Superconducting properties variation with A15 composition in Nb3Sn
  We extend the Ginzburg-Landau-Abrikosov-Gor'kov (GLAG) theory to account for the variation of the upper critical field Hc2 with Sn content in A15-type Nb3Sn. Hc2 at the vicinity of the critical temperature Tc is related quantitatively to the electrical resistivity, specific heat capacity coefficient and Tc, with inclusion of the electron-phonon coupling correction, Pauli paramagnetic limiting and martensitic phase transformation of A15 lattices. Hc2 near Tc is then extrapolated to Hc2(0) at temperature 0K, and Hc2(0) versus tin content agrees well with experiments. We find that, as Sn content gradually approaches the stoichiometry, Nb3Sn undergoes a transition from the dirty limit to clean limit, split by the phase transformation boundary. The H-T phase boundary and the pinning force show different behaviors in the cubic and tetragonal phase. Applying the theoretical formulas in the technical Nb3Sn wire, we obtain the dependence of the composition gradient on the superconducting properties variation in the A15 layer, as well as the curved tail at vicinity of Hc2 in the Kramer plot of the Nb3Sn wire. This gives a better understanding of the inhomogeneous-composition inducing discrepancy between the results by the state-of-art scaling laws and experiments. 
Yingxu Li and Yuanwen Gao
cond-mat.supr-con