In-Plane Anisotropy of Upper Critical Field inSr2RuO4

Abstract

We investigated the behavior of the spin-triplet superconductor Sr2RuO4 ( T(c) approximately 1.5 K) under the magnetic fields parallel to the quasi-two-dimensional plane. The upper critical field H(c2) exhibits a clear fourfold anisotropy of about 3% at 0.35 K. Furthermore, we detected an additional transition feature below H(c2) in both the ac susceptibility and the specific heat. These second-transition features as well as the pronounced in-plane H(c2) anisotropy disappear above 0.8 K or under intentional field misalignment of less than 1 degrees. Most of these characteristics are consistent with the predicted emergence of the second superconducting phase with a line-node gap.

Quasiclassical determination of the in-plane magnetic field phase diagram of superconductingSr2RuO4

“…Intriguingly, this analysis also points to the possibility of a tetra-critical point in the H-T phase diagram. This tetracritical point has features that agree with recent experimental measurements in high magnetic fields 14 . Given the difficulties that the chiral p-wave state has explaining the H-T phase diagram, we also address the possibility of an in-plane d-vector to see if it can account for the observed phase diagram.…”

“…While there has been no evidence of multiple superconducting transitions in the low-field regime, two superconducting transitions have been observed in the high-field range 14,44 . In one aspect these transitions are natural candidates for two transitions discussed above.…”

“…Since experimentally it has been observed that the upper critical field is relatively isotropic for in-plane fields 14 , we ask if it is possible to reproduce this. The only free parameter in the theory is ǫ which describes the anisotropy in the gap function.…”

The Intriguing Superconductivity of Strontium Ruthenate

“…Using an ingenious 17 O NMR analysis, Takashi Imai and his coworkers separated the contributions of the a,b and g sheets to the susceptibility and found that the spin susceptibility was more strongly enhanced on the g sheets. 10 Imai's result implies that, when the temperature drops below T c , superconductivity initially appears in this sheet.…”

“…The first NMR test came from Kenji Ishida and his coworkers, who found that the 17 O Knight shift was totally unaffected by the transition into the superconducting state 7 (see figure 3). Note, however, that the spin-orbit interaction blurs the distinction between spin-singlet and spin-triplet pairing.…”