Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: $^{57}$Fe M\"ossbauer study of stoichiometric iron based superconductor
  CaKFe$_4$As$_4$: a comparison to KFe$_2$As$_2$ and CaFe$_2$As$_2$   $^{57}$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.

27
The categories of this paper are: comp-gas
15
The categories of this paper are: nucl-ex
25
The categories of this paper are: patt-sol
12
The categories of this paper are: nucl-th
17
The categories of this paper are: q-fin
1
The categories of this paper are: cs
7
The categories of this paper are: hep-th
34
The categories of this paper are: chem-ph
9
The categories of this paper are: nlin
30
The categories of this paper are: q-alg
24
The categories of this paper are: dg-ga
13
The categories of this paper are: quant-ph
16
The categories of this paper are: stat
5
The categories of this paper are: gr-qc
14
The categories of this paper are: math-ph
8
The categories of this paper are: hep-ex