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There are several FeSe based superconductors, including the bulk FeSe,monolayer FeSe thin film, intercalated KxFe2-ySe2 and Li1-xFexOHFeSe, etc.Their normal states all show metallic behavior. The key player here is the FeSelayer which exhibits the highest superconducting transition temperature in theform of monolayer thin film. Recently a new FeSe based compound, CsFe4-xSe4with the space group of Bmmm was found. Interestingly the system shows a stronginsulator-like behavior although it shares the same FeSe planes as otherrelatives. Density functional theory calculations indicate that it should be ametal, in sharp contrast with the experimental observations. Here we report theemergence of unconventional superconductivity by applying pressure to suppressthis insulator-like behavior. At ambient pressure, the insulator-like behaviorcannot be modeled as a band insulator, but can be described by thevariable-range-hopping model for correlated systems. Furthermore, the specificheat down to 400 mK has been measured and a significant residual coefficientgamma_0=C/T|T->0 is observed, which contrasts the insulator-like state andsuggests some quantum freedom of spin dynamics. By applying pressure theinsulator-like behavior is gradually suppressed and the system becomes a metal,finally superconductivity is achieved at about 5.1 K. The superconductingtransition strongly depends on magnetic field and applied current, indicating afragile superfluid density. Our results suggest that the superconductivity isestablished by diluted Cooper pairs on top of a strong correlation backgroundin CsFe4-xSe4.

Unconventional superconductivity induced by suppressing an iron-selenium based Mott insulator CsFe4-xSe4