Table of contents

Cottey (see abstr. A76531 of 1971) discussed solutions of the Schrodinger equation for periodic one dimensional potentials terminated by infinitely abrupt barriers. Considering earlier calculations done by the authors, using different forms of finite surface potentials, it is shown that Cottey's results depend strongly on the special potential barrier chosen.

Up to six magnetophonon peaks are observed in the transverse magnetoresistance of high mobility samples of n-CdSe. After corrections are applied for the nonparabolicity of the band and the polaron effect, the 'bare' band edge masses are deduced to be 0.115 m for B perpendicular to c and 0.120 m for B//c. The corresponding low frequency masses are 0.122 m and 0.127 m. Magnetophonon extrema are also observed in the longitudinal magnetoresistance.

A zero-phonon line has been observed in additively coloured CaF₂ at 677-4 nm and in additively coloured SrF₂ at 769.2 nm with an associated broad absorption band on the high energy side. The broad band shows a partly resolved phonon structure related to the single phonon density of states of the crystal. The zero-phonon line and associated broad band are also observed in fluorescence. The excitation spectrum for the fluorescence has been investigated. It is suggested that the centre giving rise to the spectrum is an F₃ complex composed of three nearest neighbour F centres aligned along (100). It is shown that the F₃ complex may be reoriented by linearly polarized bleaching light.

For pt.I see ibid., vol.5, no.3, 257 of 1971. It is suggested that a zero-phonon line in CaF₂ at 677.4 nm and in SrF₂ at 769.2 nm arises from excitation of an F₃ centre composed of three nearest neighbour F centres aligned along (100). Uniaxial stress measurements on the zero-phonon line show that the transition occurs between orbital singlet states in a centre with tetragonal symmetry and that the dipole moment of the transition is parallel to the tetragonal axis of the centre. The zero-phonon lines show a quadratic Stark effect. The model proposed for the centre giving rise to the zero-phonon line is consistent with the experimental results given in the present paper and in the previous paper.

Zero-phonon lines are found at 732.7 nm in CaF₂ and 835.0 nm in SrF₂ when additively coloured crystals containing F₃ centres are exposed to ionizing radiations at low temperatures. The centres giving rise to these lines are not stable at room temperature. The maximum obtainable concentration of the centres is proportional to the number of F₃ centres present in the crystals. Measured uniaxial stress and electric field effects are similar to those found for F₃ centres. It is suggested that the new centres are negatively charged F₃ centres (i.e. F₃^-).

Methods of thermally stimulated conductivity (TSC) have been applied to study process of defect formation by nonionizing UV radiation. Assuming an excitonic model for the creation of point defects, an expression for the intensities of the conductivity glow peaks as a function of the dose of the exciting monochromatic UV light has been developed and the dependence of the TSC on the absorption coefficient of the crystal is discussed. KBr and KI crystals were irradiated at 80K by monochromatic UV light in the region 190-270 nm and the TSC was measured during the consequent heating to 400 K. The main conductivity glow peaks appeared in KBr at 110, 165 and 315 K and in KI at 125 and 260 K. Thermal activation energies of some glow peaks were evaluated and their reaction kinetics were found to be of first order.

A point defect with a long range interaction changes the dynamical matrix of the ideal crystal in infinitely many elements. Therefore it is not possible to use the same procedures as in the case of a defect with a short range interaction. A method for the computation of the Green function in the case of a long range interaction defect is shown.

The dispersion curves were calculated along various symmetry directions of sodium, a BCC structure, using both the de Launay models and the model proposed by Clark, Gazis and Wallis (CGW model) for angular forces. It was found that both models give nearly the same results, which agree very well with the experimental data. In the case of FCC structures, the two models give different results. The reason for this difference in the behaviour of the two models for FCC and BCC structures is discussed.

The spectral limits of a disordered system are given in a tight binding model, using general criteria for the localization of eigenvalues of matrices. The width of the electronic density of states is directly related to the short range order of the system and various cases of disorder are discussed. The shape of the density of states near the band edges is studied, using an expansion in its moments. Special attention is paid to some amorphous materials whose structure can be described by an ideal random covalent network. Sharp band edges are found in agreement with recent experiments on amorphous Si and Ge.

The electric-dipole intensities of Er³⁺ spectra in six different host lattices have been calculated on the basis of Judd-Ofelt theory, which takes into account the effect of a mixing of higher configurations of opposite parity into the fⁿ configuration through odd parity spherical harmonics in the ligand field interactions. The magnetic-dipole intensities have also been calculated, and the sum of the contributions from these two mechanisms has been used to account for the observed intensities in the optical spectra of Er³⁺. Tje phenomenon of hypersensitivity is discussed. The complete energy matrices of the f¹¹ electronic configuration have been diagonalized to calculate the energy levels of Er³⁺, and the eigenvectors thus obtained were used in the intensity calculations. The splittings of a few lowlying LS multiplets of the f³ configuration in the 'intermediate coupling' scheme are also shown schematically as a function of zeta /F₂.

The effect of biquadratic exchange on the asymptotic or paramagnetic Curie temperature theta is calculated using a classical-spin Heisenberg hamiltonian. Using both a Bethe-Peierls-Weiss cluster approximation, and the high temperature series expansion (Kramers-Opechowski) method, there is found to be no change in theta .

A spin wave theory of a dilute anisotropic Heisenberg ferromagnet is discussed. It is shown that it is possible to discuss the stability of the ferromagnet state in terms of the spin wave spectrum in the case where the anisotropy is small ( eta approximately=1). In this case the critical concentration is given in the lowest order by 0.296 (1-0.355(1- eta )). The criterion is valid for T=0 K and it is briefly indicated how to generalize the calculation to finite temperatures.

The second-harmonic generation coefficients of sodium chlorate and sodium bromate crystals are estimated from their optical rotatory powers by means of the anharmonic oscillator model of Condon, Altar and Eyring. The correct signs are determined, and reasonably good numerical agreement with experiment is obtained.

Nearest neighbour Mn²⁺ pairs in MgO and CaO have been studied using EPR. The isotropic exchange interaction measured in MgO cannot be written in the simple form Js_i.s_j and it is shown that although a small contribution due to intrinsic biquadratic exchange is possible, the anomalous effect arises mainly as a result of exchange striction. This exchange striction is found to be consistent with a variation of J with ionic separation r given approximately by J=J₀ exp(-19r/r₀), and this variation gives good agreement with exchange interactions deduced for MnO. Exchange striction effects also appear in the crystal field interactions and in the magnetic dipole-dipole interactions for both pair systems.