Large carbon-isotope shift of T_c in boron-doped diamond

We report the carbon-isotope effect for boron-doped diamond (BDD). Resistive as well as specific-heat measurements reveal a 0.2 K shift in the superconducting transition temperature T_c between BDDs containing ¹³C and ¹²C. This is more than two times larger than could be expected from the mass difference by the use of the simple BCS formula in case the phenomenon is related to electron-phonon mediated superconductivity in BDD.