Nonequilibrium quasiparticle dynamics in single crystals of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$

We report on measurements of the quasiparticle dynamics in single-crystals of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ in the superconducting state via ultrafast pump-probe spectroscopy. Quasiparticles are injected into the samples by ultrashort laser pulses. Once injected, they cause a reflectivity change of the sample at the laser frequency, allowing time-resolved optical measurements of their density. We describe the temperature and excitation density dependence of the quasiparticle recombination rate and discuss the implications of these measurements on the nature of the superconducting gap and the electron-phonon coupling in these materials.