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

TL;DR

This study investigates the upper critical field of KCa$_2$Fe$_4$As$_4$F$_2$ superconductors, revealing a strong Pauli paramagnetic effect influencing the in-plane critical field, which deviates from conventional models.

Contribution

The paper provides the first high-field measurements of both in-plane and out-of-plane upper critical fields in KCa$_2$Fe$_4$As$_4$F$_2$, highlighting the significant Pauli paramagnetic effect.

Findings

In-plane $H_{c2}^{ab}$ shows convex curvature with cooling.

Out-of-plane $H_{c2}^{c}$ exhibits sublinear temperature dependence.

Large Maki parameter $eta=9$ indicates strong Pauli paramagnetic influence.

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 ($H\|c$, $H_{c2}^{c}$) and in-plane ($H\|ab$, $H_{c2}^{ab}$) 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.