Ultrafast Pump-Probe Study of Phase Separation and Competing Orders in the Underdoped (Ba, K)Fe2As2 Superconductor

We report measurements of quasiparticle relaxation dynamics in the high-temperature superconductor (Ba, K)Fe2As2 in optimally doped, underdoped, and undoped regimes. In the underdoped sample, spindensity wave (SDW) order forms at similar to 85 K, followed by superconductivity at similar to 28 K. We find the emergence of a normal-state order that suppresses SDW at a temperature T* similar to 60 K and argue that this normal-state order is a precursor to superconductivity.