Evidence for s-wave superconductivity in noncentrosymmetric Re ₂₄Nb₅ from 93Nb NMR measurements

We report a 93Nb nuclear magnetic resonance (NMR) study on the noncentrosymmetric superconductor Re₂₄Nb₅. Below the superconducting temperature T_c(H), the spin susceptibility probed by the 93Nb NMR Knight shift gradually decreases with lowering temperature, accompanied by the broadening of the resonance spectrum. Such behavior is commonly observed in the BCS-type superconductors. The 93Nb NMR spin-lattice relaxation rate (1/T₁) shows a well-defined coherence peak just below T_c(H), followed by a marked decrease with further decreasing temperature. Moreover, the 1/T₁ data in the superconducting state were found to obey a single exponential expression, yielding a nodeless gap Δ/k_B=10.3 K. This value gives the ratio of 2Δ/k _BT_c(H)=3.55, that is almost identical with the value of 3.5 predicted from BCS theory. On these bases, we conclude that the noncentrosymmetric Re₂₄Nb₅ compound can be characterized as a weakly coupled BCS-type superconductor.