Anomalous correlation effects and unique phase diagram of electron-doped FeSe revealed by photoemission spectroscopy

Abstract

FeSe layer-based superconductors exhibit exotic and distinctive properties. The undoped FeSe shows nematicity and superconductivity, while the heavily electron-doped KxFe2−ySe2 and single-layer FeSe/SrTiO3 possess high superconducting transition temperatures that pose theoretical challenges. However, a comprehensive study on the doping dependence of an FeSe layer-based superconductor is still lacking due to the lack of a clean means of doping control. Through angle-resolved photoemission spectroscopy studies on K-dosed thick FeSe films and FeSe0.93S0.07 bulk crystals, here we reveal the internal connections between these two types of FeSe-based superconductors, and obtain superconductivity below ∼46 K in an FeSe layer under electron doping without interfacial effects. Moreover, we discover an exotic phase diagram of FeSe with electron doping, including a nematic phase, a superconducting dome, a correlation-driven insulating phase and a metallic phase. Such an anomalous phase diagram unveils the remarkable complexity, and highlights the importance of correlations in FeSe layer-based superconductors.

Large-moment antiferromagnetic order in overdoped high- T c superconductor 154 SmFeAsO 1− x D x

“…In sharp contrast with these direct-doping cases where electrons are supplied by an aliovalent cation substituted on the Fe site, heavy electron doping via indirect means, such as the hydride substitution in REFeAsO 1−x H x and K coating for FeSe (K y FeSe), effectively enhances the electron correlation strength (9,15). Indeed, even for the canonical BaFe 2 As 2 system, indirect electron doping via La 3+ substitution onto the Ba 2+ site doesn't lead to a quadratic resistivity ρ ≈ T 2 in the overdoped regime (25).…”

“…In the first of these systems, a quite high T c in the range of 35 K to 41 K was observed, for the electron-doped FeSe using an electric double-layer transistor, which climbs to 46 K to 48 K for the potassium-coated bulk FeSe and rises above 100 K for FeSe monolayer (6)(7)(8)(9)(10). Remarkably, in the overdoped regime of the potassium-coated bulk FeSe, an electronic-correlation-driven insulating phase is developed, where an AFM ordering is also proposed (8).…”

Temperature insensitivity of Fermi surface in electron-doped high-temperature FeSe films on SrTiO3

“…In addition to the interfacial effects, the electron doping from SrTiO 3 is essential for high-temperature superconductivity [16,17]. In fact, electron doping by alkaline-metal deposition leads to high-temperature superconductivity above 40 K in multilayer FeSe films [23] and even in bulk FeSe [24], where the interfacial effects are absent. A question naturally arises as to how the electronic structure evolves with temperature in electron-doped high-temperature FeSe superconductor; is it similar to that of non-doped FeSe?…”

Electronic structure of FeS

“…It undergoes a structural transition around 90 K [2], then becomes superconducting near 8 K. Under high pressure, its superconducting temperature (T c ) can be enhanced to 37 K [3]. When FeSe was intercalated with potassium by liquid gating [4] or dosed with potassium on the surface [5], the T c could reach 46 K. Most remarkably, FeSe monolayer film grown on SrTiO 3 (001) substrate even exhibits T c as high as 65 K [6][7][8].…”