Lower Critical Field Study on the Superconducting Phase

Abstract

The superconductivity ($T_c =1.4$ K) in the heavy-fermion $\mathrm{{URu}}_2\mathrm{{Si}}_2$ is full of mysteries. In sharp contrast to the other heavy-fermion superconductors, most of which coexist with magnetic orders, the superconductivity in $\mathrm{{URu}}_2\mathrm{{Si}}_2$ cannot coexist with the antifferomagnetic order but it coexists with the so-called “hidden-orders” state ($T_0 =17.5$ K) whose order parameter is still unknown. The clarification of the superconducting state of $\mathrm{{URu}}_2\mathrm{{Si}}_2$ therefore should provide important information on a new mechanism to induce unconventional superconductivity. In this chapter, we show an accurate determination of lower critical field down to 55 mK in ultraclean $\mathrm{{URu}}_2\mathrm{{Si}}_2$ single crystals with using our newly developed technique. The results include several remarkable features, which have never been reported in low-quality crystals. Particularly, a distinct kink structure in the temperature dependence of lower critical field is found. This is possibly attributed to multicomponent superconducting order parameter, which will be discussed with the symmetry breaking in the hidden-order phase.