Table of contents

The authors have observed striking minima in the spectrum of the output power against magnetic field of the InSb spin-flip Raman laser near 10.6 mu m. These minima, which occur in the Stokes, anit-Stokes and double Stokes emissions, are attributed to harmonic cyclotron resonance absorption of the Stokes scattered radiation in the InSb crystal. Independent studies of the transmission of the sample as a function of magnetic field support this interpretation.

A comprehensive perturbation theory of laser light modulating an electron beam within a crystal shows that the finite size of the crystal plays an essential role. In particular the theory of the central diffraction spot supports the conclusions of Van Zandt and Meyer (1970). Bremsstrahlung provides an explanation of the light emitted from a central spot if but only if the incident electron beam is sufficiently monochromatic. The intensity of the light emitted by other processes is too weak to be observed.

Spin wave theory of the magnetocrystalline anisotropy has been re-examined to incorporate the zero point correction due to the ellipticity of the spin precession. For the sixfold anisotropy of Dy, this correction was found to be very important.

The echo response of methyl protons to in-phase and in quadrature RF pulse sequences is used to show that the A-type symmetry species of a methyl group may behave at low temperatures in a similar way to a single particle of spin 3/2 subject to quadrupole interactions.

The K-absorption discontinuity of arsenic has been studied in the pure element and in six compounds in which it is trivalent. It is observed that the discontinuity shifts towards the high energy side in certain compounds and towards the low energy side in some others. It has been shown that the positive or the negative shift depends upon whether the absorbing ion is a cation of an anion. The magnitude of the shift seems to depend upon the effective ionic charge on the absorbing atom.

The exact solution of the one dimensional Dirac equation for an electron in a semi-infinite chain of Kronig-Penney type potentials (Tamm model) is presented. Expressions are derived for the bulk and surface states. It is shown that all the surface states obtained are simply relativistically corrected Tamm states. This is contrary to the prediction of Davison et al (1969-71) that a relativistic calculation yields a new type of states (called by them Dirac surface states) having no analogue in the nonrelativistic treatment of the problem. The discrepancy is explained.

The results of numerical work on the Anderson model of disordered systems are presented. The sensitivity of the eigenvalues to the choice of periodic or antiperiodic boundary conditions is used as a criterion for localization, and the theory of this criterion is discussed. For the two dimensional square lattice this criterion gives a reasonably sharp result for the onset of localization which is not in conflict with other criteria of localization, and it is found that localization occurs far more easily than Anderson's theory suggests. For the diamond lattice the onset of localization is less sharply defined, and localization occurs less easily than for the square lattice, but more easily than in Anderson's theory.

Using electronegativity differences the interactions within the primitive cell of a typical substitutional alloy of two binary compounds are estimated. It is shown that the important interactions for energy gap bowing are those between intersubstitutional atoms. From this and a virtual crystal approach, trends in the bowings are explained. Bowings in some substitutional alloys of practical interest are estimated.

A system of nonlinear integro-differential equations was constructed to describe the phenomenon of the phonon avalanche as a function of space and time in a paramagnetic crystal. The spatial effects were taken into account by a transport probability kernel which is similar to the Holstein-Biberman model of photon imprisonment in gases. The equations were then applied to the simplified one dimensional case simulating the geometry of an infinite slab. Calculations were made for dilute Ce₂Mg₃(NO₃)₁₂.24H₂O. Semiquantitative agreement between theory and earlier experimental data was obtained. The remaining discrepancies are thought to be due to the neglect of consideration of spectral spin diffusion in the present model.

For Pt. I see ibid., vol.5, 828 (1972). The theory of the phonon avalanche in paramagnetic crystals is generalized to include diffusion of the magnetic excitation in addition to the spatial transport of acoustical excitation treated in Pt.I. Quantitative agreement between theory and earlier experimental data is found to be further improved.

A set of variation states is constructed for a defect with degenerate states of E symmetry interacting with the modes of vibration of a cubic crystal. A projection operator is used to ensure that the states have the symmetry corresponding to the hamiltonian of the system. The energies of the ground and some excited states are derived together with the reduction factors for perturbations. Comparison shows that the cluster model cannot consistently give the values of these quantities except in the limit of strong interactions.

Calculations have been made of the likely ionic distortions caused by incorporation of the Tm²⁺-H^-(s) trigonal centre in the CaF₂, SrF₂ and BaF₂ lattices. The resulting trigonal contributions to the crystal field are calculated using a scaled point charge model and compared with experimental values derived from the g value anisotropies, assuming the A₂⁰(r²) distortion to dominate.

Ultraviolet absorption spectra of thin films of polyethylene and of the paraffins C₂₈H₅₈ and C₄₂H₈₆ in the range 6-12 eV have been measured at 300 K and 4 K. This range covers the onset of intrinsic electronic excitations. The significance of the results is discussed briefly.

Optical reflection and transmission measurements have been made between 6 eV and 13 eV on specimens of natural muscovite, phlogopite, biotite and a synthetic fluorphlogopite. Reflectivity peaks are observed near 9.9 eV and 11.8 eV and are thought to be due to the silicate structure of mica. An absorption edge occurs near 7.85 eV in synthetic mica. Additional structure is observed above 7.8 eV in the magnesium micas but not in the aluminium micas. The absorption of phlogopite mica is unusually low in the high energy region. Below 7.8 eV absorption is believed to depend on transition metal impurities in the mica.