Light scattering studies of phonon anomalies and superconductivity in A15 compounds

Light scattering studies of phonon anomalies and superconductivity in A15 compounds and Nb, V, and Ta are presented. The experimental methods included one-and two-phonon and electronic Raman scattering and Brillouin scattering from Surface Acoustic Waves (SAWs). The Eg optic phonon in V3Si, Nb~n, and V3Ge is shown to have an anomalous width, shape, and temperature dependence. These anomalies are attributed to coupling of the Eg phonon to interband electronic transitions between the flat bands emanating from the r 12 level. A simple model of the interaction quantitatively reproduces the temperature dependence of the Eg phonon linewidth. Raman measurements on p-irradiated V3Si reveal a disorder induced one-phonon density of states (DOS) spectrum for which the overall features and peak positions are in good agreement with the phonon DOS as determined by neutron scattering. The data suggest that irradiation produces a homogeneous distribution of defects which reduces the electronic DOS, and hence the electron-phonon coupling, due to a disorder induced reduction in the electronic mean free path. Raman scattering measurements on Nb3Sn, V3Si, and Niobium show for the first time new peaks in the superconduct1ng state close in energy to the superconducting gap 28. These are attributed to direct electronic Raman scattering by pairs of superconducting quasiparticles. A theory is presented which fits the data well and provides evidence for anisotropy of the gap (-20 %in Nb3Sn) and of the matrix element for electronic Raman scattering. It is demonstrated for the first time that Brillouin scattering from SAWs can probe bulk phase transitions in opaque materials. The measurements provide evidence for a depth dependent martensitic phase transition temperature, Tm, in Nb3Sn, with the free surface nucleating the transition in the outer -200 A -15 K above the bulk Tm. Two-phonon spectra from Nb, V, and Ta are presented and compared with a2F from tunneling measurements and with the phonon density of states from neutron measurements. There is good overall agreement, with some differences assign~d to enhanced electron-phonon matrix elements for anomalous phonons.U of I OnlyThesi