Evidence that the upper critical field of Nb3Sn is independent of whether it is cubic or tetragonal
Although 2011 marks the 50th anniversary of Nb3Sn as the first high field superconductor, real understanding of its upper critical field behavior Hc2 is incomplete. Here, we show surprising Hc2 data on highly homogeneous bulk samples that exhibit identical upper critical field Hc2(0.3 K) ∼ 29 ± 0.2 T with or without undergoing the cubic-to-tetragonal transition, a result in strong contrast to widely used multiple-source data compilations that show a strong depression of Hc2(0 K) from 29 T to 21.4 T in the tetragonal state.