Pressure-induced symmetry lowering in Nb3Sn1−x superconductor

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Cubic Pm-3n Nb3Sn0.92 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 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, 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 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.