Quasiparticle relaxation dynamics in underdoped Bi2Sr2CaCu2O8+δ by two-color pump-probe spectroscopy

We investigate the relaxation dynamics of photoexcited quasiparticles (QPs) in underdoped Bi2Sr2CaCu2O8+delta (T-c = 78 K). By changing the excitation energy and polarization of the probe beam, two different types of relaxation dynamics, associated with superconducting (SC) and pseudogap (PG) QPs, are quantitatively analyzed independently. From the temperature dependencies, we obtained the SC gap, Delta(SC)(0) = 24 meV, using BCS-type temperature-dependent gap and the pseudogap, Delta(PG) = 41 meV. The pump fluence (F) dependence of the SC-dominated transients shows a contribution of the PG component above the saturation condition of the SC component (F-th = 16 mu J/cm(2)), where Cooper pairs with long-range order are fully destroyed within the photoexcited volume. Assuming a temperature-independent PG decay time, we successfully isolate the native SC transient even above F-th by subtracting the PG response from the original data. In the saturation regime, the exponential decay (recovery of SC) is fast (tau(SC) similar to 2-3 ps), suggesting an efficient nonequilibrium phonon relaxation in this compound. We also find a flat-top response preceding the exponential decay at F > F-th, which appears as a delay of SC recovery in the original data. This response is visible over the whole temperature range below T-c and its duration increases with increasing F. The response is attributable to a photoinduced SC to non-SC phase transition arising from excitation by the nonthermal QPs and/or high-frequency phonons. The consistently near-constant magnitude of the PG response at the start of the SC state recovery from the non-SC phase suggests a correlation between the SC and PG QPs.