Layer-Dependent Superconductivity in Iron-Based Superconductors CsCa 2 Fe 4 As 4 F 2 and CaKFe 4 As 4 .

In the quasi-two-dimensional superconductor NbSe 2 , the superconducting transition temperature ( T c ) is layer-dependent, decreasing by about 60% in the monolayer limit. However, for the extremely anisotropic copper-based high- T c superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212), the T c of the monolayer is almost identical with that of its bulk counterpart. To clarify the effect of dimensionality on superconductivity, here, we successfully fabricate ultrathin flakes of iron-based high- T c superconductors CsCa 2 Fe 4 As 4 F 2 and CaKFe 4 As 4 . It is found that the T c of monolayer CsCa 2 Fe 4 As 4 F 2 (after tuning to the optimal doping by ionic liquid gating) is about 20% lower than that of the bulk crystal, while the T c of three-layer CaKFe 4 As 4 decreases by 46%, showing a more pronounced dimensional effect than that of CsCa 2 Fe 4 As 4 F 2 . By carefully examining their anisotropy and the c -axis coherence length, we reveal the general trend and empirical law of the layer-dependent superconductivity in these quasi-two-dimensional superconductors.