Evidence of electron interaction with an unidentified bosonic mode in superconductor CsCa$_2$Fe$_4$As$_4$F$_2$

TL;DR

This study identifies a unique electron-boson interaction in CsCa$_2$Fe$_4$As$_4$F$_2$ superconductor via ARPES, revealing a mode linked to superconductivity but of unknown origin, challenging existing theories.

Contribution

First observation of an orbital and momentum-dependent kink in a Fe-based superconductor linked to a mysterious bosonic mode.

Findings

Kink appears at 15 meV below Fermi level along Gamma-M

Kink disappears when superconducting gap closes

Bosonic mode at 9 meV is related to superconductivity

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 CsCa$_2$Fe$_4$As$_4$F$_2$ 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 Gamma-M direction at the band with dxz orbital character and vanishes when approaching the Gamma-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 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.