Interplay of electron-phonon coupling, pseudogap, and superconductivity in CsCa$_2$Fe$_4$As$_4$F$_2$ studied using ultrafast optical spectroscopy

TL;DR

This study uses ultrafast optical spectroscopy to explore how electron-phonon interactions, pseudogap formation, and superconductivity coexist and influence each other in the iron-based superconductor CsCa$_2$Fe$_4$As$_4$F$_2$, revealing key energy scales and dynamics.

Contribution

It provides the first detailed ultrafast spectroscopic analysis of the pseudogap and electron-phonon coupling in CsCa$_2$Fe$_4$As$_4$F$_2$, elucidating their roles in superconductivity.

Findings

Pseudogap opens below 60 K before superconductivity appears.

Superconducting gap of approximately 6.6 meV was observed.

A coherent phonon mode at 5.49 THz shows deviations below T_c.

Abstract

The quasiparticle relaxation dynamics of the iron-based superconductor CsCa$_2$Fe$_4$As$_4$F$_2$ ($T_c$ $\sim$ 29 K) were investigated using ultrafast optical spectroscopy. A pseudogap ($\Delta_{PG}$ $\approx$ 3.3 meV) was observed to open below $T^{\ast}$ $\approx$ 60 K, prior to the emergence of a superconducting gap ($\Delta$ $\approx$ 6.6 $\pm$ 0.4 meV). At high excitation fluence, a coherent A1g phonon mode at 5.49 THz was identified, exhibiting deviations from anharmonic behavior below $T_c$. The electron-phonon coupling constant for this mode was estimated to be $\lambda_{A_{1g}}$ $\approx$ 0.23. These results provide insights into the interplay between the electron-phonon interactions, pseudogap, and the superconducting pairing mechanism in CsCa$_2$Fe$_4$As$_4$F$_2$.