Nuclear Relaxation Study in Strong Coupling Superconductors –A Comparison with High-<i>T</i>_cSuperconductors–

Abstract

The nuclear relaxation behavior in s - wave superconductors has been investigated as for Chevrel-phase superconductors, TlMo 6 Se 7.5 and Sn 1.1 Mo 6 Se 7.5 with T c =12.2 K and 4.2 K, respectively. The nuclear-spin-lattice-relaxation rate, 1/ T 1 , of 119 Sn in Sn 1.1 Mo 6 Se 7.5 reveals a distinct coherence peak just below T c , followed by an exponential decrease with the isotropic energy gap of 2Δ=3.6 k B T c in a weak coupling regime. By contrast, 1/ T 1 of 205 Tl in TlMo 6 Se 7.5 possesses no coherence peak, although the exponential decrease of 1/ T 1 has also been observed with a somewhat larger value of 2Δ=4.5 k B T c in a strong coupling regime. The stronger electron-phonon coupling enhances the superconducting transition temperature and at the same time, opens a decay channel to cause an intense damping of quasiparticles, resulting in the depression of the coherence peak. The comparison with high- T c superconductors will be discussed.