Phonon softening in superconducting diamond

Hoesch, M.*; Fukuda, Tatsuo  ; Mizuki, Junichiro; Takenouchi, Tomohiro*; Kawarada, Hiroshi*; Sutter, J. P.*; Tsutsui, Satoshi*; Baron, A. Q. R.*; Nagao, Masanori*; Takano, Yoshihiko*

We observed phonon dispersions using inelastic X-ray scattering from CVD-grown high quality superconducting (=4.2 K) boron-doped and non-doped diamonds. By comparing these two, we found almost no change in the acoustic branch along both [111] and [001] directions. On the other hand, strong softening were observed in the optical branch of the boron-doped diamond in both directions: about 2 meV at Brillouin zone boundary and 8 meV at zone center ( point). From this result we estimated the electron-phonon coupling factor =0.33, and this supports theoretical models suggesting a phonon-mediated pairing mechanism via coupling of optical phonon modes to Fermi surfaces around the point.