Table of contents

The variation of electronic energy loss of low-velocity heavy ions in planar channelling is calculated as a function of the position of the ion in the channel. The model used is that applied recently to explain Z₁ and Z₂ variations in electronic stopping power, in which the heavy ion is assumed to lose energy by transferring momentum to the target electrons which scatter elastically from it. The results for 60 MeV ¹²⁷I ions in the (100) channel are compared with experimental results and with previous calculations.

In the light of an increasing number of disagreements between thermomagnetic data for bismuth and the theoretical expectations, the authors emphasize and demonstrate the basic theoretical assumptions.

Charging and discharging current transients have been measured as a function of voltage and film thickness on thin sputtered films of Al₂O₃ in Al-Al₂O₃-Al junctions. These currents decay with time as I varies as exp(-t/ tau ) for short times and t^-n1 for longer times, followed by a t^-n2 dependence. The relaxation time tau primarily increases with voltage and saturates for higher voltages. The coefficient n₁ tends to increase with decreasing voltage, while n₂ increases with voltage. Current theories of I-t response based on space charge and polarization effects do not generally predict the two-stage t^-n dependence. Their physical bases are outlined, and possible modifications to them are briefly discussed.

Contrary to previous work (see abstr. A57113 of 1972 and abstr. A29602 of 1975) the author points out that insulating states of the Peierls or Slater types are not possible in actual three-dimensional systems unless the relevant interaction strength (electron-phonon coupling, or U) exceeds some finite threshold value. There are several distinct effects which contribute to this result.

Formation of aggregates of hole centres (called E centres) in SrCl₂:M⁺ (M⁺=Rb⁺, K⁺, Na⁺) by X-irradiation between 130K and 230K is studied. The results show that the E centre is formed from two positive hole centres, the energy of formation being 0.12 eV. The dichroism induced in the E band at 80K ( lambda =263 nm) shows that the centre has a four-fold symmetry axis. The main properties of the E centre-thermal stability (230K), preferential orientation under optical excitation at 80K, thermal reorientation (160K) and photoexcitation at 5K-are described and compared to the properties of the di-interstitial centres (V₄ centres) in KCl. The analogy between the properties of the V₄ and the E centres supports the hypothesis of similar structure, these centres likely being quasi-molecular complexes.

An investigation is made of the electronic structure of disordered networks by utilization and extension of the techniques of Halperin (1965) applied to a tight-binding model. Both the averaged-local and the averaged-total density-of-states functions are determined for a Cayley tree with arbitrary cellular disorder from self-consistent integral equations derived by comparing successively larger Cayley trees. For Lorentzian disorder, it is shown that these techniques reproduce the results of Lloyd (1969) for the averaged-local density of states. The applicability of these methods to a network of arbitrary topology is then quantitatively examined, and an expression is derived for the breakdown of these techniques as a function of energy, disorder, and the local atomic environment of the network. Further it is argued that this breakdown is associated with the transition between localized and extended states; and it is found that these results compare favourably with DC electrical conductive behaviour in amorphous solids.

A solid-state phase transformation in octafluoronaphthalene has been discovered at 266.5K on cooling, and at 15K higher on heating. The symmetry of both phases is found to be the same, namely monoclinic with space group P2₁/c. The unit cell parameters change by up to 10%, but the integrity of a single crystal, which shatters on cooling, is good enough for a single-crystal structure determination. This has been done in both phases to a sufficient accuracy that a mechanism for the transformation can be proposed. Molecules which lie parallel to one another shear to a new parallel position, the shear movement being equal to one carbon-carbon bond of the naphthalene skeleton. In this process the molecules reorient, but are still related by the same symmetry operations. This transformation, although not unique, is probably the first of its kind to be discovered in molecular systems.

Statistical fluctuations of the hydrogen density in metals in thermal equilibrium are analysed in terms of eigenfunctions of the elastic interaction between the protons. A general relation connecting the density fluctuations with the anelastic relaxation strength as measured in Gorsky-effect experiments is derived. The macroscopic range of the elastic interaction leads to a strong suppression of the fluctuations and the corresponding spinodal temperatures. Examples are discussed where the spinodal temperature of a single eigenmode can be observed. The results are compared with experiment.

Measurements have been made of the angular spreading of initially well defined beams of phonons of energies h(cross) Omega /k_B<or approximately=12K in liquid ⁴He at pressures between SVP and 24 bar. For bath temperatures T<or approximately=0.15K the spreading is independent of T and is unaffected by the addition of 15 ppm of ³He. The spreading increases with increasing source temperature and decreases with pressure, becoming undetectable (<or approximately=2 degrees ) at P>or=17 bar. The decay lengths of the processes giving rise to this spreading are found to be approximately 1 mm. Various scattering mechanisms are considered and it is concluded that the results are consistent with a spontaneous three-phonon decay process allowed when the phonon dispersion curve initially deviates upward from its asymptotic linear form.

For pt.I see abstr. A80956 of 1972. A number of methods previously used for calculating the local electronic structure for tight-binding bands were based on the local atomic environment without the use of Bloch's theorem. In applying the recursion method to a variety of problems, the authors have simplified, generalized and refined their original ideas. The authors (i) present a more definitive statement of the recursion method (ii) gather together a number of practical points arising from the computation of the continued-fraction expansion of the Green function and (iii) relate the techniques to other approaches to the same problems.

The physically interpretable contributions to the valence-band energy of the linear H₂ crystal models have been calculated by the Musher-Amos-Murrell-Shaw symmetry-adapted perturbation theory. It is shown that the molecular level is shifted by the first- and second-order electrostatic contributions independent of the wavevector k. The second-order electrostatic energy is negligible in comparison to that of the first-order, which is the crystal field effect. The second-neighbour crystal-field contribution can be approximated as the electron-quadrupole interaction. The exchange contributions depend on k. Among them the only practically important term is the first-order exchange energy corresponding to the first-neighbour interaction. The extrapolation of the present results to more complex molecular crystals is discussed.

Theoretical expressions for the photo-ionization cross section of deep impurity levels are developed. With the approximations of a delta-function potential for the impurity atom and a free-electron-like mass for the donor electron it is shown that good agreement with data for oxygen-doped GaAs is obtained. The expressions are then further generalized so as to be valid for both non-spherical and non-parabolic bands and are applied to recently published photo-ionization cross sections for Si:Au and Si:Zn. For all the cases studied, the authors find effective field ratios close to unity. The optical threshold energies for Si:Zn are reevaluated and agreement with the thermal ionization energies for both acceptor levels is found. The bands in the 110 direction as determined by the kp method do not influence the cross section in the manner expected.

The dispersion relation for plasma oscillations in a thin layered film is calculated in the SCF approximation for a model in which the electron motion along the film thickness is described like a tunnelling process between adjacent planes. For a semi-infinite medium the linear term in the dispersion relation of the surface plasmon is obtained. This is done using an appropriate expansion technique and in the limit of a 'create' separation between adjacent planes.

The spin-¹/₂ XY model with quenched impurity sites is studied on three cubic lattices. Series of degree seven in inverse temperature at fixed magnetic-ion concentration and series of the same degree in magnetic-ion concentration at fixed temperature are generated for both the zero-field free energy and the zero-field fluctuation in the long-range order. The high-temperature series are too short for satisfactory analysis. The low-concentration series for the fluctuation however yield estimates for the critical exponent gamma , which appear to depend on temperature. The phase transition curve is estimated for all three lattices.

The full Stokes vector of the first-order Raman spectrum has been measured for various scattering geometries, and the results compared with theory. Hence it is shown that the long-wavelength optical vibrations in the zinc-blende structure have the symmetry of the vibrations of a tetrahedral molecule. It is also shown that most of the information contained in the first-order Raman spectrum can be obtained by measuring only a single component of the Stokes vector in a very limited number of scattering geometries.