Table of contents

A random walk model discussed recently by Marinari et al. (1983) is generalised in the case of gaussian randomness with arbitrary correlation time tau . The exact results for the mean squared displacement provide a direct interpretation of earlier results for static randomness in terms of a crossover from diffusive behaviour for t>> tau to disorder induced ballistic behaviour t<< tau . This analysis also justifies indirectly the additional boundary conditions used in the earlier treatment for static randomness.

The Madelung constants for the transition metal dichalcogenide layer compounds have previously been evaluated by Shen and Liang (1983) by a direct summation method. It is shown that the use of the two-dimensional Poisson summation formula gives a much more efficient method of evaluating the lattice sums.

Previous results for arbitrary clusters of spin ¹/₂ Anderson impurities are generalised to an arbitrary number N of spin components. The model assumes that the number of electrons of each spin component is conserved. The specific heat and the charge and spin susceptibilities are related by Fermi-liquid parameters, which are discussed for some special cases. In the Kondo limit the author obtains N_gamma =(N-1)( pi ²/3)(12 chi _s/(N²-1)) for any spatial configuration of impurities, in particular an isolated impurity and the 'Kondo lattice'. The results are finally generalised to a 'true' Anderson system with (non-conserved) orbital angular momentum.

The authors have analysed the self-consistent charge density of a seven-layer tungsten (001) slab by calculating contributions to the 4f core eigenvalue difference between surface and bulk atoms. This analysis shows that at the surface, charge is positioned farther from the nucleus than in the bulk-implying d to s charge conversion at the surface. This conversion is consistent with a monotonic change in configuration from bulk to free atoms; with Pettifor's theory (1977, 1978) of transition metal cohesion; with increased magnetic moments at transition metal surfaces; and with the possibility of magnetic atoms on the surface of paramagnetic hosts.

A low-temperature proton magnetic resonance study of beta PdH_0.82 in the temperature range 30K<or=T<or=70K is reported. The free induction decay signal is temperature independent over this temperature range. The signal has a characteristic shape which is well represented by the mathematical function f(t)=f(0)exp(-a²t²)J₃2/(bt)/(bt)^3/2 where J_3/2(bt) is a Bessel function of the first kind. Using this function, accurate values of the second and fourth moments are deduced. The second moment is approximately 35% less than the Van Vleck prediction for a randomly occupied rigid lattice of point magnetic dipoles. The ratio M₄/M₂² is as expected for an FCC lattice. The significance of these results is discussed.

The temperature dependence of DC conductivity in granular systems is analysed in terms of the Coulomb quasigap. It is pointed out that whereas the stability criterion proposed by Efros and Shklovskii (1975) gives BE² as the upper bound for the quasigap density of states rho (E), in granulars the functional dependence of rho (E) does not necessarily follow that of the E²-bound. Instead, the detailed shape of rho (E) is a function of the grain size distribution. The authors show that sigma approximately exp(-A/T¹2/) behaviour observed experimentally could arise from a density of states which is a combination of an impurity contribution and the Coulomb quasigap for the grains. The existence of a second quasigap is conjectured.

Optical hole-burning and optically detected nuclear magnetic resonance have been used to determine the hyperfine splitting in the ground, ⁷F₀, and excited, ⁵D₀, states of Eu³⁺ in the stoichiometric material KEu(WO₄)₂. In addition it is shown how the use of an applied RF field during hole-burning can greatly simplify a complex hole-burning spectrum.

Optically detected magnetic resonance (ODMR) has been observed via electroluminescence for the first time. AC electroluminescence was excited by both forward and reverse bias and a quenching signal was observed at resonance. The results are compared with photoluminescence ODMR and PDMR (photovoltaic detected magnetic resonance) signals taken under the same conditions.

A satellite structure to high energy of the K-L_2,3V band spectrum in metallic Mg has been resolved using soft X-ray excitation. The observed energy of the structure is shown to be consistent with that predicted for KL_2,3-L_2,3²V transitions on the basis of excited atom Delta SCF Hartree-Fock calculations. The spectral profile is shown to be adequately reproduced using the K-L_2,3V lineshape and the multiplet structure of the initial and final states.

Photoelectron spectra of the 4f core level in polycrystalline hafnium were recorded using synchrotron radiation. The emission from both surface atoms and bulk atoms was identified in the spectra. The surface core levels were found to be shifted 0.42+or-0.04 eV towards higher binding energies, in agreement with current theoretical calculations.

The soft pure transverse acoustic modes and dimensionality of the soft acoustic subspace have been tabulated for all elastic phase transitions involving a homogeneous deformation of the crystal lattice in the absence of external fields.

The authors investigate numerically the behaviour of the elastic constant of incommensurate ground states of the discrete Frenkel-Kontorova (FK) model below their transition by breaking of analyticity (corresponding to the lattice locking of the incommensurate modulation). They show that the elastic constant is critical and vanishes with an exponent that depends on the irrational commensurability ratio. They propose a scaling relation between this exponent zeta and the exponents of the phonon gap and of the correlation length which are critical from above the transition and which were previously calculated. These results allow one to confirm a previous conjecture which asserts that the Devil's staircase of the discrete FK model is incomplete when the incommensurate configurations are unlocked by the lattice and is complete when they are locked.

Incoherent neutron scattering results obtained near the order-disorder phase transition of p-terphenyl (C₁₈H₁₄) are presented. In this system coherent scattering experiments had shown the appearance of pretransitional short-range order and critical slowing-down of orientational fluctuations. The results represent the first observation of related critical phenomena by incoherent neutron scattering experiments. The measured spectra simultaneously exhibit the effect of single-molecule reorientations as well as the effect of the appearance of short-range order on the single-molecule motion. These results are discussed in terms of the dynamics of orientationally short-range-ordered clusters. Measurements of the order parameter of C₁₈H₁₄ by coherent neutron scattering as well as diffuse scattering data are also presented.

The authors present a simple and efficient method for calculating the wavefunctions and the density of states for the extremely narrow bands of a one-dimensional system with incommensurate lattice potentials. The method is based on the interesting convergence properties of the Green function for this system.

The authors have studied the influence of hydrostatic pressure on the free and bound states in tellurium by submillimetre-magnetospectroscopy. Transmission and photoconductivity spectra were recorded as a function of an external magnetic field for several radiation energies up to pressure values of 5.6 kbar. For P=0 a set of impurity resonances was found, originating from a deep level impurity with a binding energy of 2 meV. The pressure-induced changes in the free-carrier system were found to be consistent with previous measurements under uniaxial stress. The impurity states, however, reveal a more complex response to the hydrostatic pressure.

The eigenvalue equation relevant for investigating Stark ladders (SL) in finite crystals on the basis of the tight-binding approximation (TBA) is derived from a difference equation, which is established using (localised) Wannier functions. The treatment makes use of a two-band model, includes the effect of the electric field on both the Coulomb and the resonance integrals and incorporates the finiteness of the crystal via the use of the periodic boundary conditions. Numerical explorations of the eigenvalue equation are carried out with the help of Householder's tridiagonalisation method in combination with the Q-L algorithm. The energy eigenvalues are computed for various values of the electric field and other pertinent parameters, the parameters involving the interband terms being determined on the basis of the (k.p) method. A thorough scanning of the energy eigenvalues reveals the circumstances under which SL would occur in finite crystals conforming with the conditions adopted. The features of the results are discussed critically, both independently and in the light of other similar results. In particular, the treatment predicts well the features expected of SL in finite systems for low fields; for higher fields, which are likely to introduce Zener tunnelling, the application of the TBA to the problem of SL in finite crystals appears to leave room for question.

A field theoretic model treating weak dense frustration as a perturbation about a Mattis-like theory is motivated through Dzyaloshinskii-Moriya random anisotropy and Callan-Symanzik renormalisation-group equations (1970) are derived up to the one-loop level. No stable fixed point is found. This is shown to hold true up to infinite order due to the symmetry of the model. The critical properties with respect to the frustration parameter at the Mattis transition temperature are studied.

X-band ESR spectra of the binary semiconducting glasses 55 mol.% V₂O₅-45 mol.% MO₂ (M=Ge, Se, Te) have been examined in the range 298-498K. The three spectra at 298K have distinct differences in their V⁴⁺ resonances which are attributed to a monotonic increase in the covalency of the V-O bond on going from GeO₂ to TeO₂ to SeO₂. All three compositions show dramatic and reversible temperature dependences by way of the progressive broadening and eventual disappearance of the ⁵¹V (I=⁷/₂) hyperfine structure. This behaviour has been attributed to a thermally activated delocalisation of the 3d¹ electron spin leading to hopping electronic conduction.

A linear-equations algorithm for determination of the reflectivity extrapolation for the low-energy range in the Kramers-Kronig analysis has been presented. The results obtained for GaTe make possible the comparison of this algorithm with the Leveque method. A better agreement with the experimental data for alpha and R has been noticed. The method of the extrapolation error determination is described.

The temperature dependence of the infrared reflectivity spectra in KNbO₃ is reported for the cubic, tetragonal and orthorhombic phases in a temperature range extending from 300 to 1200K. Spectra have been fitted with a model based on the factorised form of the dielectric function. The temperature dependence of the mode frequencies, the dampings and the oscillator strengths through the successive phase transitions is reported. The experimental data exhibit an unstable mode which decreases continuously in frequency upon cooling across the diverse transitions. The ionic effective charges, the spontaneous polarisation and the static dielectric constant are calculated from these data. All results are discussed in terms of a displacive/order-disorder crossover, which appears especially when the transition from the cubic to the tetragonal phase is approached. A particular phase transition mechanism is proposed, which leads to an understanding of the discrepancy between the experimental and the calculated values of the dielectric constant, the large damping of the soft mode, the incomplete softening of the unstable phonon and the special sequence of phase transitions.

The effect of X-irradiation on the absorption spectra of pure and impurity (Cu, Mn, Ni, Eu)-doped LiNbO₃ has been systematically investigated. Three main bands with peaks at 1.6, 2.5 and 3.2 eV have been observed in pure crystals or Mn-, Ni- and Eu-doped crystals after low-temperature (10K) irradiation. For the Cu-doped samples, a competition is apparent between the production of these bands and the conversion of Cu²⁺ (1.2 eV band) into Cu⁺ (3.1 eV band). From this information and the thermal annealing behaviour of optical bands together with a comparison with the EPR data of Schirmer and Von der Linde (1978), the following assignment of optical bands has been made: the 1.6 eV band to polaron (Nb⁴⁺) states; the 2.5 and 3.2 eV bands to holes trapped at Li vacancies or other lattice defects. The reduction treatments induce a broad absorption made up of two main bands at 2.6 and 3.2 eV. Optical bleaching experiments at 10K, turn these bands into the same 1.6 eV (polaron) band of irradiated crystals. For Mn- and Cu-doped samples those treatments induce, as a first stage, the reduction of Mn³⁺ and Cu²⁺ which is then followed by the formation of the same bands of pure crystals. These results have led us to associate both bands with F centres (2.6 eV) and F⁺ centres (3.2 eV).

In recent years, Alvarez Rivas and co-workers (1972, 1974) have proposed a model for thermoluminescence in alkali halides in which the mobile entity is an interstitial, and not a hole or an electron as assumed in the conventional models. Recently, Sagastibelza and Alvarez Raivas (1981) have extended the model to LiF-TLD 100. In this paper some anomalies are pointed out. It is suggested that the model has to be suitably modified to remove such anomalies before it can be generally accepted.

Typical spectra from experiments involving reflectivity, ellipsometry, attenuated total reflection, and electron energy-loss measurements are calculated for a metal overlayer system. The response is computed in all cases via hydrodynamic models of the electrons' collective motion. Common features of the various spectra are emphasised, in particular the presence of standing plasma waves trapped in the overlayer. Although a quantitative description of these modes is a sensitive function of surface boundary conditions, their appearance is assured by the mere allowance of spatial dispersion. The structure they produce in various spectra is illustrated.

The influence of the occupation of higher sub-bands on the screening in the ground sub-band of n-type inversion layers at the (100) silicon surface is studied. Some numerical results are presented for commonly used inversion carrier concentrations, finite temperatures and transverse magnetic fields. It is shown that many-body effects increase the screening above 40K for a magnetic field of 10 T, especially for small values of the wavevector.

Reports a series of experiments to investigate resistive inhomogeneity in discontinuous metal films. It is shown that in the highly activated regime of conduction, inhomogeneity is on the scale of small groups of islands or of individual islands; but, as the metal-insulator transition is approached the scale becomes very large, many tens of micrometres, with dramatic macroscopic consequences. The transition is shown to be a percolation threshold phenomenon with large areas of films remaining poorly connected to the metallic network. The metallic channels are highly tortuous with critical links at separations of tens of micrometres. It is argued that similar inhomogeneity must occur in all systems that show Anderson transitions so that such transitions should always be viewed as critical phenomena with a diverging range of fluctuations as the transition is approached. This conclusion has important implications as regards consequences to size of experimental structures. It provides a possible explanation for differing experimental values of maximum metallic resistivity; but the experiments show that it is not a general explanation for fractional temperature-dependence of resistivity in systems to which other proposed models cannot apply.