All the iron-based superconductors identified to date share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square lattice structures but also in ladder structures. Yet iron-based superconductors without a square lattice motif have not been found despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe 2 S 3 , a Mott insulator with striped-type magnetic ordering below ~120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below T c = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity.The discovery of iron-based superconductors had a significant impact on condensed matter physics and led to extensive study of the interplay between crystal structure, magnetism and superconductivity 1 . All the iron-based superconducting materials discovered to date share the same structural motif: a two-dimensional square lattice formed by edge-shared FeX 4 tetrahedra (X = Se, P and As). The Fe atoms are nominally divalent in most of the parent materials. These parent compounds undergo a magnetic transition at low temperatures, typically exhibiting striped-type ordering.Superconductivity appears when the magnetic order is fully suppressed by the application of pressure or by the addition of doping carriers through chemical Purpose of this studyThe application of pressure is often a useful means of changing the electronic structure of a compound so as to induce a metallic state without simultaneously introducing any degree of disorder 17 . In this study, we investigated in detail the magnetic properties of a sulphur-analogue of the Fe-based ladder materials, BaFe 2 S 3 (space group: orthorhombic, Cmcm) 18,19 , and undertook experimental trials in which this compound was subjected to high pressures to obtain the metallic state. The electronic properties of this material depend on the manner in which the samples are synthesized, and thus we present data for sample 1 describing magnetic properties, and data for a range of samples 1 to 6 describing high-pressure effects. The details of the sample preparation process are given in the Method section. Electronic properties under ambient pressureFigure 2a displays the temperature dependence of the electrical resistivity (ρ) of BaFe 2 S 3 along the leg direction under ambient pressure. The observed insulating behaviour, which occurs despite the expected metallic behaviour in an unfilled 3d manifold, is caused by the Coulomb repulsion between Fe 3d electrons, which becomes prominent in a quasi-one-dimensional ladder structure. Figure 2b shows the magnetic susceptibility (χ) at 5 T along the three orthorhombic...

We report the discovery of a new superconducting phase in highly correlated 3d electron systems. The compound is beta-vanadium bronze, beta- Na0.33V 2O5, in which the charge-ordered phase collapses under hydrostatic high pressure and a pressure-induced superconducting phase appears around T(S C)=8 K, P=8 GPa. This report presents the first observation not only of superconductivity in vanadium oxides but also of a phase transition from charge ordered to superconducting on a pressure-temperature (P- T) plane. The phase diagrams seem to have universal aspects across the classes of materials. This indicates a profound physics of superconductivity in highly correlated electron systems.

The hollandite chromium oxide K2Cr8O16 has been synthesized in both powder and single-crystal form under high pressure. Combining electrical resistivity, magnetic susceptibility, and x-ray diffraction, we found that K2Cr8O16 is a ferromagnetic metal (or half-metal) with T(C)=180 K and shows a transition to an insulator at 95 K without any apparent structural change but retaining ferromagnetism. K2Cr8O16 is quite unique in three aspects: It has a rare mixed valence of Cr3+ and Cr4+; it has a metal (or half-metal)-to-insulator transition in a ferromagnetic state; and the resulting low-temperature phase is a rare case of a ferromagnetic insulator. This discovery could open a new frontier on the relation of magnetism and conducting properties in strongly correlated electron systems.

We performed high-pressure study for a Mott insulator BaFe_{2}S_{3}, by measuring dc resistivity and ac susceptibility up to 15 GPa. We found that the antiferromagnetic insulating state at the ambient pressure is transformed into a metallic state at the critical pressure, P_{c}=10  GPa, and the superconductivity with the optimum T_{c}=24  K emerges above P_{c}. Furthermore, we found that the metal-insulator transition (Mott transition) boundary terminates at a critical point around 10 GPa and 75 K. The obtained pressure-temperature (P-T) phase diagram is similar to those of the organic and fullerene compounds; namely, BaFe_{2}S_{3} is the first inorganic superconductor in the vicinity of bandwidth control type Mott transition.

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