Evidence of electron interaction with an unidentified bosonic mode in superconductor CsCa2Fe4As4F2

Abstract

The kink structure in band dispersion usually refers to a certain electron-boson interaction, which is crucial in understanding the pairing in unconventional superconductors. Here we report the evidence of the observation of a kink structure in Fe-based superconductor CsCa₂Fe₄As₄F₂ using angle-resolved photoemission spectroscopy. The kink shows an orbital selective and momentum dependent behavior, which is located at 15 meV below Fermi level along the $\mathrm{\Gamma }-\mathrm{M}$ direction at the band with d_xz orbital character and vanishes when approaching the $\mathrm{\Gamma }-\mathrm{X}$ direction, correlated with a slight decrease of the superconducting gap. Most importantly, this kink structure disappears when the superconducting gap closes, indicating that the corresponding bosonic mode (~$9\pm 1$ meV) is closely related to superconductivity. However, the origin of this mode remains unidentified, since it cannot be related to phonons or the spin resonance mode (~15 meV) observed by inelastic neutron scattering. The behavior of this mode is rather unique and challenges our present understanding of the superconducting paring mechanism of the bilayer FeAs-based superconductors.

The kink structure in the band dispersion can be linked to electron-boson interaction and has been reported in unconventional superconductors. Here the authors observe an orbital selective and momentum dependent kink feature in an Fe-based superconductor, which is highly correlated with superconductivity.