Ground State of BaFe2S3 from Lattice and Spin Dynamics

TL;DR

This study reveals that in BaFe2S3, a quasi-one-dimensional Mott system, magnetic fluctuations drive structural transitions through magnetoelastic coupling, with lattice symmetry lowering and dynamic magnetic correlations playing key roles.

Contribution

It demonstrates that short-range magnetic correlations can induce static structural changes, highlighting a magnetically driven mechanism in a low-dimensional iron-based system.

Findings

Lattice symmetry is lower than previously thought, consistent with P1 at low temperature.

Structural transition coincides with magnetic fluctuations, not static order.

Magnetic correlations are dynamic and short-range above the Nel temperature.

Abstract

We investigate the interplay between lattice symmetry, phonons, and magnetism in the quasi-one-dimensional ladder compound BaFe$_2$S$_3$ by combining polarized synchrotron infrared spectroscopy, hybrid-functional density functional theory calculations, and inelastic neutron scattering. Lattice-dynamics analysis reveals that the crystal symmetry is lower than previously proposed and is consistent with a $P1$ space group at low temperature. Several infrared-active phonon modes exhibit pronounced anomalies at both the structural transition temperature $T_S \approx 125$--$130$~K and the N\'eel temperature $T_N \approx 95$~K. First-principles calculations show that the modes affected at $T_S$ predominantly involve displacements that modulate magnetic exchange pathways. Neutron scattering demonstrates that below $T_N$ the magnetic order is three-dimensional, long-ranged, and static. Between $T_N$ and $T_S$, the system displays three-dimensional short-range dynamic magnetic correlations, which disappear above $T_S$. The structural transition thus coincides with the onset of magnetic fluctuations rather than with static magnetic order. Our results indicate that short-range, dynamical magnetic correlations are sufficient to drive a static structural instability, providing a magnetically driven mechanism reminiscent of the iron-pnictide 122 family, yet realized here in a quasi-one-dimensional Mott system. These findings highlight the central role of magnetoelastic coupling in iron-based superconductors beyond the itinerant regime.