Superconducting Subphase In The Layered Perovskite Ruthenate Sr2ruo4 In A Parallel Magnetic Field

Magnetic torque measurements using a microcantilever have been performed to investigate the superconducting phase of Sr2RuO4 down to 40 mK. For high-quality single crystals with the transition temperature (T-c) of 1.48-1.49 K, an abrupt jump of the torque signal is found near 1.5 T in field parallel to the conducting RuO2 planes below similar to 0.8K. The jump corresponds to the first order transition recently revealed by magnetocaloric and magnetization measurements [Yonezawa et al., Phys. Rev. Lett. 110, 077003 (2013); Kittaka et al., Phys. Rev. B 90, 220502(R) (2014)]. Furthermore, weak diamagnetic and irreversible signals are found to persist above the first order transition up to 1.85 T. The result indicates the presence of a subphase boundary separating low-and high-field phases in the superconducting phase. The high-field subphase disappears when the field is tilted from the conducting planes only by a few degrees. Quantum oscillation measurements are also reported to clarify the strong sample-quality dependence of the high-field subphase.