Yassine OUBAID : Study of the Interplay of Magnetism, Structure, and Electronic Properties in the Iron-Based Spin Ladder BaFe₂S₃

Recently, a family of quasi-one-dimensional Fe-based spin-ladder compounds with the formula BaFe₂X₃ (where X represents a chalcogen, Se or S) has been shown to exhibit superconductivity under pressures exceeding 10 GPa, with critical temperatures of approximately 10 K or 26 K, depending on the chalcogen species [1]. However, the intriguing properties of these compounds extend far beyond superconductivity. At ambient pressure, they behave as Mott insulators and display exotic structural and magnetic phenomena, including multiferroicity near room temperature. As such, these materials encompass key characteristics of strongly correlated quantum systems.
Building on extensive investigations of BaFe₂Se₃, this work presents a comprehensive experimental study of BaFe₂S₃. A detailed examination of the structural, magnetic, and electronic properties of BaFe₂S₃ as a function of temperature and pressure was carried out using a multi-technique approach. This included synchrotron X-ray diffraction, neutron diffraction, inelastic neutron scattering, and synchrotron-based infrared spectroscopy, performed on both powder and single crystals from the same synthesis batch. These experiments enabled the construction of a comprehensive picture of the underlying physics as a function of temperature and pressure.
At ambient pressure, our study revealed novel phases featuring new atomic structures and a previously unreported structural transition to a polar structure, demonstrating that BaFe₂S₃ hosts a combination of exotic tilted stripe magnetic order, multiferroicity, and magnetoelasticity [2].
Under pressure, by combining complementary structural, magnetic, optical, and transport measurements, we obtained a comprehensive picture of the magnetic and atomic evolution of BaFe₂S₃. Our results provide evidence for an intriguing dichotomy between localized and itinerant Fe 3d electrons, which helps explain both the persistence of magnetic order and the emergence of superconductivity. In particular, we have significantly revised the interpretation of several experimental observations previously reported in the literature, substantially enhancing our understanding of magnetoelastic coupling, electronic gap opening, the insulator-to-metal transition, structural degrees of freedom, and superconductivity.