Abstract
The temperature dependencies of the lower critical field of several filled-skutterudite superconductors were investigated by local magnetization measurements. While and exhibit the dependencies consistent with the single-band BCS prediction, for (the superconducting temperature ) with a similar three-dimensional Fermi surface, we observe a sudden increase in deep in a superconducting state below about . Remarkably, a rapid rise of at approximately the same reduced temperature is also found for the heavy-fermion compound (), in fair accord with the earlier macroscopic study. We attribute the unusual dependencies of and to a kink structure in their superfluid densities due to different contributions from two nearly decoupled bands. Whereas is established as a two-band isotropic -wave superconductor, nonsaturating behavior of is observed for , indicative of an anisotropic structure of a smaller gap. For this superconductor with broken time-reversal symmetry, our findings suggest a superconducting state with multiple symmetries of the order parameters.
- Received 4 June 2019
- Revised 4 November 2019
DOI:https://doi.org/10.1103/PhysRevLett.124.027001
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