Suppression of superconductivity in Sr₂RuO₄ caused by defects

We have investigated depairing effects in Sr₂RuO₄, the unconventional superconductor in the layered perovskite structure. We prepared crystals of Sr₂RuO₄ with very low levels of impurity elements, and systematically controled their superconducting transition temperature T_c ranging from 1.5 to 0.6 K by adjustments of crystal growth conditions. The dependence of T_c on the residual resistivity ρ₀ in these crystals suggests that the defects are strong pair breakers, in addition to impurities. We further characterized the effects of pair breaking in this unconventional superconductor. We found that the in-plane coherence length ξ_ab(0) evaluated from H_c2(T) is inversely proportional to T_c, and decreases with increasing mean free path l; the latter behavior is opposite to that of conventional superconductors. In addition, we examined the temperature dependence of H_c2∥c which substantially deviates from the BCS theory and even from a recent theory of a p-wave superconductor.