Sharp magnetization jump at the first-order superconducting transition inSr2RuO4

Abstract

The magnetization and magnetic torque of a high-quality single crystal of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ have been measured down to 0.1 K under precise control of the magnetic-field orientation. When the magnetic field is applied exactly parallel to the $ab$ plane, a sharp magnetization jump $4\ensuremath{\pi}\ensuremath{\delta}M$ of $(0.74\ifmmode\pm\else\textpm\fi{}0.15)$ G at the upper critical field ${H}_{\mathrm{c}2,ab}\ensuremath{\sim}15$ kOe with a field hysteresis of 100 Oe is observed at low temperatures, evidencing a first-order superconducting-normal transition. A strong magnetic torque appearing when $H$ is slightly tilted away from the $ab$ plane confirms an intrinsic anisotropy $\ensuremath{\Gamma}={\ensuremath{\xi}}_{a}/{\ensuremath{\xi}}_{c}$ of as large as 60 even at 100 mK, in contrast with the observed ${H}_{\mathrm{c}2}$ anisotropy of $\ensuremath{\sim}20$. The present results raise fundamental issues in both the existing spin-triplet and spin-singlet scenarios, providing, in turn, crucial hints toward the resolution of the superconducting nature of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$.