Ground State of BaFe2S3 from Lattice and Spin Dynamics

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 Néel 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.

Figures (5)

• Figure 1: Schematic picture of the atomic and magnetic structure of BaFe$_2$S$_3$. The axes follow the $Cmcm$ setting. Ba: green, Fe: brown, S: yellow. The arrows show the magnetic moments on different Fe sites.
• Figure 2: Temperature-dependent reflectance spectra. (Top row) Reflectance measured at room temperature (red) and at the lowest accessible temperature [20]K (blue), with the electric field $\vec{E}$ applied along the three crystallographic directions of the $Cmcm$ space group. The arrows mark the fitted IR-active phonon modes at [20]K. (Bottom row) Heat maps showing the evolution of the reflectance as a function of temperature for the three directions, after background substraction. Horizontal black dashed lines, marking $T_{N}$, $T_{S}$, and $T^{*}$ in ascending order, are added to guide the eye. The reflectance intensity (in arbitrary unit) scales from low (blue) to high (red).
• Figure 3: Temperature evolution of the [124]cm$^{-1}$, e [145]cm$^{-1}$, [312]cm$^{-1}$ and [355]cm$^{-1}$ phonons modes. The two [124]cm$^{-1}$ modes disappear at $T_S$. Blue curves : oscillator frequencies, $\omega_k$. Red curves : normalized plasma frequencies, $\omega_p$. Green curves : width, $\Gamma_k$.
• Figure 4: Neutron scattering. maximum intensity, $A$ of eq. 2 in SI (top) and FWHM (bottom) evolutions as a function of temperature for $HH$, $L$ and $E$ scans around the magnetic Bragg peak $(1/2,1/2,1)$. Vertical gray dashed lines mark $T_{N}$ and $T_{S}$, respectively. Inset in top panel displays a zoom in for $T_N\le T\le T_S$
• Figure 5: Summary of the multi-technique ambient pressure temperature-dependent results. The diagram compares the structural and magnetic information extracted from x-ray diffraction, neutron diffraction (same group previous work), infrared spectroscopy and inelastic neutron scattering (from this work) and contrasts them with the literature consensus Takahashi2015Zheng2018IR_bafe2sse3_prb.