Evidence for High-frequency Phonon Mediated S-wave Superconductivity : 11B-NMR Study of Al-doped MgB2

We report 11 B-NMR study on Al-doped MgB 2 that addresses a possible mechanism for a high superconducting (SC) transition temperature ( T c ) of ∼ 40 K in recently discovered MgB 2 . The result of nuclear spin lattice relaxation rate 1 /T 1 in the SC state revealed that the size in the SC gap is not changed by substituting Al for Mg. The reduction on T c by Al-doping is shown to be due to the decrease of N ( E F ). According to the McMillan equation, the experimental relation between T c and the relative change in N ( E F ) allowed us to estimate a characteristic phonon frequency ω ∼ 700 K and an electron-phonon coupling constant λ ∼ 0 . 87. These results suggest that the high- T c superconductivity in MgB 2 is mediated by the strong electron-phonon coupling with high-frequency phonons.