# Low temperature specific heat of 12442-type KCa_2Fe_4As_4F_2 single crystals

## TL;DR

This study measures the low-temperature specific heat of KCa₂Fe₄As₄F₂ single crystals, revealing a superconducting transition, evidence of multi-gap or anisotropic gaps, and a line-nodal gap structure, with implications for understanding its superconducting nature.

## Contribution

It provides detailed specific heat measurements under magnetic fields, revealing multi-gap features and a line-nodal gap structure in 12442-type superconductors, advancing understanding of their pairing mechanisms.

## Key findings

- Superconducting transition observed at T_c with ΔC/T|_{T_c} = 130 mJ/mol K^2.
- Electronic SH coefficient Δγ(H) shows rapid increase below 3 T, then slows down.
- Presence of T^2 term supports line-nodal gap structure.

## Abstract

Low-temperature specific heat (SH) is measured for the 12442-type KCa$_2$Fe$_4$As$_4$F$_2$ single crystal under different magnetic fields. A clear SH jump with the height of $\Delta C/T|_{T_c}$ = 130 mJ/mol K$^2$ is observed at the superconducting transition temperature $T_c$. It is found that the electronic SH coefficient $\Delta\gamma (H)$ quickly increases when the field is in the low-field region below 3 T and then considerably slows down the increase with a further increase in the field, which indicates a rather strong anisotropy or multi-gap feature with a small minimum in the superconducting gap(s). The temperature-dependent SH data indicates the presence of the $T^2$ term, which supplies further information and supports the picture with a line-nodal gap structure. Moreover, the onset point of the SH transition remains almost unchanged under the field as high as 9 T, which is similar to that observed in cuprates, and placed this system in the middle between the BCS limit and the Bose-Einstein condensation.