Table of contents

The phase boundary of the bond diluted s-state Potts model on the square lattice is calculated for the case of quenched randomness. Using a duality transformation and the replica method the author finds the phase boundary to be given by exp(-sJ/T_c)=(s^xp/-1)/(s-1), where x=p-p_c.

The analysis of the angular momentum relaxation time data of SiCl₄ along the liquid-vapour coexistence curve from 223K to 343K confirms the existence of as yet unknown molecular interactions consistent with the results obtained previously by other techniques.

Zero-sound excitations in liquid ³He at 0.02K and at the pressure of the saturated vapour have been studied using neutron inelastic scattering techniques. The results for the frequencies of the zero-sound excitations are in agreement with the measurements of Skold and co-workers. The results also show a non-zero linewidth for the excitations with wave-vector transfers 0.45 AA^-1<Q<0.8 AA^-1, which suggests that modification to the current theories is required.

An X-ray scattering study of xenon physi-adsorbed onto the (0001) surfaces of graphite is reported. Coverages between 0.3 and 2.0 monolayers are examined and unambiguous evidence is obtained for the existence of a 2D triple point at 99K. Above 99K incommensurate monolayer solid and liquid phases are observed. The liquid is well described by an exponential droplet model.

The density of levels of a disordered system falls off exponentially at large negative energies in the region of localised states if the randomness is gaussian. The authors compute the absolute normalisation of the pre-exponential factor in order to make contact with available two-dimensional numerical results.

The proton resonance line in (NH₄)₂IrCl₆ powder is narrow due to rapid NH₄⁺ tunnelling, even below the antiferromagnetic temperature T_N=2.15K, and proton relaxation T₁ gives information about Ir⁴⁺ psin dynamics. Spin correlation above T_N results in T₁ varies as )(T-T_N)/T_N)^0.27+or-0.07 which may be explained with the dynamic scaling hypothesis of critical exponents. From 2.15K to 2.00K T₁ increases by a factor of the order of 200, and below 2.00K the authors find the direct relaxation process T₁ varies as T^-1 for the tunnelling NH₄⁺ ions (NH₄)₂ReCl₆ behaves similarly, with T_N approximately=10K.

Infrared absorption at 2K of non-stoichiometric potassium beta -alumina shows three peaks at 69, 84 and 107 cm^-1. In materials with a relatively high degree of stoichiometry the 84 cm^-1 band is present but the 69 and 107 cm^-1 bands are absent. It is concluded that the 84 cm^-1 band is due to K⁺ ions in singly-occupied Beevers-Ross site and it is suggested that the 69 and 107 cm^-1 bands are due to K⁺ ions in mid-oxygen sites, possibly associated with charge-compensating interstitial oxygen ions.

As-grown sodium beta alumina is non-stoichiometric and shows strong absorption in the region 20-200 cm^-1 with partly resolved structure. The authors show that in material close to stoichiometry sharp absorption lines are observed in the same spectral range. They conclude from these studies that a sharp intense peak at 59 cm^-1 is due to vibrations of Na⁺ ions near singly occupied Beevers-Ross sites. They suggest that other absorption peaks in the far infrared are due to complexes involving sodium ions and charge-compensating interstitial oxygen ions, and propose that complexes responsible for absorption at 102 cm^-1 are particularly effective in contributing to the ionic conductivity.

The problem of completely frustrated spin systems with occupancy disorder-an example of which is the orientational problem in solid hydrogen-can be approached at T=0 through the simpler problem of colouring a randomly occupied lattice. Within the framework of this colour model, it can be proved that there exists a critical concentration of filled sites below which long-range order disappears. A numerical estimate of this critical colouring concentration in the triangular lattice in p_cc=0.72+or-0.03. This model termed 'percoloration' is applicable to the prediction of the existence of a critical concentration for long-range order in a random array of quadrupoles such as solid hydrogen.

The dynamics of a binary alloy vibrational system of simple cubic (monotonic) structure with both mass and force constant disorder (algebraic, degenerate, nearest neighbour) is studied in terms of the configurationally averaged lattice Green function. In the random alloy case, the CPA subseries has been summed. A similar subseries is summed in the correlated alloy case and the self-energy is expressed in terms of pair correlations.

The Grimley method, using the Green function, is adapted for a simple qualitative analysis of the effect on the electronic properties of graphite intercalation compounds of the interaction between one or two inserted atoms and their neighbouring carbon atoms. The rough shape of the energy band of these compounds is obtained from the chemisorption matrix which describes two s orbitals inserted in graphite. The most important results are the distortion of the graphite valence band and the appearance of a characteristic band shouldering the conduction band and containing the Fermi level.

For determining the electronic quasiparticle spectrum of a ferromagnetic semiconductor a new spectral density approach is proposed, which explicitly takes into account quasiparticle damping by a gaussian ansatz for the one-electron spectral density A_{k sigma} ( omega ). The method is checked by some exact results known for the strong coupling region and by higher spectral moments. In the ferromagnetic saturation (T=0) the conduction band is split into one stable spin up-sub-band, and two different spin down-quasiparticle sub-bands. Lifetimes and spectral weights of the two quasiparticles strongly depend on the bandwidth W of the Bloch band, and also on the value of the f-spin S. For an s-f coupling constant g=0.2 eV the quasiparticle structure is well defined up to bandwidths of order W approximately=1.0 eV (realistic for EuS). The often used picture that the conduction band of a ferromagnetic semiconductor splits into two sub-bands (one spin up-and spin down-band) is even as T=0 true only for S to infinity (classical limit), because then the lower spin down-quasiparticle disappears.

A diagram useful for representing the electronic levels arising from transition-metal impurities in tetrahedral semiconductors is proposed. Computer calculations of carrier equilibrium effects are described, highlighting the problems of including all relevant physical effects. The calculations are believed to be amongst the most complete attempted to date. Semi-insulating behaviour, conduction activation energy, relative occupations of Cr valence states, and compensation dependence of Cr solubility in Cr-doped GaAs are elucidated. The calculations suggest that the Cr levels tend to follow the valence band edge, rather than the conduction band, as temperature varies, and that the saturated solubility of the neutral species Cr^III at the melting point is about 10¹⁷ cm^-3. The calculated variation of electrical resistivity with compensation shows a structured 'resistivity edge', in the region of which the Fermi potential, fractional occupations of Cr oxidation states, and the apparent conduction activation energy all vary rapidly.

The low-temperature polarised Zeeman measurements of the ⁴A_2g to ⁴T_2g transition in Cs₂TiF₆:Mn⁴⁺ are reported. Three zero-phonon lines associated with the three lowest levels of the ⁴T_2g state are identified. Their separation is explained in terms of spin-orbit and trigonal field splitting both appreciably quenched by a strong Jahn-Teller interaction.

The contribution of exchange scattering to the Hall effect of liquid Fe, Co and Ni is evaluated. Two models of the electronic states are examined: the local magnetic order model (Korenman, Murray and Prange), and the paramagnetic model. Although the effect of exchange scattering is an order of magnitude larger than that of spin-orbit scattering, it is still an order of magnitude too small to account for the Hall effect in Fe and Co.

The indirect exchange interaction between two localised magnetic moments is calculated for Hg_1-xMn_xTe alloys near the zero-gap semiconductor transition. For such Mn concentrations the non-parabolic effects are overwhelming and have been accounted for properly. The electron-heavy-hole virtual transitions are shown to be larger than the electron-light-hole transitions. The net exchange integral is antiferromagnetic; the numerical value of the Curie-Weiss temperature is calculated and compared with experimental determinations.

The bicritical point occurring in disordered spin-flop antiferromagnets is novel in two respects: (i) The critical exponents are, to all orders in epsilon =6-d, those of an Ising system in a random ordering field; (ii) In contrast to the pure case, the upper (u) and lower (l) phase boundaries are described by different shift exponents, psi _u> psi ₁. The upper shift exponent psi _u is equal to the crossover exponent phi = gamma (1+ epsilon ²/36+...), where epsilon =6-d and gamma is the staggered susceptibility exponent. The lower shift exponent is unity, to all orders in epsilon .

A recently designed refractometer employing the phenomenon of total internal reflection is used to measure the refractive indices of the condensed phases of the rare gases argon, krypton and xenon in the temperature ranges 35-90K, 60-117K and 30-178K, respectively. The values obtained for argon and xenon are in good agreement with those obtained by the method of minimum deviation, but values for krypton obtained by the two methods differ by more than the combined experimental error. This difference does not appear to be due to impurities. Experimental values of the refractive indices have been used to compute values of the Lorentz-Lorenz function for comparison with predictions arising from the theoretical models of C.J.F. Bottcher and of H. Block and S.M. Walker and to obtain revised values of the parameters for the theoretical treatment of S. Doniach and R. Huggins. The effect of vacancy concentration on density and hence on the Lorentz-Lorenz function is discussed.

Optically-detected magnetic resonance experiments at 2K on indium-doped hexagonal zinc sulphide distinguished two broad emission bands that are due to recombination between donors and acceptors. For the first band, which peaks at 1.94 eV, the recombination is between indium centres that act as deep-lying donors and A centres that act as deep-lying acceptors. For the second band, which peaks at 2.34 eV, the indium centres again act as donors but the acceptor centres are in this case unidentified. The indium donor level is estimated to lie at 0.4 to 0.6 eV beneath the conduction band and the indium ions are associated with perturbing defects at the nearest cation sites.

Luminescence spectra due to the ⁴T₂ ( Gamma ₈^phi ) to ⁴A₂ ( Gamma ₈) no-phonon crystal-field transition are analysed in ZnS, ZnSe, ZnTe, CdTe and CdS doped with cobalt. The no-phonon line intensity, extrapolated to 0K, ranges from 0.020 (for CdTe:Co) to 0.24 (for ZnS:Co) of the total luminescence intensity. The effect of temperature on the intensity f and position Omega of the no-phonon line is interpreted according to McCumber's model of the ion-phonon interaction. The numerical value of the quadratic coupling constant alpha h(cross) ranges from 4.5*10^-2 cm^-1 (for ZnS:Co) to 4.4*10^-1 cm^-1 (for ZnTe:Co), being greater for systems with smaller f values. However, the effect of temperature on the luminescence line halfwidth 2 Gamma could not be explained within the framework of the McCumber model. Numerical analysis of the experimental data was also made in the Debye approximation. In this model the thermal dependence of f and Omega was reproduced with values of the Debye temperature by some 20-30% lower than those characteristic of pure crystals.

Four theoretical models have been considered for the discussion of low-field impact ionisation and Auger recombination rates involving a single trap. For shallow levels two hydrogenic models are considered. These are then modified for application to deep levels by the use of the Lucovsky delta -function potential model. It is also shown how atomic results on hydrogen ionisation can be used to compare with the theoretical solid state results. Particular attention is paid to problems induced by the non-orthogonality of approximate wavefunctions, and it is found that the use of orthogonal wavefunctions significantly reduces the impact-ionisation probability per unit time P(2'), for higher energies of the ionising carriers in state 2'. These results are highly model sensitive. When the Auger recombination coefficient is inferred from P(2') by integration, the results obtained are much less model dependent.

The electronic states of CO molecularly adsorbed (1000 Langmuirs exposure) on Mo (110) are studied by metastable helium atom de-excitation spectroscopy (MDS) founded on the metastable-surface interaction model described by Hagstrum. The spectrum obtained after deconvolution, compared with the direct photoemission spectrum, shows a density-of-states profile corresponding to this adsorption and suggests a possible mechanism for the CO-Mo bond involving the 5 sigma +1 pi and 2 pi * orbitals of the CO molecule. The results obtained show that MDS is very well suited to research on the electronic properties of surfaces.