Magnetic field-induced quasiparticle excitation in Nb3 Sn: Evidence for anisotropic s -wave pairing

The response of vortex state to the magnetic field in Nb3 Sn 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 T. The gradual increase of the magnetic penetration depth with increasing field provides microscopic evidence for anisotropic (or multigapped) s -wave pairing suggested by the Raman scattering experiment. This result renders need for careful examination on the difference of electronic properties between Nb3 Sn and V3 Si.