Unusual temperature dependence of the in-plane critical field in Sr2RuO4

We have studied the upper critical field Hc2 of the spin-triplet superconductor Sr2RuO4. For H || c, Hc2 increases linearly with decreasing temperature down to well below Tc = 1.5 K, consistent with the orbital depairing effect. However, for H || ab, the increase of Hc2 is strongly suppressed below 1 K. The origin of this in-plane Hc2 limit has been actively discussed, but its mechanism is still unclear. In the present study, we performed ac susceptibility measurements in the applied fields with higher accuracy than in previous studies to obtain the field-temperature phase diagrams for H || c and H || ab. We revealed that Hc2 || ab (T) does not exhibit a linear temperature dependence even above 1 K, which indicates that the Hc2 suppression is operative even close to Tc. In addition, we found that the Hc2 anisotropy ratio Hc2 || ab/Hc2 || c decreases monotonically from 60 to 20 with decreasing temperature.