Pronounced anharmonicity in the classical high-T_c superconductor Nb₃Sn

Precision recoil-free fraction values f_a=0.204(7) and f_a=0.365(12) for Sn atoms in Nb₃Sn, have been measured at 297 K and 77 K, respectively. These measurements were compared with previous results obtained by other groups, all of which disagree with one another. The results reported here corroborate the Mössbauer data of Boidachenko et al. who find a weak temperature dependence for the recoil-free fraction in the temperature range of 300 K down to 10 K. In addition, the Debye-Waller factor (-Inf_a) is anomalously large at low temperatures when compared with that of a Debye solid, and cannot be extrapolated to zero at T=0 K. This is indicative of pronounced anharmonicity in the system and lends further support to the local phonon picture of superconductivity, in which enhanced superconducting transition temperatures are thought to be a consequence of highly anharmonic interatomic potentials.