Magnetic field-induced quasiparticle excitation in Nb 3 Sn: Evidence for anisotropic s-wave pairing

The response of vortex state to the magnetic field in ${\mathrm{Nb}}_{3}\mathrm{Sn}$ 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 $2\char21{}3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. 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 ${\mathrm{Nb}}_{3}\mathrm{Sn}$ and ${\mathrm{V}}_{3}\mathrm{Si}$.