Ultrafast dynamics of quasiparticles and coherent acoustic phonons in slightly underdoped (BaK)Fe₂As₂

We have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe₂As₂ 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 BaFe₂As₂ is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe₂As₂.