# $^{57}$Fe M\"ossbauer study of stoichiometric iron based superconductor CaKFe$_4$As$_4$: a comparison to KFe$_2$As$_2$ and CaFe$_2$As$_2$

## TL;DR

This study uses $^{57}$Fe Mössbauer spectroscopy to analyze the hyperfine properties of the superconductor CaKFe$_4$As$_4$, revealing no magnetic order and comparing its features with related compounds to understand its structural and electronic characteristics.

## Contribution

First Mössbauer study of CaKFe$_4$As$_4$, providing detailed hyperfine parameters and comparison with related iron-based superconductors.

## Key findings

- CaKFe$_4$As$_4$ shows no static magnetic order.
- Hyperfine parameters are intermediate between KFe$_2$As$_2$ and CaFe$_2$As$_2$.
- No clear hyperfine anomalies at the superconducting transition.

## Abstract

$^{57}$Fe M\"ossbauer spectra at different temperatures between $\sim 5$ K and $\sim 300$ K were measured on an oriented mosaic of single crystals of CaKFe$_4$As$_4$ . The data indicate that CaKFe$_4$As$_4$ is a well formed compound with narrow spectral lines, no traces of other, Fe - containing, secondary phases in the spectra and no static magnetic order. There is no discernible feature at the superconducting transition temperature in any of the hyperfine parameters. The temperature dependence of the quadrupole splitting approximately follows the empirical "$T^{3/2}$ law". The hyperfine parameters of CaKFe$_4$As$_4$ are compared with those for KFe$_2$As$_2$ measured in this work, and the literature data for CaFe$_2$As$_2$, and were found to be in between those for these two, ordered, 122 compounds, in agreement with the gross view of CaKFe$_4$As$_4$ as a structural analog of KFe$_2$As$_2$ and CaFe$_2$As$_2$ that has alternating Ca - and K - layers in the structure.