Pressure-induced symmetry lowering in Nb3Sn1−x superconductor

Abstract

Cubic Pm-3n Nb₃Sn_0.92 superconductor (T_c ~ 16 K) was found to exhibit tetragonal instabilities at the superconducting state (T = 10 K). These instabilities are manifested through the appearance of reflections which are forbidden in the Pm-3n symmetry but are compatible with the P4₂/mmc structure which is observed in the Nb₃Sn_1-x system for higher Sn content at temperatures lower than ~ 43 K. Nevertheless, the low-temperature structure of Nb₃Sn_0.92 remains metrically fully cubic, as concluded from single crystal synchrotron radiation diffraction experiments. Subsequent application of external pressure amplifies the observed instabilities with a resulting pseudo-cubic – tetragonal transformation at P = 3 GPa at 10 K and this transition is energy driven, as concluded from ab initio calculations. The electronic structures of the corresponding phases are virtually identical and, therefore, the pseudo-cubic – tetragonal transformation does not influence significantly the underlying electronic interactions. Consequently, no anomalies in the behavior of the critical temperature, T_c, are expected at this pressure. However, anomalies in the upper critical field are anticipated during this transition, in analogy to the corresponding behavior observed during the cubic-tetragonal transformation in Nb₃Sn_1-x induced by increase in Sn content. Therefore targeted changes in composition could be used to enhance upper critical field of Nb₃Sn_1-x for specific extreme conditions of temperature and pressure.