Evidence for large gap anisotropy in superconducting Pb from phonon imaging

We report the first ballistic phonon images of superconducting Pb. Unusual absorption lines are observed for phonon wave vectors in 111 planes. We show that a highly anisotropic energy gap can lead to sharply defined directions of phonon attenuation. Overhauser and Daemen [Phys. Rev. Lett. 61, 1885 (1988)] postulated a spin-density-wave ground state for Pb that leads to directions of strongly reduced gap. By applying their idea to the actual Fermi surface of Pb, we predict phonon attenuation directions consistent with the data.