Direct Observation of the Superconducting Gap in Phonon Spectra

We show that the superconducting energy gap 2 Delta can be directly observed in phonon spectra, as predicted by recent theories. In addition, since each phonon probes the gap on only a small part of the Fermi surface, the gap anisotropy can be studied in detail. Our neutron scattering investigation of the anisotropic conventional superconductor YNi(2)B(2)C demonstrates this new application of phonon spectroscopy.