paper

Giant anisotropy in superconducting single crystals of CsCa $_{2}$ Fe $_{4}$ As $_{4}$ F $_{2}$

arXiv:1811.05706 · doi:10.1103/PhysRevB.99.144501

Abstract

CsCa $_{2}$ Fe $_{4}$ As $_{4}$ F $_{2}$ is a newly discovered iron-based superconductor with $T_{c} \sim$ 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 $I ¨ _{c} (T) / I ¨ _{ab} (T)$ , which increases with decreasing temperature, from 750 at 300 K to 3150 at 32 K. The $I ¨ _{c} (T)$ 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 $I ¨ _{ab}$ is 0.7 K higher than that in $I ¨ _{c}$ , suggesting two-dimensional (2D) superconducting fluctuations. The lower and upper critical fields also show an exceptional anisotropy among iron-based superconductors. The $H_{c 1}^{⊥} (T)$ data are well fitted using the model with two $s$ -wave-like superconducting gaps, $I^_{1} (0) = 6.75$ meV and $I^_{2} (0) = 2.32$ meV. The inter-plane coherence length $I^¾_{c} (0)$ is $3.6$ Ã , remarkably smaller than the distance between conducting layers (8.6 Ã ), consolidating the 2D nature in the title material.

8 pages, 5 figures, 1 Table

Giant anisotropy in superconducting single crystals of CsCa2​Fe4​As4​F2​

Abstract

Giant anisotropy in superconducting single crystals of CsCa $_{2}$ Fe $_{4}$ As $_{4}$ F $_{2}$