Evidence for s-wave superconductivity in noncentrosymmetric Re24Nb5 from Nb-93 NMR measurements

Abstract

We report a Nb-93 nuclear magnetic resonance (NMR) study on the noncentrosymmetric superconductor Re24Nb5. Below the superconducting temperature T-c(H), the spin susceptibility probed by the Nb-93 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 Nb-93 NMR spin-lattice relaxation rate (1/T-1) shows a well-defined coherence peak just below T-c(H), followed by a marked decrease with further decreasing temperature. Moreover, the 1/T-1 data in the superconducting state were found to obey a single exponential expression, yielding a nodeless gap Delta/kB = 10.3 K. This value gives the ratio of 2 Delta/kBT(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.