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 {mu}0Hc2 is incomplete. Here we show surprising {mu}0Hc2 data on highly homogeneous bulk samples examined both by small-current, transport and by volumetric-averaging specific heat and the reversible magnetization techniques, which exhibit identical upper critical field {mu}0Hc2(0.3 K) ~ 29{pm} 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 {mu}0Hc2(0K) from 29 T to 21.4 T in the tetragonal state.