Strong Pauli paramagnetic effect in the upper critical field of KCa${}_2$Fe${}_4$As${}_4$F${}_2$

Abstract

Recently, 12442 system of Fe-based superconductors has attracted considerable attention owing to its unique double-FeAs-layer structure. A steep increase in the in-plane upper critical field with cooling has been observed near the superconducting transition temperature, $T_c$, in KCa${}_2$Fe${}_4$As${}_4$F${}_2$ single crystals. Herein, we report a high-field investigation on upper critical field of this material over a wide temperature range, and both out-of-plane ($H\Vert c$, $H_{c2}^c$) and in-plane ($H\Vert ab$, $H_{c2}^{ab}$) directions have been measured. A sublinear temperature-dependent behavior is observed for the out-of-plane $H_{c2}^c$, whereas strong convex curvature with cooling is observed for the in-plane $H_{c2}^{ab}$. Such behaviors could not be described by the conventional Werthamer--Helfand--Hohenberg (WHH) model. The data analysis based on the WHH model by considering the spin aspects reveals a large Maki parameter $\alpha =9$, indicating that the in-plane upper critical field is affected by a very strong Pauli paramagnetic effect.

6 pages, 4 figures, accepted by SCIENCE CHINA Physics, Mechanics & Astronomy

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• Charge Density Wave order and electron-phonon coupling in ternary superconductor Bi${}_2$Rh${}_3$Se${}_2$
• Single particle tunneling spectroscopy and superconducting gaps in layered iron based superconductor KCa${}_2$Fe${}_4$As${}_4$F${}_2$
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• Nodal multigap superconductivity in the anisotropic iron-based compound RbCa2Fe4As4F2
• Disorder-robust high-field superconducting phase of FeSe single crystals
• Observation of Two-dimensional Superconductivity in an Ultrathin Iron-Arsenic Superconductor
• Pseudogap and Strong Pairing Induced by Incipient and Shallow Bands in the Quasi-Two-Dimensional KCa${}_2$Fe${}_4$As${}_4$F${}_2$
• Spectroscopic studies of the superconducting gap in the 12442 family of iron-based compounds
• Second magnetization peak, anomalous field penetration, and Josephson vortices in KCa${}_2$Fe${}_4$As${}_4$F${}_2$ bilayer pnictide superconductor
• Nematic fluctuations in the non-superconducting iron pnictide BaFe${}_{1.9-x}$Ni${}_{0.1}$Cr${}_x$As${}_2$
• Anomalous metallic oxygen band in the potential superconductor KCa${}_2$Fe${}_4$As${}_4$O${}_2$
• Unusually weak irradiation effects in anisotropic iron-based superconductor RbCa2Fe4As4F2
• The upper critical field and its anisotropy in RbCr${}_3$As${}_3$
• Preformed Cooper Pairs in a Triclinic Iron Pnictide Superconductor