Anomalous Temperature Dependence of Lower Critical Field in Ultraclean URu2Si2

To investigate a mysterious superconducting state of URu 2 Si 2 embedded in the so-called hidden order state, the lower critical field H c1 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 ( T c = 1.4 K) with residual resistivity ratio exceeding 700. We find that the temperature dependence of H c1 significantly differs from that of any other superconductors. The whole H c1 ( 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 H c1 ( T ), which cannot be accounted for by two gaps, is observed. This behavior for H ∥ c sharply contrasts with the cusp behavior of H c1 ( T ) associated with a transition into another superconducting phase found in UPt 3 and U 1- x Th x Be ...