In-plane anisotropy of upper critical field in Sr2RuO4

We investigated the behavior of the spin-triplet superconductor Sr2RUO4 (T-c approximate to 1.5 K) under the magnetic fields parallel to the quasi-two-dimensional plane. The upper critical field H-c2 exhibits a clear Fourfold anisotropy of about 3 % at 0.35 K. Furthermore, we detected an additional transition feature below H-c2 in both the ac susceptibility and the specific heat. These second-transition features as well as the pronounced in-plane H-c2 anisotropy disappear above 0.8 K or under intentional field misalignment of less than 1 degrees. Most of these characteristics are consistent with the predicted emergence of the second superconducting phase with a line-node gap.