The role of disorder in maximizing the upper critical field in the Nb-Sn system

Abstract

We have measured the transition temperature T c , upper critical field H c2 (T) and the resistivity above T c , ρT c , for thin films of electron beam co-evaporated Nb-Sn which were deposited as Nb 3 Sn at various substrate temperatures, or off-stoichiometry, or with tertiary additions of Al or Zr. The T c and H c2 correlate with the resistivity, no matter how the materials were made. A maximum H c2 (0) of about 300 kOe occurs when ρT c ∼ 30 μΩ-cm and the corresponding T c ∼ 16 K; also H c2 shows no sign of Pauli limiting.