Superconducting Transition Temperatures of Cu (Co_<1-x>M_x)_2S_4(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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Superconducting transition temperatures T_c have been measured for s-wave superconductors Cu (Co_<1-x>M_x)_2S_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 (Co_<1-x>M_x)_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 Na_zCoO_2・yH_2O, which has two-dimensional layers of CoO_2 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 Na_zCoO_2・yH_2O. Brief arguments of a possible mechanism of the T_c suppression of the present system are given.
2004-07-15