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 Hc2 at low temperatures. In this study, we clarified the dependence of Hc2 on the angle theta 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 Hc2 for |theta| > 5 is well explained by the orbital pair-breaking effect, Hc2(T) for |theta| < 5 is clearly limited at low temperatures. We also revealed that the Hc2 limit for |theta| < 5 is present not only at low temperatures, but also at temperatures close to Tc. These features may provide additional hints for clarifying the origin of the Hc2 limit. Interestingly, if the anisotropic ratio in Sr2RuO4 is assumed to depend on temperature, the observed angular dependence of Hc2 is reproduced better at lower temperature with an effective-mass model for an anisotropic three-dimensional superconductor. We discuss the observed behavior of Hc2 based on existing theories.