Competing energy scales in high-temperature superconductors: Ultrafast pump-probe studies

We present a review of photoexcited quasiparticle dynamics of cuprate and pnictide high-temperature superconductors in regimes (temperature, doping) where different phases such as superconductivity, spin-density-wave (SDW) and pseudogap phases coexist or compete with one another. We start with the overdoped cuprate superconductor Y_1-xCa_x Ba₂Cu₃O_7-δ, where the superconducting gap and pseudogap coexist in the superconducting state. In another cuprate Tl₂Ba₂Ca₂Cu₃O_y, we ob- serve a competition between SDW and superconducting orders deep in the superconducting state. Finally, in the underdoped iron pnictide superconductor (Ba,K)Fe₂As₂, SDW order forms at 85 K, followed by superconductivity at 28 K. We also find the emergence of a normal-state order that suppresses SDW at a temperature T * ~ 60 K and argue that this normal-state order is a precursor to superconductivity. (