Symmetry of order parameters in multiband superconductors LaRu$_4$As$_{12}$ and PrOs$_4$Sb$_{12}$ probed by local magnetization measurements
Résumé
The temperature dependencies of the lower critical field Hc1ðTÞ of several filled-skutterudite superconductors were investigated by local magnetization measurements. While LaOs4As12 and PrRu4As12
exhibit the Hc1ðTÞ dependencies consistent with the single-band BCS prediction, for LaRu4As12 (the
superconducting temperature Tc ¼ 10.4 K) with a similar three-dimensional Fermi surface, we observe a
sudden increase in Hc1ðTÞ deep in a superconducting state below about 0.32Tc. Remarkably, a rapid rise of
Hc1ðTÞ at approximately the same reduced temperature 0.27Tc is also found for the heavy-fermion
compound PrOs4Sb12 (Tc ≃ 1.78 K), in fair accord with the earlier macroscopic study. We attribute the
unusual Hc1ðTÞ 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 Hc1ðTÞis 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.
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