Magnetic field-induced quasiparticle excitation in Nb3Sn: Evidence for anisotropic s-wave pairing

The response of vortex state to the magnetic field in Nb3Sn is probed using muon spin rotation and small-angle neutron scattering. A transformation of vortex structure between hexagonal and squared lattice is observed over a relatively low field range of 2-3 Tesla. The gradual increase of the magnetic penetration depth with increasing field provides microscopic evidence for anisotropic (or multi-gapped) s-wave pairing suggested by the Raman scattering experiment. This result renders need for careful examination on the difference of electronic properties between Nb3Sn and V3Si.