Anomalous low-field diamagnetic response in ultraclean URu2Si2 superconductor

The lower critical field Hcl in ultraclean URu2Si2 single crystals (Tc = 1.4K) with residual resistivity ratio exceeding 700 is precisely determined by the positional dependence of the local magnetic induction measured down to 55mK. We show that the whole Hcl(T) for H || a can be explained by the two gaps with line and point nodes. In contrast, for H || c we find a distinct kink in the slope of Hc1 at 1.2 K, which cannot be accounted for by the two gaps. This outstanding anomaly demonstrates a highly unusual superconducting state embedded in the hidden order state of URu2Si2 possibly related to the time reversal symmetry breaking.