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. 
Elbert E. M. Chia, D. Talbayev, J.-X. Zhu, H. Q. Yuan, T. Park, J. D.   Thompson, C. Panagopoulos, G. F. Chen, J. L. Luo, N....