A complex structure of the superconducting order parameter in Ln2C3 (Ln=La,Y) is demonstrated by muon spin relaxation measurements in their mixed state. The muon depolarization rate sigma v(T)] exhibits a characteristic temperature dependence that can be perfectly described by a phenomenological double-gap model for nodeless superconductivity. While the magnitude of two gaps is similar between La2C3 and Y2C3, a significant difference in the interband coupling between those two cases is clearly observed in the behavior of sigma v(T).

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The local magnetic field distribution B͑r͒ in the mixed state of a boride superconductor, YB 6 , is studied by muon spin rotation ͑SR͒. A comparative analysis using the modified London model and Ginzburg-Landau ͑GL͒ model indicates that the latter exhibits a better agreement with SR data at higher fields and thus demonstrating the importance of reproducing the field profile near the vortex cores when the intervortex distance becomes closer to the GL coherence length. The temperature and field dependence of the magnetic penetration depth ͑͒ does not exhibit any indications of the nonlocal effect or of low-lying quasiparticle excitation. This suggests that the strong coupling of electrons to the rattling motion of Y ions in the boron cage, as suggested by bulk measurements, induces a conventional superconductivity with isotropic s-wave pairing. Considering the present results, a review is provided for probing the anisotropy of the superconducting order parameters by the slope of against the field ͑ ϰ d / dH͒.

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