Re-emerging superconductivity at 48 kelvin in iron chalcogenides

Pressure has an essential role in the production and control of superconductivity in iron-based superconductors. Substitution of a large cation by a smaller rare-earth ion to simulate the pressure effect has raised the superconducting transition temperature T_c to a record high of 55 K in these materials. In the same way as T_c exhibits a bell-shaped curve of dependence on chemical doping, pressure-tuned T_c typically drops monotonically after passing the optimal pressure. Here we report that in the superconducting iron chalcogenides, a second superconducting phase suddenly re-emerges above 11.5 GPa, after the T_c drops from the first maximum of 32 K at 1 GPa. The T_c of the re-emerging superconducting phase is considerably higher than the first maximum, reaching 48.0-48.7 K for Tl_0.6Rb_0.4Fe_1.67Se₂, K_0.8Fe_1.7Se₂ and K_0.8Fe_1.78Se₂.