Non-equilibrium Pair Breaking in Ba(Fe1-xCox)2As2 Superconductors: Evidence for Formation of Photo-Induced Excitonic Spin-Density-Wave State

TL;DR

This study uses ultrafast THz spectroscopy to reveal complex non-equilibrium dynamics in iron pnictide superconductors, showing evidence of a transient state where superconductivity and spin-density-wave correlations compete after optical excitation.

Contribution

It provides experimental evidence and modeling for a photo-induced excitonic SDW state that coexists and competes with superconductivity in Ba(Fe1-xCox)2As2.

Findings

Observation of a two-step SC gap quench with slow and fast components

Dependence of pair breaking dynamics on doping, fluence, and temperature

Evidence for transient excitonic SDW state formation

Abstract

Ultrafast terahertz (THz) pump{probe spectroscopy reveals unusual out-of-equilibrium Cooper pair dynamics driven by femtosecond (fs) optical quench of superconductivity (SC) in iron pnictides. We observe a two{step quench of the SC gap, where an abnormally slow (many 100's of ps) quench process is clearly distinguished from the usual fast (sub-ps) hot{phonon{mediated scattering channel. This pair breaking dynamics depends strongly on doping, pump uence, and temperature. The above observations, together with quantum kinetic modeling of non-equilibrium SC and magnetic correlations, provide evidence for photogeneration of a transient state where SC competes with build{up of spin-density-wave (SDW) excitonic correlation between quasi-particles (QP).