Interface superconductivity in the eutectic Sr₂RuO₄-Ru: 3-K phase of Sr₂RuO₄

Enhanced superconductivity in the eutectic system Sr₂RuO₄-Ru is referred to as the 3-K phase of the spin-triplet superconductor Sr₂RuO₄ because of its enhanced superconducting transition temperature T_c of ∼3K. We have investigated the field-temperature (H-T) phase diagram of the 3-K phase for fields parallel and perpendicular to the ab plane of Sr₂RuO₄, using out-of-plane resistivity measurements. We have found an upturn curvature in the H_c2(T) curve for H‖c, and a rather gradual temperature dependence of H_c2 close to T_c for both H‖ab and H‖c. We have also investigated the dependence of H_c2 on the angle between the field and the ab plane at several temperatures. Fitting the Ginzburg-Landau effective-mass model apparently fails to reproduce the angle dependence, particularly near H‖c and at low temperatures. We propose that all of these characteristic features can be explained, at least in a qualitative fashion, on the basis of a theory by Sigrist and Monien that assumes surface superconductivity with a two-component order parameter occurring at the interface between Sr₂RuO₄ and Ru inclusions. This provides evidence of the chiral state postulated for the 1.5-K phase by several experiments.