Nuclear Relaxation Study in Strong Coupling Superconductors –A Comparison with High-TcSuperconductors–

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.