Evidence for s-wave superconductivity in noncentrosymmetric Re(24)Nb(5) from (93)Nb NMR measurements

Abstract

We report a (93)Nb nuclear magnetic resonance (NMR) study on the noncentrosymmetric superconductor Re(24)Nb(5). Below the superconducting temperature T(c)(H), the spin susceptibility probed by the (93)Nb 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 (93)Nb 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 Re(24)Nb(5) compound can be characterized as a weakly coupled BCS-type superconductor.