Superconducting subphase in the layered perovskite ruthenate Sr 2 RuO 4 in a parallel magnetic field

Magnetic torque measurements using a microcantilever have been performed to investigate the superconducting phase of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ down to 40 mK. For high-quality single crystals with the transition temperature $({T}_{\mathrm{c}})$ 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 ${\mathrm{RuO}}_{2}$ planes below $\ensuremath{\sim}0.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. 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.