Superconducting transition temperatures of Cu(Co1-xMx)2S4

Superconducting transition temperatures T-c have been measured for s-wave superconductors Cu(Co1-xMx)(2)S-4 with varying x. The system is often considered to be a three dimensional version of the triangular lattice, because it has a three dimensional network of corner-sharing (Co1-xMx)(4) tetrahedra. As the M atom, Cr, Ir and Rh, which can substitute for Co, are used. Although T-c decreases linearly in x, the suppression rates \dT(c)/dx\ are much smaller than the values expected from the pair breaking due to magnetic impurities in s-wave superconductors. They are, of course, much smaller than those expected for the pair breaking due to non-magnetic impurities in superconducting systems with anisotropic order parameter. The behavior is very similar to that observed in the newly found superconductor NazCoO2 . yH(2)O, which has two-dimensional layers of CoO2 triangular lattice. It suggests that a same mechanism of the T-c suppression is working in the two systems and presents useful information on the symmetry of the superconducting order parameter of NazCoO2 . yH(2)O. Brief arguments of a possible mechanism of the T-c suppression of the present system are given.