Symmetry of Order Parameters in Multiband Superconductors LaRu4As12 and PrOs4Sb12 Probed by Local Magnetization Measurements

The temperature dependencies of the lower critical field H-c1(T) of several filled-skutterudite superconductors were investigated by local magnetization measurements. While LaOs4As12 and PrRu4As12 exhibit the H-c1(T) dependencies consistent with the single-band BCS prediction, for LaRu4As12 (the superconducting temperature T-c = 10.4 K) with a similar three-dimensional Fermi surface, we observe a sudden increase in H-c1(T) deep in a superconducting state below about 0.32T(c). Remarkably, a rapid rise of H-c1(T) at approximately the same reduced temperature 0.27T(c) is also found for the heavy-fermion compound PrOs4Sb12, (T-c similar or equal to 1.78 K), in fair accord with the earlier macroscopic study. We attribute the unusual H-c1(T) dependencies of LaRu4As12 and PrOs4Sb12 to a kink structure in their superfluid densities due to different contributions from two nearly decoupled bands. Whereas LaRu4As12, is established as a two-band isotropic s-wave superconductor, nonsaturating behavior of H-c1(T) is observed for PrOs4Sb12, 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.