μSR measurements on the vortex lattice of CaAlSi: Anisotropic response in magnetic penetration depth

The vortex state defined by two characteristic length scales, magnetic penetration depth (λ) and coherence length (ξ), in a layered hexagonal superconductor CaAlSi was probed using muon spin rotation. From preliminary analysis using modified London model, we found that the in-plane λ increases with increasing magnetic field while inter-plane λ is almost independent of magnetic field. The presence of field-induced quasiparticle excitation manifested in the field dependence of λ suggests that CaAlSi has an anisotropic gap structure (nodes/dips or multi-gapped).