Abstract
Ultrafast terahertz (THz) pump-probe spectroscopy reveals an unusual out-of-equilibrium Cooper pair nonlinear dynamics and a nonequilibrium state driven by femtosecond (fs) photoexcitation of superconductivity (SC) in iron pnictides. Following fast SC quench via hot-phonon scattering, a second, abnormally slow (many hundreds of picoseconds), SC quench regime is observed prior to any recovery. Importantly, a nonlinear pump fluence dependence is identified for this remarkably long prebottleneck dynamics that are sensitive to both doping and temperature. Using quantum kinetic modeling we argue that the buildup of excitonic interpocket correlation between electron-hole (e-h) quasiparticles (QP) quenches SC after fs photoexcitation leading to a long-lived, many-QP excitonic state.
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