Ultrafast dynamics of quasiparticles and coherent acoustic phonons in slightly underdoped (BaK)Fe(2)As(2)

We have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe(2)As(2) crystals without magnetic transition. The photoelastic signals due to coherent acoustic phonons have been quantitatively investigated. According to our temperature-dependent results, we found that the relaxation component of superconducting quasiparticles persisted from the superconducting state up to at least 70 K in the normal state. Our findings suggest that the pseudogaplike feature in the normal state is possibly the precursor of superconductivity. We also highlight that the pseudogap feature of K-doped BaFe(2)As(2) is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe(2)As(2).