Anomalous Temperature Dependence of Lower Critical Field in Ultraclean URu2Si2
Abstract
To investigate a mysterious superconducting state of URu2Si2 embedded in the so-called hidden order state, the lower critical field Hc1 is precisely determined down to 55 mK for H || a and H || c. For this purpose, the positional dependence of the local magnetic induction is measured on ultraclean single crystals (Tc = 1.4 K) with residual resistivity ratio exceeding 700. We find that the temperature dependence of Hc1 significantly differs from that of any other superconductors. The whole Hc1(T) for H || a are well explained by the two superconducting gap structures with line and point nodes, which have been suggested by the recent thermal conductivity and specific heat measurements. On the other hand, for H || c, a change of slope with a distinct kink in Hc1(T), which cannot be accounted for by two gaps, is observed. This behavior for H || c sharply contrasts with the cusp behavior of Hc1(T) associated with a transition into another superconducting phase found in UPt3 and U1-xThxBe13. The observed anomalous low-field diamagnetic response is possibly related to a peculiar vortex dynamics associated with chiral domains due to the multicomponent superconducting order parameter with broken time reversal symmetry.