Table of contents

A simple expression for the heat flux between sintered silver powder and ³He-B is derived. The heat flux in the superfluid state is expressed in terms of that in the normal state, without the precise mechanism of heat transfer needing to be specified. A comparison with recent measurements suggests that the energy gap involved is a factor of 0.65 of the bulk value. This implies that the gap is depressed near a boundary and that processes occurring close to the boundary dominate the thermal exchange mechanism.

New data on the temperature dependence of pulsed photocurrents are presented for the one-dimensional semiconductor single crystal PDATS. The quantum efficiency for carrier creation is initially temperature-sensitive due to the role of geminate recombination, but new results show the carrier creation to become very weakly temperature dependent below 100K and remain so down to helium temperatures. This new result shows that the Onsager description of geminate recombination is inadequate at low temperatures and new models are discussed.

XANES, X-ray photo-electron and optical spectra of divalent nickel in NiCr₂O₄ and Ni_1-xCu_xCr₂O₄ compounds are reported. The characteristic parameters of the Ni(II)_tetra-O chemical bond in spinels are not very sensitive to the Jahn-Teller effect. The tetragonal distortion c/a<1 for the Ni_1-xCu_xCr₂O₄ system for the range 0.2<or=x<0.5 is due to the simultaneous influence of Ni²⁺ spin-orbit coupling and the Cu²⁺ Jahn-Teller effect.

Neutron scattering experiments were carried out on the quasi-one-dimensional conductor (TaSe₄)₂I. At the superlattice point (0.05, 0.05, 0.085) a soft temperature-dependent transverse acoustic (TA) mode polarised in the c plane was observed. A 'spoon-like' anomaly of the dispersion curve of the TA branch was observed around the superlattice point. In addition, an extremely low-lying TA mode was found.

By means of neutron and X-ray four-circle diffractions, evidence for two structural transitions in Mo₈O₂₃ has been found. The incommensurate phase appears at a value of T_cl higher than room temperature and the incommensurate-commensurate transition takes place at T_c2 approximately=285K. The idea that the transitions are associated with the charge-density wave formation can explain the temperature dependences of the magnetic susceptibility and the electrical resistivity consistently. The characteristic features of the condensed mode in the commensurate phase are discussed.

Various state equations derived by expanding energy as function of volume in a Taylor series and using the difference order Pade approximants have been combined with the quasi-harmonic approximation for free energy to reproduce the thermodynamic properties of the crystal on its P-V-T surface. The authors have used these state equations to compute the pressure-volume behaviour, the isothermal bulk modulus, the pressure derivative of bulk modulus and the Anderson-Gruneisen parameter of NaCl at high pressure (up to 80 kbar) and various temperatures (T=298, 550 and 800K). The results obtained are reasonably good, lending support to the state equations and the method used to extend their applicability at elevated temperatures. The significant results obtained in the present study include the unified reduced equation of state for NaCl which generates a single curve for the P-V behaviour at all temperatures.

The thermodynamics of the six-vertex models in direct and staggered electric fields is investigated using the cluster field method. The scheme of the successive approximations of the method is described and the result of the first and second approximations are presented. The cluster field method is shown to combine simplicity with high accuracy, in particular, in the description of ferroelectric phase transitions where the basic results agree with the exact ones in all the exactly soluble cases. The properties of the models in the staggered field s not=0 are investigated in detail. The expressions obtained for the ferroelectric phase transition point T_F(s) and for the polarisability near T_F(s) are apparently exact for the six-vertex models.

Using an extension of Fermi liquid theory appropriate to electron-spin-polarised deuterium (D down arrow ), the Landau-Silin kinetic equations are derived. The relevance of both Elliot's SU(3) and Gell-Mann's SU(3) symmetries to the hydrodynamics of D down arrow is discussed and the free precession and diffusion behaviour of the coupled modes of spin polarisation and alignment are studied. A generalised form of both the Larmor precession and the Leggett-Rice effect familiar from the study of spin-¹/₂ systems, are found in D down arrow .

An exact real-space rescaling transformation is used to calculate the Green functions and densities of states for a one-dimensional binary system with long-range exponentially decaying interactions between the atoms. The model is applied to both diatomic and s-p hybrid crystals.

Thermoreflectance spectra of single-crystal ZrS₂ and ZrSe₂ were obtained at 60K. A Kramers-Kronig analysis was carried out and fits of experimental curves calculated. The resulting parameters are interpreted on the basis of existing band-structure and spin-orbit splitting calculations. A simplified band picture is presented.

Measurements of the Shubnikov-de Haas effect in Cd₃As₂ single crystals as a function of crystal orientation were performed by Blom and co-workers (1980) who analysed their results in the light of the Bodnar model (1977). It was found that the experimental Lande g-factor was 15-20% lower than the predicted one. More recently, Singh and Wallace (1983) calculated the contribution of the free-electron term, which is neglected in Bodnar's model, and concluded that an excellent fit with experiment could be obtained by taking this contribution into account. The authors present a more careful treatment and conclude, in contrast, that its inclusion in the calculation worsens the fit with experiment. It is shown that the numerical values chosen by Bodnar in his model should be modified to take into account the free-electron term. To this end, new values are proposed that fit the data of Blom and those used by Bodnar.

Methods for the theoretical prediction of the static equilibrium properties of solids by means of total-energy minimisation are investigated. The authors discuss practical aspects and the pitfalls connected with finite plane-wave basis sets, and show how a number of ambiguities can be overcome with the aid of the stress theorem. The procedures developed are very efficient for achieving full convergence of calculated properties. Precautions necessary when performing calculations that are not fully converged, which are unavoidable for complex systems, are discussed.

It was shown earlier that the field-dependent drift mobility in some organic compounds may be interpreted in terms of the so-called 'difficult' transitions. A computer simulation of the dielectric losses in a hopping system containing the 'difficult' transitions is described in this paper. It has been shown that the recently measured dielectric properties of p-terphenyl pellets may be interpreted by means of a previously published 'difficult' transition model.

A detailed Monte Carlo simulation of hot-electron transport at 77K in a AlGaAs/GaAs quantum well has been carried out in the bulk-phonon approximation taking into account quasi-two-dimensional electron-electron scattering along with other quasi-two-dimensional scattering mechanisms and the effects of degeneracy, but neglecting dynamic screening and plasmon interactions. It is shown that for an intermediate range of AlGaAs donor concentrations (10¹⁷-10¹⁸ cm^-3) a scattering-induced negative differential resistance (SI-NDR) occurs as previously predicted. The threshold for the onset of SI-NDR is significantly less than the thresholds for either inter-valley or real-space transfer; hence the latter mechanisms are thereby delayed to higher fields. It is pointed out that SI-NDR has a much faster relaxation time than either inter-valley or real-space transfer.

The influence of a magnetic field on microwave photon-assisted quasiparticle tunnelling has been studied experimentally. The experimental results for a superconducting tin-oxide-tin tunnelling junction are analysed. It is realised that for sufficiently high magnetic field, quantum effects in the tunnel characteristic phase out. Instead, the junction behaviour becomes like that of a classical RF energy detector. These results are found to be in good agreement with the Tien-Gordon theory (1963) as modified by Hamilton and Shapiro (1970) and the Abrikosov-Gor'kov theory (1960).

The authors apply the semiclassical approach to calculate the combined effect of quantum corrections and out-of-plane fluctuations on the soliton contribution to the specific heat in the one-dimensional easy-plane ferromagnet. They show that the importance of quantum corrections is greatly enhanced by going beyond the sine-Gordon approximation. They argue that the remaining discrepancy in experiments in CsNiF₃ and CHAB arises from the approximations required in the semiclassical approach. The quantitative interpretations of neutron scattering and specific heat data on CsNiF₃ are shown to be closely parallel.

Using the Bethe ansatz technique the authors have diagonalised Coleman's (1984-5) generalisation of the single-impurity infinite-U Anderson model. A special limit of their solution extends earlier results for the SU(N) Coqblin-Schreiffer model to multiple f occupancy. The authors have calculated the thermodynamics of the generalised Anderson model and find that as they take the degeneracy to infinity maintaining a finite filling factor of the f state the cross-over to strong coupling sharpens into a phase transition and the thermodynamics converges to the mean-field solution valid at infinity degeneracy.

A simple one-dimensional model is used for electronic polarisability calculations. First-principles results are compared on the one hand with variational many-body molecular-like ones, and on the other with the harmonic oscillator approximation. Limitations of applicability of the latter two methods are evaluated.

The optical energy transfer between Cr³⁺ ions from cubic centres to tetragonal centres is measured. Transfer is observed only when the donor is very close to the acceptor and the measured transfer times are relatively short ( approximately 100 mu s). The tetragonal acceptors are Cr³⁺-ion pairs which form different triads with the donor. It is deduced that transfer to rhombic sites is also rapid only from very close cubic centres. The shortness of the transfer times explains why a spin memory is observed during the energy transfer from cubic to rhombic centres. The localisation of optical excitation in triads is due to the presence of charge-compensating Mg²⁺ vacancies and the inefficiency of the dipole-dipole interaction.

The luminescence spectra and decay curves of Sb³⁺-doped Cs₂NaMCl₆ (M=Sc,Y,La) are presented. In these compounds the 5s² ion is octahedrally surrounded by six chloride ions like the s² ions in alkali halides. Therefore, they can be considered as an ideal model system. It appears that the Sb³⁺ luminescence in Cs₂NaMCl₆ is comparable with the luminescence of the isoelectronic s² ion In⁺ in potassium chloride. The emission consists of one band moving to lower energies from scandium to lanthanum, suggesting that the Stokes shift increases when the space available for the Sb³⁺ ion increases. The excitation spectra can be explained by assuming a dynamical Jahn-Teller effect to be operative in the excited state of the Sb³⁺ ion. The consequences of this interaction are discussed.