Giant anisotropy in superconducting single crystals of CsCa$_2$Fe$_4$As$_4$F$_2$

Abstract

CsCa$_2$Fe$_4$As$_4$F$_2$ is a newly discovered iron-based superconductor with $T_\mathrm{c}\sim$ 30 K containing double Fe$_2$As$_2$ layers that are separated by insulating Ca$_2$F$_2$ spacer layers. Here we report the transport and magnetization measurements on CsCa$_2$Fe$_4$As$_4$F$_2$ single crystals grown for the first time using the self flux of CsAs. We observed a huge resistivity anisotropy $ρ_c(T)/ρ_{ab}(T)$, which increases with decreasing temperature, from 750 at 300 K to 3150 at 32 K. The $ρ_c(T)$ data exhibit a non-metallic behavior above $\sim$140 K, suggesting an incoherent electronic state at high temperatures due to the dimension crossover. The superconducting onset transition temperature in $ρ_{ab}$ is 0.7 K higher than that in $ρ_c$, suggesting two-dimensional (2D) superconducting fluctuations. The lower and upper critical fields also show an exceptional anisotropy among iron-based superconductors. The $H_{c1}^\bot(T)$ data are well fitted using the model with two $s$-wave-like superconducting gaps, $Δ_1(0)=6.75$ meV and $Δ_2(0)=2.32$ meV. The inter-plane coherence length $ξ_c(0)$ is $3.6$ Å, remarkably smaller than the distance between conducting layers (8.6 Å), consolidating the 2D nature in the title material.