Quasi-particles dynamics in underdoped Bi2212 under strong optical perturbation.

In this work an optical pump-probe set-up is used to study the photo-induced non-equilibrium dynamics of a superconducting underdoped Bi2212 single crystal in a strong excitation regime (10<[lowercase_phi_synonym]<600 μJ/cm²). The use of a tunable repetition rate 120 fs pulsed laser source allows us to avoid significant average heating of the sample and to optimize the signal-to-noise ratio in the detection of the transient reflectivity variation. A discontinuity of the transient reflectivity is observed at high excitation intensities ([lowercase_phi_synonym]∼70 μJ/cm²). Numerical simulations of the heat diffusion problem indicate that, in this regime, the local temperature of the sample is lower than T_C, confirming the impulsive nature of this phenomenon. The quasi-particles (QP) dynamics in the strongly perturbed superconducting state (10<[lowercase_phi_synonym]<70 μJ/cm²) is analysed within the framework of the Rotwarf-Taylor model. The picture emerging from the data is consistent with a dynamics governed by high-frequency phonon (HFP) population, which causes a “bottleneck” effect in the QP recombination. [ABSTRACT FROM AUTHOR]