Pressure-induced symmetry lowering in Nb3Sn1-x superconductor

Abstract

Cubic Pm-3n Nb3Sn0.92 superconductor (Tc ~ 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 Nb3Sn1-x system for higher Sn content at temperatures lower than ~ 43 K. Nevertheless, the low-temperature structure of Nb3Sn0.92 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, Tc, 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 Nb3Sn1-x induced by increase in Sn content. Therefore targeted changes in composition could be used to enhance upper critical field of Nb3Sn1-x for specific extreme conditions of temperature and pressure.