Ultrafast transient response and electron-phonon coupling in the iron-pnictide superconductor Ba(Fe(1-x)Co(x))2As2

TL;DR

This study investigates ultrafast electron relaxation and electron-phonon interactions in Ba(Fe(1-x)Co(x))2As2, revealing weak coupling that suggests other mechanisms contribute to superconductivity.

Contribution

It provides the first detailed analysis of electron-phonon coupling dynamics in this iron-pnictide superconductor using pump-probe spectroscopy.

Findings

Fast electron relaxation time < 1 ps

Electron-phonon coupling constant lambda ~ 0.12

Weak electron-phonon coupling suggests other pairing mechanisms

Abstract

The transient response of Ba(Fe(1-x)Co(x))2As2, x=0.08 was studied by pump-probe optical reflectivity. After ultrafast photoexcitation, hot electrons were found to relax with two different characteristic times, indicating the presence of two distinct decay channels: a faster one, of less than 1 ps in the considered pump fluence range, and a slower one, corresponding to lattice thermalization and lasting ~6 ps. Our analysis indicates that the fast relaxation should be attributed to preferential scattering of the electrons with only a subset of the lattice vibration modes, with a second moment of the Eliashberg function lambda(omega^2)~64 meV^2. The simultaneous excitation of a strong fully symmetric A(1g) optical phonon corroborates this conclusion and makes it possible to deduce the value of lambda~0.12. This small value for the electron-phonon coupling confirms that a phonon mediated process cannot be the only mechanism leading to the formation of superconducting pairs in this family of pnictides.