 to 80 kG, the low-velocity XY collisions generally result in pair annihilation to spin waves. For fields less than 20 kG, however, the low-velocity KK XY pair transmit. Experimental results for TMMC are discussed in the light of these features.

A 'non-relativistic' Kondo model is presented which is cut off from the outset but includes an interaction between the s-wave electrons. The model is justified in detail. The model Hamiltonian is exactly diagonalised by Bethe ansatz techniques, and the ground state at given down-spin number is found. Since the interacting bulk system also allows for an exact treatment, the impurity magnetisation is obtained by subtraction. The corresponding integral equations are derived and analyzed in the limit of large Fermi momentum. The impurity versus bulk magnetisation exhibits typical Kondo behaviour in this limit.

The new hydrated mixed-valence fluoride Fe^IIFe₂^IIIF₈(H₂O)₂ presents an 'idle-spin behaviour' attributed to a topological frustration: the magnetic ordering of the Fe^III sublattice occurs at T_N=157 K whereas the Fe^II spins only order below 36 K. In this work, the magnetic properties of polycrystalline samples in the 1.6-300 K temperature range are presented. From magnetic susceptibility experiments, the Curie constant is found, above 200 K, to be smaller than the free-ions value. Under high continuous magnetic fields up to 200 kOe, characteristics are observed: in the 1.6-24 K temperature range a very abrupt transition attributed to the Fe^II-spin reorientation is found; it is characterised by a large-field hysteresis. To explain this magnetic behaviour, a magnetic structure is proposed and exchange integrals are evaluated. All this work confirms the importance of the role of the frustration.

The authors analyse the critical behaviour of two simple compressible Ising models in terms of the effective spin Hamiltonians. By considering separately the effect of pure volume and pure ion position fluctuations they show that a long-range pair-pair interaction term, -J₄( Sigma _(ij) sigma _i sigma _j)², is obtained in either case, with corresponding coefficients of the opposite sign. The radically different critical behavior in the two cases (J₄>0-first transition, J₄<0-second-order transition) indicates the opposite role played by each kind of fluctuation. In a real system both kinds of fluctuations are present and the two effects will compete. The authors also show that different effective spin Hamiltonians may be arrived at according to the choice of boundary conditions or ensemble. They use exact as well as renormalisation group calculations to show that different types of critical behaviour may be predicted.

The authors present a spin wave calculation of the memory function for the classical anisotropic one-dimensional antiferromagnetic Heisenberg model. Their calculation is made for all wave-vectors at low temperature. Their theory is used to analyse experimental data for neutron scattering in (CD₃)₄NMnCl₃ (TMMC).

Optically detected ESR and ENDOR experiments in p-type GaAs under photoexcitation show that an arsenic-anti-site-As-interstitial complex has energy levels E_v+0.52 eV (As_Ga⁺2+/-As_I) and E_v+0.74 eV (As_Ga⁰+/-As_I). From the occupied As_Ga⁰+/-As_I level an absorption band starts peaking at 1.2 eV. From the investigation of the photoquenching behaviour and metastability of the As_Ga⁺-As_I pair is concluded that the diamagnetic As_Ga-As_I complex can be associated with EL2.

The introduction of a small amount of orthorhombic anisotropy into a uniaxial antiferromagnet results in an extension of the region of coexistence of spin-flopped and antiferromagnetic phases along a hyperbola in the hard-easy plane. Mean-field theory calculations on the orthorhombic system show that the thickness of the extension decreases very rapidly beyond the edge of the uniaxial spin-flop asteroid unless the orthorhombic energy is large. It is also found that for small values of the orthorhombic anisotropy energy three different phases (one antiferromagnetic and two spin flopped) may coexist for certain applied fields.

Magnon and phonon Raman scattering is studied in the canted ferromagnet Rb₂CrCl₄. The phonon spectra obtained at about 8 and 200 K are discussed on the basis of the exact crystal structure (D_2h¹⁸) and of the appropriate quadratic one (D_4h⁵) previously proposed for the isostructural crystal K₂CuF₄. Among the twelve modes predicted in D_4h⁵ symmetry, only one (belonging to Gamma ₄⁺) is missing. The most important feature of this work is the first direct observation of the optic gap predicted by the theory at about 170 cm^-1 and the study of its magnetic field dependence for B along the easy direction (110) and the hard direction (001). The behaviour of the line is discussed in relationship with spin wave calculations by considering a possible slight misorientation of the field off the principal axes. The acoustic magnon line, previously observed directly via ferromagnetic resonance, is also seen for B>2T along (110). Polarisation measurements on the two lines indicate that the acoustic and optic modes must be assigned to gamma ₂⁺ and gamma ₁⁺ (C_2h) respectively. A quantitative analysis of the authors results shows that the average Lande factor is close to the spin-only value and it suggests that g_{perpendicular to} is slightly larger than g/sub ///. It also leads to an estimate of P+2D where P and D stand for the two anisotropy parameters.