Date of Award
Doctor of Philosophy (PhD)
Professor B. N. Brockhouse
The technique of inelastic scattering of thermal neutrons offers a versatile, yet powerful, method of observing the spectra of elementary excitations in condensed matter. In this thesis, this technique was used in the determination of the phonon spectra of three metals, using monocrystalline specimens.
The first study concerns the temperature dependence of the phonon spectrum of Cu in the range 20ºC to 1063ºC, i.e. up to 20ºC below the melting point, and in the various directions of high symmetry accessible in a (001) scattering plane. Resolution effects were taken into account in the determination of intrinsic phonon frequencies and linewidths. The results indicate a general softening of the lattice as a function of temperature, the vibrations of transverse polarization being significantly more affected (on a relative basis) than those of longitudinal polarization. Most of the signals have remained observable up to the vicinity of the melting point. A high resolution study of the small wavevector domain was also performed in order to determine the temperature dependence of the (adiabatic) elastic constants in a temperature range inaccessible to the usual ultrasonic methods. Of particular interest is the fact that the T1 branch was found to manifest a peculiarity at the approximate reduced wavevector ζ = 0.2; a careful examination did not reveal any corresponding distinctive behaviour in the linewidth (lifetime). A search for possible manifestations of the Kohn effect at high temperature was also performed at other positions in reciprocal space.
A similar study (extending up to 1050ºC) was also carried on the small wavevector phonons in Pd. It is known, from other workers, that this substance shows a peculiar temperature dependence in the T1 branch, this dependence being opposite to that observed in Cu.
Finally, the third study deals with an unsuccessful attempt to observe a predicted weak Kohn effect in the room temperature phonon spectrum of W.
Technical details concerning the apparatus built for the high temperature measurements are to be found in chapter III following a review chapter on the crystal dynamics of metals and the technique of triple-axis spectroscopy, with particular emphasis on the optimization of experimental variable and the correction of resolution induced effects.
In appendix, various projects not having an immediate relevance to this research are discussed, i.e. triple-axis spectrometer automation by paper tape, compilation of a bibliography covering most of the literature concerning thermal neutron scattering, various computer programmes.
Larose, André, "Phonons in Copper at Elevated Temperatures" (1975). Open Access Dissertations and Theses. Paper 932.