$^{57}$Fe Mössbauer study of stoichiometric iron based superconductor CaKFe$_4$As$_4$: a comparison to KFe$_2$As$_2$ and CaFe$_2$As$_2$

Abstract

57 Fe Mössbauer spectra at different temperatures between ∼ 5 K and ∼ 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.

Spin Resonance in the New-Structure-Type Iron-Based Superconductor CaKFe4As4

“…CaKFe 4 As 4 undergoes superconductivity below T c ≃ 35 K without other phase transitions for 1.8 ≤ T ≤ 300 K. [15][16][17] The magnetic ground state of the normal state in CaKFe 4 As 4 is paramagnetic. 16,18) Various measurements 15,16,[18][19][20][21] indicated that CaKFe 4 As 4 shows behaviors similar to optimally or slightly overdoped (Ba 1−x K x )Fe 2 As 2 .…”

“…CaKFe 4 As 4 undergoes superconductivity below T c ≃ 35 K without other phase transitions for 1.8 ≤ T ≤ 300 K. [15][16][17] The magnetic ground state of the normal state in CaKFe 4 As 4 is paramagnetic. 16,18) Various measurements 15,16,[18][19][20][21] indicated that CaKFe 4 As 4 shows behaviors similar to optimally or slightly overdoped (Ba 1−x K x )Fe 2 As 2 . Angle-resolved photoemission spectroscopy (ARPES) measurements supported by density functional theory calculations indicated that the Fermi surface consists of three-hole pockets at the Γ point and two electron pockets at the M point.…”