Ultrafast pump-probe study of phase separation and competing orders in the underdoped (Ba,K)Fe${}_2$As${}_2$ superconductor

We report measurements of quasiparticle relaxation dynamics in the high-temperature superconductor (Ba,K)Fe${}_2$As${}_2$ in optimally doped, underdoped and undoped regimes. In the underdoped sample, spin-density wave (SDW) order forms at $\sim$85 K, followed by superconductivity at $\sim$ 28 K. We find the emergence of a normal-state order that suppresses SDW at a temperature $T^{\ast }\sim$ 60 K and argue that this normal-state order is a precursor to superconductivity.