Layer-Dependent Superconductivity in Iron-Based Superconductors CsCa₂Fe₄As₄F₂ and CaKFe₄As₄

In the quasi-two-dimensional superconductor NbSe₂, 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 Bi₂Sr₂CaCu₂O_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 CsCa₂Fe₄As₄F₂ and CaKFe₄As₄. It is found that the T_c of monolayer CsCa₂Fe₄As₄F₂ (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 CaKFe₄As₄ decreases by 46%, showing a more pronounced dimensional effect than that of CsCa₂Fe₄As₄F₂. 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.