In-Plane Anisotropy of Upper Critical Field in Sr₂RuO₄

We investigated the behavior of the spin-triplet superconductor Sr₂RuO₄ (T_c≈1.5 K) under the magnetic fields parallel to the quasi-two-dimensional plane. The upper critical field Hc2 exhibits a clear fourfold anisotropy of about 3% at 0.35 K. Furthermore, we detected an additional transition feature below Hc₂ in both the ac susceptibility and the specific heat. These second-transition features as well as the pronounced in-plane Hc₂ anisotropy disappear above 0.8 K or under intentional field misalignment of less than 1°. Most of these characteristics are consistent with the predicted emergence of the second superconducting phase with a line-node gap.