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Register a new userMagnetic-order-driven metal-insulator transitions in the quasi-one-dimensional spin-ladder compounds BaFe$_2$S$_3$ and BaFe$_2$Se$_3$
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The quasi-one-dimensional spin ladder compounds, BaFe$_2$S$_3$ and BaFe$_2$Se$_3$, are investigated by infrared spectroscopy and density functional theory (DFT) calculations. We observe strong anisotropic electronic properties and an optical gap in the leg direction that is gradually filled above the antiferromagnetic (afm) ordering temperature, turning the systems into a metallic phase. Combining the optical data with the DFT calculations we associate the optical gap feature with the $p$-$d$ transition that appears only in the afm ordered state. Hence, the insulating ground state along the leg direction is attributed to Slater physics rather than Mott-type correlations.
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Since the discovery of pressure-induced superconductivity in the two-leg ladder system BaFe$_2X_3$ ($X$=S, Se), with the 3$d$ iron electronic density $n = 6$, the quasi-one-dimensional iron-based ladders have attracted considerable attention. Here, we use Density Functional Theory (DFT) to predict that the novel $n = 6$ iron ladder BaFe$_2$Te$_3$ could be stable with a similar crystal structure as BaFe$_2$Se$_3$. Our results also indicate that BaFe$_2$Te$_3$ will display the complex 2$times$2 Block-type magnetic order. Due to the magnetic striction effects of this Block order, BaFe$_2$Te$_3$ should be a magnetic noncollinear ferrielectric system with a net polarization $0.31$ $mu$C/cm$^2$. Compared with the S- or Se-based iron ladders, the electrons of the Te-based ladders are more localized, implying that the degree of electronic correlation is enhanced for the Te case which may induce additional interesting properties. The physical and structural similarity with BaFe$_2$Se$_3$ also suggests that BaFe$_2$Te$_3$ could become superconducting under high pressure.
We report a comprehensive study of the spin ladder compound BaFe$_2$S$_{2.5}$Se$_{0.5}$ using neutron diffraction, inelastic neutron scattering, high pressure synchrotron diffraction, and high pressure transport techniques. We find that BaFe$_2$S$_{2.5}$Se$_{0.5}$ possesses the same $Cmcm$ structure and stripe antiferromagnetic order as does BaFe$_2$S$_3$, but with a reduced N{{e}}el temperature of $T_N=98$ K compared to 120 K for the undoped system, and a slightly increased ordered moment of 1.40$mu_B$ per iron. The low-energy spin excitations in BaFe$_2$S$_{2.5}$Se$_{0.5}$ are likewise similar to those observed in BaFe$_2$S$_{3}$. However, unlike the reports of superconductivity in BaFe$_2$S$_3$ below $T_c sim 14$~K under pressures of 10~GPa or more, we observe no superconductivity in BaFe$_2$S$_{2.5}$Se$_{0.5}$ at any pressure up to 19.7~GPa. In contrast, the resistivity exhibits an upturn at low temperature under pressure. Furthermore, we show that additional high-quality samples of BaFe$_2$S$_3$ synthesized for this study likewise fail to become superconducting under pressure, instead displaying a similar upturn in resistivity at low temperature. These results demonstrate that microscopic, sample-specific details play an important role in determining the ultimate electronic ground state in this spin ladder system. We suggest that the upturn in resistivity at low temperature in both BaFe$_2$S$_3$ and BaFe$_2$S$_{2.5}$Se$_{0.5}$ may result from Anderson localization induced by S vacancies and random Se substitutions, enhanced by the quasi-one-dimensional ladder structure.
The electronic structure of BaFe$_2X_3$ ($X$ = S and Se) and CsFe$_2$Se$_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe L edges for BaFe$_2X_3$ exhibit two components, indicating that itinerant and localized Fe 3$d$ sites coexist. Substantial x-ray linear dichroism (XLD) is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe-ladder even above the N{e}el temperature $T_N$. Direct exchange interaction along the legs of the Fe-ladder stabilizes the orbital and antiferromagnetic orders in BaFe$_2$S$_3$, while the ferromagnetic molecular orbitals are realized between the rungs in CsFe$_2$Se$_3$.
The majority of the iron-based superconductors (FeSCs) exhibit a two-dimensional square lattice structure. Recent reports of pressure-induced superconductivity in the spin-ladder system, BaFe$_2$X$_3$ (X =S,Se), introduce a quasi-one-dimensional prototype and an insulating parent compound to the FeSCs. Here we report X-ray, neutron diffraction and muon spin relaxation experiments on BaFe$_2$Se$_3$ under hydrostatic pressure to investigate its magnetic and structural properties across the pressure-temperature phase diagram. A structural phase transition was identified at a pressure of 3.7(3) GPa. Neutron diffraction measurements at 6.8(3) GPa and 120 K show that the block magnetism persists even at these high pressures. A steady increase and then fast drop of the magnetic transition temperature $Trm_N$ and greatly reduced moment above the pressure $P_s$ indicate potentially rich and competing phases close to the superconducting phase in this ladder system.
BaFe$_2$S$_3$ is a quasi one-dimensional Mott insulator that orders antiferromagnetically below 117(5),K. The application of pressure induces a transition to a metallic state, and superconductivity emerges. The evolution of the magnetic behavior on increasing pressure has up to now been either studied indirectly by means of transport measurements, or by using local magnetic probes only in the low pressure region. Here, we investigate the magnetic properties of BaFe$_2$S$_3$ up to 9.9,GPa by means of synchrotron $^{57}$Fe Mossbauer spectroscopy experiments, providing the first local magnetic phase diagram. The magnetic ordering temperature increases up to 185(5) K at 7.5,GPa, and is fully suppressed at 9.9,GPa. The low-temperature magnetic hyperfine field is continuously reduced from 12.9 to 10.3,T between 1.4 and 9.1,GPa, followed by a sudden drop to zero at 9.9,GPa indicating a first-order phase transition. The pressure dependence of the magnetic order in BaFe$_2$S$_3$ can be qualitatively explained by a combination of a bandwidth-controlled insulator-metal transition as well as a pressure enhanced exchange interaction between Fe-atoms and Fe 3textit{d}-S 3textit{p} hybridization.
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