Ru NQR Probe of Superconducting Property in Impurity-Doped CeRu2

Superconducting characteristics of CeRu 2 have been investigated by systematic measurements of the nuclear-spin lattice relaxation rate, 1/ T 1 of 101 Ru on pure and impurity-substituted CeRu 2 , employing an NQR technique in zero magnetic field. (1/ T 1 ) for CeRu 2 shows a broad coherence plateau just below T c followed by an exponential decrease well below T c . By contrast, a distinct coherence peak of (1/ T 1 ) has been found just below T c for the impurity-substituted CeRu 2 , even though La- and Al-substitutions for the Ce and Ru sites make T c slightly increase and decrease, respectively. It is considered that the impurity scattering averages out the anisotropy of the energy gap, in good agreement with the theory of dirty superconductors by Anderson. 101 (1/ T 1 T ) in the normal state, which is in proportion to the square of the density of states at the Fermi level, N 2 ( E F ), is found to decrease with the Al content. The suppression in T c by substituting nonmagnetic Al impurity for the Ru sites is well accounted for by an appreciable reduction in N ( E F ). The present Ru NQR study has revealed that CeRu 2 is characterized as a conventional s -wave superconductor with a large anisotropic energy gap.