Table of contents

In a random triangular cactus, with the end spins in each triangle constrained parallel, the probability that frustration does not inhibit an ordered phase is equal to percolation probability on an equivalent Bethe lattice. This equivalence permits a check on Ono's method (1980) of calculating the random ordered phase boundary.

A simple procedure to extract pseudopotentials from ab initio atomic calculations is presented. The main body of the resulting pseudopotential is completely analytic and fitting the remaining parts to analytic functions is very easy and accurate. The final form of the pseudopotential reduces to a modified form of the optimised Heine-Abarenkov model potential.

Results of the low-temperature electrical resistivity measurements on annealed single Fe₃O₄ crystals are reported in the temperature range T_V/2<T<T_V below the Verwey transition. The Arrhenius law is obeyed throughout this temperature range. The effects of magnetisation and ageing of the samples are also discussed.

A two-dimensional model is proposed to explain, through the many-body properties of a strong correlated electron gas, the main electronic features of a (111)-1*1 surface of highly doped silicon.

4d core levels of InSb(110) cleaved surface have been measured by ultraviolet photoemission spectroscopy with the aid of synchrotron radiation. The authors have recently observed surface core levels of In shifted to larger binding energies by +0.22 eV and those of Sb shifted to smaller binding energies by -0.29 eV, relative to their bulk components. The location of the cation-derived empty surface state is re-evaluated to be at 0.33 eV above the valence band maximum.

Two quite different values of the g-factor of Co²⁺ in InP have been reported in the literature recently. Using both conventional and thermally detected EPR on several different samples, the authors have obtained a g-value of 2.191+or-0.001, which agrees with the value quoted by Lambert and co-workers (1982) and is consistent with that found in GaAs and GaP.

The Mossbauer spectrum of dilute ⁵⁷Fe²⁺in MgCO₃ shows a well resolved magnetic hyperfine pattern at 4.2K due to a very slow rate of electronic spin relaxation. Above 60K only pure quadrupole splitting is obtained as a result of very fast spin-lattice relaxation rate. The spin-lattice relaxation behaviour of Fe²⁺:MgCO₃ is very similar to that of Fe²⁺:ZnCO₃, which shows that the slow spin relaxation of Fe²⁺ is a consequence of its trigonal site symmetry in a carbonate lattice.

Both the changes in the optical absorption spectrum and the F*-centre emission thermoluminescence induced by F photostimulation at low temperatures in pure KCl samples previously irradiated at room temperature have been studied. It has been found that there is F aggregate centre formation during F photostimulation at 80K without any clear F-centre formation. Aggregate centres are also formed at temperatures at which F*-centre emission glow peaks occur. It is concluded that the F* centre can act as a mobile entity in the F-centre aggregation processes.

The ultrasonic attenuation has been measured from 4 to 270K in single crystals of pure bismuth germanium oxide, Bi₁₂GeO₂₀(BGO) and in crystals doped with Al, Ga, Zn, Pb, Cr, Ga+Cr, P+Ga. The relaxation attenuation peak near 50K in pure BGO is increased in the Cr-doped sample, reduced in the Zn-doped sample and absent in the other samples, and this correlated with the optical absorption measured by Oberschmid (1981). The point defect responsible for the attenuation is shown to have trigonal symmetry and the data are analysed in terms of the deformation tensor of the elastic dipole of the defect. A discussion is given of the possible defects which may produce the optical and acoustic absorptions.

The authors formulate a theory of indirect excitons in GaP under uniaxial stress including the valence-band degeneracy and the camel's back structure of the conduction band. In the high-stress limit they separate relative and centre-of-mass motion and obtain analytic expressions for the exciton dispersion, which are then used to calculate the exciton lineshapes. Comparison with experimental line shapes from wavelength-modulated spectra yields energy band parameters in particular for the lowest conduction bands. In addition, they analyse the strength of different phonon replicas to determine the relative magnitude of the electron-phonon coupling to the zone-boundary phonons assisting the indirect-gap absorption.

A classical theory of melting based on the lowest-order Bogolyubov-Born-Green-Kirkwood-Yvon equation is presented. This equation is decoupled in a fashion closely analogous to the mean-field approximation used in other fields of physics. The nonlinearity of the equation is treated seriously and the free-energy difference between the solid and the liquid at low pressure is calculated. This leads to a first-order phase transition. The melting temperature and the entropy of melting at low pressure are calculated and compared with experiment. The theory also demonstrates that melting occurs when the amplitude of the lattice vibrations in the solid reaches a fixed fraction of the nearest-neighbour distance. It thus provides a justification of Lindemann's result.

Discusses the statistical distribution of internal fields due to random defects in solids. It extends previous discussions by separating explicitly the contribution of the nearest defect from that due to all others. Possible applications include those to preferential defect pairing, to hopping conduction in randomly doped crystals, and to questions of the existence of phase transitions in random dipole systems. As a by-product, it is noted that the random strains in solids have a statistically negligible probability of localising a muon (assumed not self-trapped) except at the sites of defects themselves.

The extended pair approximation is used to compare the predictions of the Miller-Abrahams rate equation theory (1960) for AC and DC hopping conductivity with experiment. For an analysis of impurity conduction the authors consider data for n-type silicon and gallium arsenide. In both cases the theory accurately predicts the DC conductivity when reasonable assumptions concerning the density of states are made. In the AC regime the theory provides a good description of the data on silicon but in the case of the gallium arsenide data the theory predicts an onset of AC behaviour at a frequency about six orders of magnitude higher than that observed. Data for amorphous germanium are also analysed. The theory fits the DC conductivity which follows a T^-1/4 law, with a characteristic frequency R₀, of 10²¹ Hz. The AC conductivity is qualitatively in agreement but the onset of AC behaviour is two orders of magnitude higher in frequency than is observed.

Ultrasonic attenuation at 9 GHz has been studied as a function of magnetic field at 4.2K in TmVO₄, which undergoes a Jahn-Teller phase transition at 2.15K. The ultrasonic attenuation is shown to be caused by the relaxation properties of the Tm³⁺ electronic states, which are coupled by the Jahn-Teller interaction to the acoustic phonons. The observed dependence of attenuation on magnetic field is well described by a linear response function theory. Values for the electronic pseudo-spin damping parameters are Gamma ₁/2 pi =(1.8+or-0.5) GHz and Gamma ₂/2 pi =(2.5+or-0.7) GHz in pure TmVO₄, and Gamma ₁/2 pi approximately equal to 9 GHz for the dilute salt Tm_0.82Lu_0.18VO₄.

The DC G(T) has been measured in the approximate range 4-320K for crystalline samples of the series Ti_nO_2n-1 with 4<or=n<or=9. This includes the first results reported for n=7, 8 and 9. The G(T) for n=4, 5 and 6 show qualitative differences from the behaviours reported previously, including extra transitions at 127K and 119K for n=5 and 6, respectively. Despite the homologous crystal structures and the similar transition temperatures, the G(T) for this Magneli series show few systematic trends. This implies that no single conduction mechanism dominates across the series, but for n=5, 6 and 7 the authors find evidence that quantum tunnelling and thermally activated jumping may be the dominant mechanisms over limited temperature ranges.

A new approach is presented to deal with magnetic properties of the singlet ground state systems with nuclear spins of arbitrary magnitude. The electronic spin polarisation of a magnetic atom consists of the exchange-induced part and the HF-induced part. The latter is assumed to follow adiabatically the nuclear spin of the interested atom and gives rise to the I-I coupling and the pseudoquadrupole interaction while the former is responsible for the internal field effect to nuclear spins. The new method succeeds in constructing the effective nuclear spin Hamiltonian in such a scheme and treating the problem by taking the nuclear spin polarisation as the representative LRO parameter. It gives a unified treatment of magnetism over all regimes (electronic and nuclear regimes) and phases (ordered and paramagnetic states) irrespective of relative magnitudes of the parameters in the Hamiltonian. Some numerical results of the present formalism are presented.

⁵⁷Fe Mossbauer spectroscopy is used to investigate the low-temperature behaviours of alpha -MnS and MnSe, and the results are successfully interpreted in terms of a crystal field model. In alpha -MnS there are at least two magnetic spin arrangements. At 131K<T<T_N the spins are along a cubic (110) direction, whereas at T<117K a multiaxial arrangement is likely. MnSe is found to exhibit two distinct crystallographic structures which coexist at certain temperatures. In a NaCl form the spin arrangement is multiaxial. The other form is a NiAs modification, in which the spins are along a hexagonal (210) direction.

The occurrence of a photoinduced reduction of the V⁴⁺ ESR signal intensity in Ca₂NaMg₂V₃O₁₂ is reported. Its time and temperature dependences are considered and a comparison is made with the photomagnetic effects observed in Y₃Fe₅O₁₂. The intensity reduction is attributed to light-induced electron transport within the vanadium sublattice. In this respect the results of photoconductivity and optical reflectance measurements are also presented and discussed.

Electron spin resonance and electron spin echo studies on Cu²⁺ in four A-zeolites exchanged with K⁺, Cs⁺ and NH₄⁺ indicate the formation of a new tetrahedral species, Cu(O_z)₃(H₂O)²⁺ where O_z is a zeolite oxygen and the water molecule is probably in the beta cage. The preferential formation of this tetrahedral species in these zeolites instead of the usually observed Cu(H₂O)₆²⁺ or Cu(O_z)₃(H₂O)₃²⁺ species is ascribed to the bulkiness and positions of the co-cations Cs⁺, NH₄⁺ and K⁺, resulting in a strong electrostatic repulsion in the alpha cage of the zeolite structure which effectively prevents Cu²⁺ from occupying either S2 or S2* positions in the alpha cage and forces it to occupy the S2' position in the beta cage.

The crossover scaling behaviour for the lattice anisotropy of the susceptibility of the quasi-two-dimensional Ising models has been studied using high-temperature series expansions. The predictions of the extended scaling hypothesis are verified and the plots of the effective exponent are presented in the scaling regime. These results should provide a basis for comparison with experiments on quasi-two-dimensional Ising systems.

The 3d absorption spectra of gaseous and solid xenon are compared. The spectrum of the gas agrees substantially with earlier data apart from two additional weak peaks about 1.3 and 5.0 eV above the M₅ threshold. Comparison with the ³P₁ and ³D₁ lines in the absorption spectra of Ce⁴⁺, La³⁺, Ba²⁺ and Cs⁺ suggests that the weak xenon peaks represent transitions into resonantly localised continuum f states. No corresponding structure has been reported in the 4d spectrum. The gross structure of the spectrum of the solid is similar to that of the gas with the dominating M₄ and M₅ continuum peaks at the same energies and with similar cross sections. Notable differences are disappearance of the 1.3 eV peak, enhancement of the 5.0 eV peak and the appearance of weak maxima extending from 7.8 eV above the M₅ threshold to higher energy, across both M₄ and M₅ continuum peaks. This structure, which lies above the range of existing density of states calculations, is attributed to scattering from surrounding atoms. No corresponding structure has been reported either in the 4d spectrum of solid xenon or in the isoelectronic M_4,5 spectra of I^-, Cs⁺, Ba²⁺ or La³⁺ in solids.

The authors report measurements of angle-resolved photoemission and of transport properties for the same samples of nearly stoichiometric TiS₂, and of Ti_1.05S₂. It is shown that the carrier densities derived from the two techniques are consistent, and the geometries of the Fermi surfaces are mapped out. The data also show that there is an indirect gap of 0.3+or-0.1 eV between the S p and Ti d bands, for both materials.

The apparent suppression of plasmon satellites in core-level photoemission at low electron kinetic energy is studied. The process of photoemission is studied in the Golden Rule formulation, and the final-state wavefunction corresponding to the photoemitted electron leaving a plasmon excited in the solid is found using perturbation theory. It is found that interference between intrinsic and extrinsic excitation suppresses the long-wavelength surface and bulk plasmon excitation, but the shorter-wavelength plasmons are still excited even at low electron kinetic energies. However the satellites are featureless and are probably lost in the background until the electron kinetic energy is about 40 eV, with parameters appropriate to Al. Subtracting the featureless 'background', total satellite intensities of 14% for the surface plasmons and 31% for the bulk plasmons are obtained, at an electron kinetic energy of 100 eV.

The electron energy-wavevector (E-k) dispersion relation for linear chains of thallium atoms adsorbed on the copper (100) surface has been determined with angle-resolved photoelectron spectroscopy. The behaviour of the Tl 6s and Tl 6p emission features is consistent with a simple one-dimensional band structure. For k parallel to the chains the 6s dispersion curve is sinusoidal and in good agreement with the result of an LCAO calculation using relativistic Hartree-Fock-Slater wavefunctions. Electrons emitted from the Cu 3d surface resonance are coherently scattered for wavevectors parallel to the chains.

Present methods of measuring electronic band structure of solids by photoemission suffer from reduced resolution in k_z because the escaping electron has a finite escape depth which broadens its momentum. Thin films grown epitaxially on substrates are predicted to show fine oscillations in their spectra caused by quantisation of hole states. The spacing of peaks is determined by the number of layers and by the group velocity of the band concerned and the peaks are broadened only by the intrinsic lifetime of the hole state. Realistic calculations are presented for silver grown epitaxially on a palladium (001) surface. They show strong differences from pure silver up to thicknesses of at least 7 layers due to the quantisation effects.