Sharp magnetization jump at the first-order superconducting transition in Sr2RuO4

Abstract

The magnetization and magnetic torque of a high-quality single crystal of Sr${}_2$RuO${}_4$ have been measured down to 0.1 K under a precise control of the magnetic-field orientation. When the magnetic field is applied exactly parallel to the $ab$ plane, a sharp magnetization jump $4\ddot{I}€\hat{I}´M$ of $(0.74\pm 0.15)$ G at the upper critical field $H_{c2,ab}\sim 15$ 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 $\Gamma =\hat{I}{¾}_a/\hat{I}{¾}_c$ of as large as 60 even at 100 mK, in contrast with the observed $H_{c2}$ anisotropy of $\sim 20$. 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 Sr${}_2$RuO${}_4$.

5 pages, 4 figures (main text) + 2 pages, 2 figures (supplemental material), published in Phys. Rev. B