In-Plane Anisotropy of Upper Critical Field inSr2RuO4

We investigated the behavior of the spin-triplet superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ ( $T_{\mathrm{c}}\approx 1.5\mathrm{K}$) under the magnetic fields parallel to the quasi-two-dimensional plane. The upper critical field $H_{c2\mathrm{}}$ exhibits a clear fourfold anisotropy of about 3% at 0.35 K. Furthermore, we detected an additional transition feature below $H_{c2\mathrm{}}$ in both the ac susceptibility and the specific heat. These second-transition features as well as the pronounced in-plane $H_{c2\mathrm{}}$ 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.