Evidence that the upper critical field of Nb3Sn is independent of whether it is cubic or tetragonal

TL;DR

This study demonstrates that the upper critical field of Nb3Sn remains approximately 29 T at 0.3 K regardless of whether the material is in the cubic or tetragonal phase, challenging previous data suggesting a significant decrease in the tetragonal state.

Contribution

It provides new experimental evidence showing the independence of Nb3Sn's upper critical field from its cubic or tetragonal crystal structure, contradicting earlier reports.

Findings

{0}Hc2(0.3 K) b1 0.2 T for both phases

No significant difference in b0Hc2 between cubic and tetragonal Nb3Sn

Contradicts previous data indicating lower b0Hc2 in tetragonal phase

Abstract

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.