Table of contents

Neutron scattering measurements on DCsDA (CsD₂AsO₄) have failed to show ferroelectric critical scattering above T_c, but have provided evidence for a complex structure in the ferroelectric phase with periodicities of approximately 30, 15, 10 and 5 lattice spacings.

The original arguments by Hernandez and Ziman (see abstr. A52796 of 1973) regarding the possibility that electrons may be trapped by low density fluctuations in helium are shown to be too restrictive. Trapping may occur providing that the fluctuations are several standard deviations less than the average. The probability for such trapping is still large enough to account for the experimental results.

A crystal field analysis of the optical spectra of Nd³⁺ in YAG observed by Feofilov et al. (1965) has been made. The parameters obtained by the least-squares method reproduce 24 experimental energy levels of the ⁴I term with a standard deviation of 3.7 cm^-1. The calculated g-values, g_x(1.784, g_y=-1.094, g_z=-3.953, are in good agreement with mod g_x mod =1.733, mod g_y mod =1.179, mod g_z mod =3.915, the EPR values reported by Wolf et al. (1962).

The direct correlation function (DCF) of liquid methane has been evaluated from X-ray diffraction data at 186K. The Mikolaj and Pings method was applied to obtain the estimated DCF below the hard-core diameter. A small valley has been observed at 4.6 AA in the DCF curve and this has been attributed to possible octopole-octopole interactions. From the calculated DCF, the potential energy parameters are obtained using hypernetted chain Percus-Yevick and the mean spherical model equations. The DCF was found to give a maximum at 4.3 AA and this has been ascribed to the nearest-neighbour distance. The present calculations clearly show that the DCF decays to near-zero values in about two molecular diameters, thus proving the short-range nature of this function even for a polyatomic molecular liquid like methane.

It has been observed that the thermoluminescence of heavily gamma-irradiated crystals of NaCl and KCl is inhibited up to temperatures higher than those corresponding to less irradiated crystals. In NaCl it is clearly seen that the vacancy-centre aggregation processes including the formation of the colloidal band are operative before thermoluminescence occurs. During the thermal bleaching of the colloidal band F-centres are produced which in turn are annihilated by interstitials. The temperature of the maxima of the glow peak agrees well with the temperature at which the radiation-induced hardening anneals out. As in the model proposed in a previous paper (see abstr. A16630 of 1972 and A8677 of 1973), it is concluded that the thermoluminescence in irradiated alkali halides is due to the recombination of mobile interstitials with vacancy centres; at some stage in this process light is emitted.

The lattice spacings of thin films of bismuth have been measured (as a function of film thickness) using an electron diffraction method. The films, which varied in thickness between about one and twenty nanometres, were deposited on carbon and formvar bases. Referring the structure of bismuth to hexagonal axes, it was found that as the film thickness decreased the lattice parameter a decreased by up to 1.3%, while the c/a ratio increased by up to 2.5%. The changes in a and c were observed to be greater on formvar bases than on carbon bases.

The Hubbard hamiltonian is analysed by means of Gutzwiller's variational scheme for both the nondegenerate and the degenerate band models. The single-particle excitations in the correlated ground state, which is determined by a balance between the electron hopping energy and the intra-atomic interaction energy, are investigated for arbitrary electron density and arbitrary correlation strength. It is found that the single-particle excitation energies forming a quasi-energy band. The centre of gravity of the quasi band is shifted from that of the bare band, and the bulk part of the density of states function for the quasi particles is narrowed by the correlation effect. Both the shift and the narrowing are spin-dependent. In the limit of strong correlation, our results agree with those obtained by Harris and Lange (1967) and Roth (1969).

The decoupling transformation is used to establish approximate relations between Coulomb matrix elements referred to Wannier and Bloch states in a linear lattice. This suggests the introduction of several alternative parametrization schemes which generalize the Hubbard hamiltonian. An exact solution for the energy eigenstates of a two-site lattice is obtained, which gives an approximate criterion for the existence of an antiferromagnetic ground state for a linear lattice of arbitrary length.

A model containing two hybridized narrow electron bands is considered. It is shown that in such a model a metal-insulator transition occurs connected with a jump in the conductivity. Some analogies to the Lifshitz instabilities are discussed. As a possible example for such a behaviour, the properties of VO are studied both qualitatively and quantitatively.

The influence of a homogeneous electric field on the energy levels of two-dimensional Wannier excitons has been calculated. First of all, the unperturbed effective-mass equation has been solved in parabolic coordinates. The energy levels of the perturbed Schrodinger equation are then approximated by the WKB method. The validity of the method and the results are discussed.

Conductivity measurements have been made on carefully prepared compactions of solid solutions of violanthrene-iodine. The results for these charge-transfer complexes have been compared with those reported for other amorphous solids. The data, which satisfy the Meyer-Neldel rule, may be explained in terms of a narrow-band semiconductor in which charge movement occurs via an activated hopping process with mobility approximately 10^-3 cm² V^-1s^-1.

A theoretical study of the tunnelling phenomenon through a metal-insulator-metal junction containing interacting paramagnetic impurities, is made. The impurity spins are assumed to be interacting with each other through 'super exchange interaction'. Expressions for conductance-voltage-temperature characteristics have been obtained starting from magnetic parameters of impurity interaction in an 'isolated pair model', considering that impurities are randomly distributed in a two-dimensional plane near metal-insulator interface in the barrier region. Agreement between experimental and theoretical results is satisfactory.

A ferromagnetic binary alloy is considered where each component has spin S_A, S_B and concentration X_A, X_B=1-X_A, respectively. The interaction is given by the Heisenberg hamiltonian with exchange constants J^{alpha beta} between nearest-neighbour spins S_alpha , S_beta ( alpha , beta =A or B). It is shown that neglecting fluctuations in the static field seen by a spin S_A (S_B), generalized CPA equations can be written for the configurational average matrix G_ij, with components G_ij^{alpha beta} . The formalism is symmetric in both components and valid for arbitrary values of the exchange constants and spins. In the general case it is expected to be good only for intermediate values of the concentrations, because it fails to reproduce the known exact results in the dilute limit X_alpha <<1( alpha =A or B) due to the approximate treatment of the local field that suppresses the p and d modes and modifies the energy of the s modes. However, it is shown that the theory reproduces the exact results when the values of the impurity and host spins are restricted by the condition (S_alpha J^{alpha alpha} /S_alpha J^{alpha alpha} )=1, where alpha =A or B if alpha =B or A.

The superposition model is used to analyse ESR data for Gd³⁺ and Eu²⁺ in strained cubic crystals. The quadrupolar (n=2) parameters derived from this data are shown to be consistent with the model proposed by Newman and Urban (1972). It is also shown that the n=4 parameters are consistent with the hypothesis that they represent a simple superposition of contributions from the coordinated ligands.

The principal EPR spectrum from AgCl:Cr³⁺ at 95K is orthorhombic with spin hamiltonian parameters g=1.980+or-0.004, D=30.5+or-0.2 mT and E=14.8+or-0.1 mT. The spectrum is compatible with the monoclinic centre of two vacancies associated with the Cr³⁺ ion suggested by Kunze and Muller (1970). At 400K the vacancies are no longer bound to the Cr³⁺ ion and the spectrum reduces to a single line with an intensity approximately equal to the total integrated intensity at 95K once allowance is made for the Boltzmann factor. At temperatures between 95K and 400K the outer fine structure lines broaden due to vacancy motion and an activation energy of 0.21+or-0.04 eV is derived. Similar spectra are observed from AgBr:Cr³⁺ but with larger field splittings. The principal spectrum from AgBr:Cr³⁺ at 95K is orthorhombic with (100) axes and spin hamiltonian parameters g=1.990+or-0.003, D=110.8+or-0.4 mT and E=66.8+or-1 mT. The larger g value and linewidth in AgBr are attributed to an increase in covalency.

The EPR spectrum of ⁶³Cu²⁺ in NH₃Cl has been remeasured as a function of temperature between 5K and 300K, and is found to be in substantial agreement with all previously reported results. An intensity anomaly and the otherwise apparently tetragonal symmetry above 7K are satisfactorily accounted for by means of a qualitative dynamical model based on a site symmetry which is orthorhombic. No discontinuity in g-values or hyperfine splittings occurs at 7K, below which the rhombic Cu²⁺ sites are frozen in.

The validity of a four-parameter semi-quantum (FPSQ) model to fit infrared reflectivity spectra of crystals having a large number of phonon branches is discussed in view of the results of anharmonic quantum theories. The form of the quantum damping function may indeed be simplified when the two-phonon density of states resembles a broad continuum in the vicinity of the resonance frequency. Infrared reflectivity data of corundum obtained at temperatures ranging from 77 to 1775K are fitted with the aid of the FPSQ model. The temperature dependence of damping of TO and LO modes is analysed and it is found that phonon lifetimes are essentially limited by anharmonic three-phonon coupling described by the cubic hamiltonian consistent with two qualitative arguments.

Raman spectra of KCl:Pb crystals have been measured after heating. They show a change in the intensity depending upon the temperature to which the crystals are heated. Comparing the projected density of states of pure KCl with these spectra, a model explaining the data is proposed.

Quasi normal-incidence reflectivity measurements in the 2-11 eV region at 300 and 80K are reported for the magnetic insulators NiCl₂ and NiBr₂. The data has been discussed in relation to a possible energy-band scheme. The authors conclusions can be summarized as follows: the peaks up to approximately=7.7 eV in NiCl₂ and up to approximately=7 eV in NiBr₂ are due to a charge transfer process from the p states of the halogen (valence band) to the localized d states of the Ni²⁺ ion; the structures at higher energies are probably due to excitonic and band-to-band transitions (halogen p states to metal s states).

From energy-loss measurements of fast electrons on solid cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, benzene, toluene, monochlorobenzene and pyridine the energy-loss functions Im (- epsilon ^-1) are calculated. The epsilon ₂ spectra of these polycrystalline substances are obtained by means of a Kramers- Kronig analysis and are compared with the results of measurements taken on the same substances in the vapour phase. Some qualitative rules for the different behaviour of the optical properties in the two states of aggregation are deduced.