Absorption of ballistic phonons by quasiparticles in superconducting Pb

Abstract

Phonon imaging of single-crystal Pb at T=2 K shows strong absorption of ballistic longitudinal phonons with wave vectors in {1 1 1} planes. A calculation of the phonon-scattering from quasiparticles on the anisotropic Fermi surface of Pb indeed shows concentrated scattering in these planes; however, the temperature dependence of absorption suggests an anisotropic gap with a minimum that is much lower than the accepted gap of Pb.