BaFe2Se3 a quasi-unidimensional non-centrosymmetric superconductor

Abstract

The spin-ladder compounds of the BaFe2X3 (X = chalcogen) family may be viewed as dimensional reductions (along stripe-like motifs) of the two-dimensional iron-based pnictide planes extensively studied since 2006. Remarkably, despite their reduced dimensionality, these materials retain the capacity for unconventional ground states, exemplified by the emergence of superconductivity in \bfse\ under applied pressure beyond 10 GPa, following a structural phase transition at 4 GPa. Here, we report a comprehensive investigation combining high-resolution single-crystal X-ray diffraction, infrared spectroscopy, and ab initio calculations, which together elucidate the true crystallographic nature of this pressure-induced superconducting phase. While X-ray diffraction alone reveals a symmetry lowering from the widely accepted orthorhombic Cmcm group to a monoclinic structure, it lacks sufficient sensitivity to resolve the precise space group. By integrating vibrational spectroscopy with density functional theory, we provide unambiguous evidence that the high-pressure phase is non-centrosymmetric, adopting the polar space group P2_1. These findings not only revise the structural assignment of \bfse\ in its superconducting state but also establish its non-centrosymmetric character (an essential ingredient for potential unconventional pairing mechanisms- thereby opening new perspectives on the interplay between lattice symmetry, dimensionality, and superconductivity in iron-based materials.