Table of contents

Considers LEED by a metal preceded by its image potential barrier and derives the corresponding S-matrix for energies near the threshold where a beam has grazing emergence. The authors establish that the electric surface resonances of the Rydberg series type caused by the image potential have a perfectly smooth continuation above the threshold.

Quantum corrections to the classical statistical mechanics of quasi-one-dimensional easy-plane ferromagnets (e.g. CsNiF₃) in the presence of an easy-plane magnetic field are outlined. Criteria for the validity of a classical description are suggested on the basis of complete quantisation of strongly non-linear excitations. One of these criteria agrees well with one suggested by Mikeska and Patzak (1977) on the basis of a low-order quantum perturbation theory for essentially linear modes in the absence of a magnetic field. It is confirmed that CsNiF₃ can be treated substantially classically in typical experimental circumstances.

The thermal resistivity r(T) of ice Ih has been re-measured from 120K to as close as practicable to melting. Measurements were made under isobaric conditions at a pressure of 0.07 GPa, and corrected to isochoric conditions. The resultant deviation from the simple theoretical prediction, r/T=constant, has been plausibly accounted for by interaction between optic and acoustic phonons. Thermal resistivity arising from the presence of point defects has been shown to be of negligible importance.

Derives a new integro-differential equation for the electron density and an exact formula for the surface tension of the jellium model of a metal surface. The authors' results constitute a proper generalisation of Samoilovich's pressure tensor approach to the jellium surface. The corresponding results for a two-component charged fluid are also presented; these provide insight into the theory of electron-hole liquid droplets in semiconductors and form a basis for a theory of the surfaces of molten salts.

Amplitude ratios near tricritical points in three dimensions are discussed and shown to be universal and in agreement with the classical values when logarithmic corrections are included. The spherical model of a tricritical point is shown to be a special case exhibiting non-universality because of the absence of logarithmic corrections.

During investigations of donor-acceptor recombination radiation in II-VI compounds that are photoexcited with radiation of photon energy in excess of the band gap, the authors have found that the ODMR signals decrease in intensity as the excitation power is increased, and that for sufficiently high laser power the signals disappear completely. This behaviour is shown to be consistent with the concept of well-defined pair recombination, which has been found necessary to account for previous ODMR phenomena in II-VI materials, and has important practical consequences for these investigations.

Two different renormalisation-group (RG) approaches are used to determine approximate solutions for the paramagnetic-ferromagnetic transition line of the square-lattice bond-dilute first-neighbour-interaction Ising model. The first approach (in two different versions, named RG1 and RG2) consists of substituting a single bond for an H-shaped cluster. In the second approach (RG3, RG4, RG5 and RG6) the authors take advantage of the self-duality of the square-lattice and define a duality renormalisation operation. All six renormalisation groups are defined as operations on the p-t space, where p is the independent occupancy probability, and t=tanh(J/k_bT). Both approaches yield very good results, including the exact values t_c= square root 2-1 and p_c=¹/₂ (for all six RGs) and dt₀/dp mod _p=1=8-6 square root 2 (for RG5 and RG6), as well as the correct asymptotical behaviour in the neighbourhood of t=1 (that is, T=0). The transition line obtained by RG5 might be extremely close (probably better than 0.5% in the most unfavourable case) to the unknown exact solution.

For pt.I see ibid., vol.8, no.23, p.4124 (1975). The low-energy satellites of K beta _1,3 lines of metallic Co, Ni, Cu, and Zn have been measured. These spectra are interpreted in terms of the K to M² radiative Auger (RA) effect because the energies of the satellite edges are in accordance with Auger electron observation and the intensities compare favourably with the theoretical RA shake-off calculation. The intensities of interfering lines due to resonant Raman, K alpha ^h hypersatellite, and radiative rearrangement (RER) transitions, are found to be negligible in comparison with RA intensity in the whole period 15<or=Z<or=30. Thus, using photon or electron excitation, the low-energy structures of K beta _1,3 lines designated K eta in the literature can be ascribed almost completely to the K to M² RA effect.

Reports detailed experimental studies of the onset of hard-mode Raman activity near the cubic-tetragonal structural phase transitions in KMnF₃ and RbCaF₃. The results are discussed in the context of a theory taking into account the influence not only of the long-range order, reflected in the macroscopic order parameter, but also that of the precursor order reflected in the central peak and phonon sideband of the soft-mode spectral function. The precursor order, dominant near the phase transition, is responsible for the persistence of quasi-first-order hard-mode features above the transition temperature. The extent of this persistence is used as a basis for specific estimates of the degree of precursor order in KMnF₃ and RbCaF₃.

A study has been made of the adsorption of molecular oxygen, principally at room temperature, on Al(111). On the basis of LEED and CPD measurements supported by symmetry considerations and the calculation of several possible oxygen overlayer structures, it is suggested that the oxygen may form an instep ordered overlayer in tetrahedral sites before substantial incorporation takes place.

The redox process of Co^+z ions (z=2,3) substituted into the sapphire alpha -Al₂O₃ structure produced by heat treatment in H₂ and O₂ atmosphere is correlated with ESR and ENDOR spectra of (Co⁺², H⁾) and with UV-VIS and IR spectra of Co⁺³: alpha -Al₂O₃ and (Co⁺², H⁺): alpha -Al₂O₃ respectively. From ENDOR spectra the charge compensator H⁺ for Co⁺²(c): alpha -Al₂O₃ is uniquely identified and the reduction of the crystal-field symmetry by it from D_3d to C₁ is demonstrated. Location of the charge compensator and structural parameters of the defect (Co⁺²,H⁺): alpha -Al₂O₃ are determined. Furthermore, it is shown that the proton-ENDOR spectra of (Co⁺², H⁺): alpha -Al₂O₃ are described by a hyperfine interaction term characterised by an asymmetric coupling tensor originating from dipole-dipole interaction.

Utilises continuum theory to investigate the occurrence of thermal instabilities when a vertical temperature gradient is applied across a sample of nematic liquid crystal confined between two horizontal flat plates, the molecules being uniformly aligned obliquely with respect to the plates. Two distinct types of roll solution are examined, one being independent of the direction perpendicular to the vertical planes containing the molecules initially and the other independent of the horizontal direction in these planes. Both a Fourier series method and an orthonormalisation method are employed to obtain the critical values of the temperature gradient at which the initial configuration becomes unstable. It is shown that the type of roll instability likely to occur depends on whether heating is from above or below. The variation of the critical temperature gradient with the boundary orientation of the molecules is also investigated.

Ultrasonic attenuation in KMnF₃ has been measured at frequencies ranging from 10-170 MHz near 186-2K. The frequency and temperature dependences of the critical attenuation are determined and the dynamic scaling hypothesis is examined. Above 186-2K, it is found that the critical attenuation alpha is described by a scaling function, alpha = omega ^yF( omega / Omega _kappa ), where y=0.13+or-0.05 and Omega _kappa is the characteristic frequency of the order parameter fluctuation with the wave number equal to the inverse correlation length. The temperature dependence of Omega _kappa is discussed in the light of the Murata's (1976) theory for the sound attenuation of the perovskite crystal near the structural phase transition point.

Magneto-optical and magnetophonon experiments are reported which demonstrate the influence of the structural phase transition in Pb_1-xGe_xTe (x approximately=0.012) on the band structure. Above T_c ( approximately 45K) the band structure is similar to that of PbTe with four equivalent valleys at the L-point in both conduction and valence bands. At T_c a rhombohedral phase transition occurs and the valleys are split into a singly degenerate (T) valley and three equivalent (L) valleys. The band edge effective masses in the T valley is 0.015 m₀ in the conduction band and 0.039 m₀ in the valence band, and those in the L valleys are 0.026 m₀ in the conduction band and 0.023 m₀ in the valence band. A detailed analysis of the amplitudes of the magneto-optical resonances gives information on the relative band populations and on the orientation of structural domains.

The universality of the jump in the superfluid density of He films at T_c, predicted by Kosterlitz and Nelson (1977), is investigated when higher vortex configurations are taken into account. An extension of the real-space renormalisation group analysis carried out by these authors leads to a possible non-universality. However it is shown independently that this conclusion is incorrect and that universality must hold in an exact calculation.

By representing f orbitals by cubic harmonics, overlap (or energy) integrals for f electrons have been calculated in terms of two-centre integrals. Results of s-f, p-f, d-f and f-f integrals are useful in making the tight-binding calculation as an interpolation method for substances with rare-earth or actinide elements.

The coexistence of tight-binding and exchange-correlation effects inside each chain of a model quasi-one-dimensional metal was studied for both plasmon and charge-density wave properties. The results obtained, while in qualitative agreement with other treatments of the problem at long wavelengths, indicate a strong tendency for plasmons to turn into excitons at larger momenta and to exhibit an 'excitonic' charge-density wave instability at k approximately 2k_F. The nature of the plasmon branches and of the excitonic charge distortion is examined and the relevance to existing quasi-one-dimensional materials is also discussed.

An economical way of using the Lanczos algorithm on the very large sparse matrices associated with absorption band-shape calculations in dynamic Jahn-Teller and similar systems is described, and the sources of error are identified and discussed. Band shapes are calculated for a two-mode E(X)( epsilon ₁(+) epsilon ₂) system which show a remarkable similarity with those calculated using a single effective frequency. Band shapes are also calculated for T(X)( tau (+) epsilon ) with and without spin-orbit coupling, with the resolution of very narrow spikes in the band under favourable circumstances.

A model is proposed for systems exhibiting the cooperative Jahn-Teller effect, in which the concentration of the Jahn-Teller ions is variable. The importance of the lattice imperfections in such systems is pointed out. If the concentration of Jahn-Teller ions is less than a certain critical value, the interaction with the imperfections prevents the cooperative ordering. Taking this interaction into account, the authors calculate the critical concentration, the concentration dependence of the temperature at which the cooperative Jahn-Teller effect appears, and some other quantities of interest. The analysis is performed for systems containing the Jahn-Teller ions with double degenerate E-type electronic states and triply degenerate T-states interacting with the doubly degenerate epsilon -deformation. The calculations are compared with the experimental results on the spinel system Mn_xCr_3-xO₄.

The photoconductivity of GaAs:O (2 Omega cm at 300K) has been studied between 90-200K using a pulsed HF gas laser, in addition to conventional steady-state and chopped-light techniques. The substantial power available has been exploited to study photoresponse up to saturation level. Besides confirming the existence of a broadened trap distribution of width 160 meV below the conduction band, the authors have determined for the 0.4 eV centre the density N_T, the volume capture rate c_n, the absolute photoionisation cross section sigma _n, the effective radius of the centre r_T*, the Huang-Rhys factor S, the Franck-Condon shift d_FC, the optical ionisation energy E_T and its dependence on temperature. They are N_T=1.4*10¹⁴ cm^-3, c_n=1.3*10^-8 cm³ s^-1 at 100K, sigma _n=7.1*10^-18 cm² at 100K and at 0.4682 eV, r_T*=11 AA, S=3.5, d_FC=0.11 eV, E_T=0.420-7*10^-5 TeV. The spectral dependence of sigma _n agrees with a new model by Ridley (1980) rather than with the Lucovsky model, and this new model is exploited in the derivation of the above quantities. It is pointed out that the Franck-Condon shift may be more a property of the host crystal than of the defect.

The energies of the spin waves are first calculated numerically for short chains. The results indicate a periodicity in the spin-wave spectrum with period Delta k=2 pi r/Na where r is the number of deviations from the aligned (ferromagnetic) state. The method of des Cloiseaux and Pearson (1962) and of Griffiths (1964) is then used to obtain the corresponding energies in the limit as N to infinity . The amplitude of the periodic spectrum decreases as the applied field is increased. The authors' results agree with the 'hole-like' excitations given recently by Ishimura and Shiba (1977) but only for one period of the spectrum. The periodic spectrum consists of 'hole-like' excitations over the whole range of wavenumbers with lower energies than the 'particle-like' states.

To apply the layer iteration method of low-energy electron diffraction to the diffraction of UV and visible light, the transmission and reflection coefficients of the electromagnetic waves are derived for a two-dimensional periodic array of dielectric spheres with an isotropic dielectric constant. The concepts of phase shifts and t matrix are introduced for light scattering. The theory is a direct extension of Kambe's LEED theory (1968). Also, the secular equation obtained for the bandstructures of photons is a generalisation of the Korringa-Kohn-Rostoker formalism of electronic band calculation.

The recoilless fraction f_a has been determined as a function of temperature for small iron particles (average diameter 100 to 150 AA). The effect of the motion of the particle as a whole on the observed f_a is averted by embedding the particles in matrices. The particles have an oxide surface layer 1-2 layers thick. It is concluded, from the comparison of the temperature dependences of f_a for the particles and a bulk specimen, that the lattice vibration of the metallic core of the particles is not softened as compared with the bulk, while the lattice vibration of the oxide surface layer is very soft. The Debye temperature of the metallic core is found to be 470+or-30K.

The interaction of titanium films with oxygen has been studied by observing their change of resistance R and workfunction phi over the range of gas pressures from UHV to 5 Torr and exposures from 0-10⁶ Torr s. The changes in R during the fast stage of the interaction indicated that oxygen dissolved in the Ti lattice in a few seconds. The maximum increase of 1.1 eV in phi was reached at an equilibrium pressure p approximately 10^-4 Torr. When p was raised above 10^-4 Torr phi decreased by 0.4-0.8 eV which suggests that crystals of an oxide were being formed on the film. For p>1 Torr and exposure >10⁴ Torr s there were further changes approximately=0.2 eV in phi which showed that the oxide was not stable. There was also a change approximately=0.1 eV due to a weakly bound adspecies which was desorbed when the gas was pumped off.

Self-consistent calculations based on the Hartree approximation for electric sub-bands in inversion layers of p-type tellurium are presented. Two surface orientations, (0001) and (1100), in the presence of a magnetic field perpendicular to the surface planes are considered. The main result obtained was that the spin degeneracy of the electric sub-bands is found to be lifted by spin-orbit coupling. With this in mind, a distribution of the electrons over two or three electric sub-bands was obtained. From Shubnikov-de Haas oscillations, the same carrier distribution was deduced within the range of applied electric fields. Two strong resonance lines are expected in cyclotron resonance absorption for both surface orientations and in the (1100) surface, this was indeed found to be the case. In the (1000) case, however, the cyclotron resonance data are obscured by bulk contributions.