Angular dependence of the upper critical field of Sr₂RuO₄

One of the remaining issues concerning the spin-triplet superconductivity of Sr₂RuO₄ is the strong limit of the in-plane upper critical field H_c2 at low temperatures. In this study, we clarified the dependence of H_c2 on the angle θ between the magnetic field and the ab plane at various temperatures, by precisely and accurately controlling the magnetic field direction. We revealed that, although the temperature dependence of H_c2 for |θ|≥5° is well explained by the orbital pair-breaking effect, H_c2(T) for |θ|<5° is clearly limited at low temperatures. We also revealed that the H_c2 limit for |θ|<5° is present not only at low temperatures but also at temperatures close to T_c. These features may provide additional hints for clarifying the origin of the H_c2 limit. Interestingly, if the anisotropic ratio in Sr₂RuO₄ is assumed to depend on temperature, the observed angular dependence of H_c2 is reproduced better at lower temperature with an effective-mass model for an anisotropic three-dimensional superconductor. We discuss the observed behavior of H_c2 based on existing theories.