Phonon softening in boron doped diamond

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

Diamond is well known for its maximum hardness, large energy gap, high thermal conductivity, and so on. In addition to these, the superconductivity was discovered recently in boron doped (B-doped) diamond, and this was soon confirmed by other groups. We performed inelastic X-ray scattering experiments at BL35XU of SPring-8 to measure phonons in a highly B-doped CVD grown diamond having superconducting transition at 4.2 K. By comparing the experimental results with those from nitrogen doped CVD grown diamond and/or pure diamond, we found that the optical modes are softened especially at around the Brillouin zone center along with a strong broadening, while the acoustic modes do not show both softening and broadening. This confirms the strong electron-phonon coupling in superconducting diamond.