Sign reversal of the oxygen isotope effect on T_c in Sr₂RuO₄

We investigated the oxygen isotope effect (IE) on the transition temperature T_c in the spin-triplet superconductor Sr₂RuO₄ (with the intrinsic T_c0=1.5 K). A clear IE shift in T_c was observed. Moreover, we found that the IE coefficient alpha exhibits an unusual variation with T_c. For lower-T_c crystals containing impurities and defects, α is positive and increases with decreasing T_c; α(T_c) is described well by the universal behavior expected theoretically. However, for crystals with T_c approaching T_c0, α deviates from the universal α(T_c) and becomes negative. We discuss its possible mechanism on the basis of existing models.