Phonon softening in superconducting diamond

We observe strong softening of optical-phonon modes in superconducting (T-c=4.2 K) boron-doped diamond near the Brillouin-zone center using inelastic x-ray scattering from a chemical vapor deposition grown highly oriented sample. The magnitude of the softening, and our observation that it becomes stronger approaching zone center, supports theoretical models suggesting a phonon-mediated pairing mechanism via coupling of optical-phonon modes to Fermi surfaces around the zone center. The electron-phonon coupling parameter is determined as approximately lambda=0.33.