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. 
R. Kadono, K. H. Satoh, A. Koda, T. Nagata, H. Kawano-Furukawa, J.   Suzuki, M. Matsuda, K. Ohishi, W. Higemoto, S. Kuroiwa,...