Anomalous Temperature Dependence of Lower Critical Field in Ultraclean URu$_2$Si$_2$

TL;DR

This study precisely measures the lower critical field in ultraclean URu2Si2 down to 55 mK, revealing anomalous temperature dependence and complex vortex dynamics possibly linked to chiral domains and multicomponent superconductivity.

Contribution

It provides the first detailed low-temperature Hc1(T) data for URu2Si2, uncovering unusual behavior that challenges existing models and suggests novel vortex and order parameter phenomena.

Findings

Hc1(T) for H || a fits two-gap models with nodes.

Hc1(T) for H || c shows a kink not explained by two gaps.

Anomalous diamagnetic response linked to chiral domain vortex dynamics.

Abstract

To investigate a mysterious superconducting state of URu_2Si_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_xBe_{13}. 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.