Nonequilibrium superconductivity and quasiparticle dynamics in YBa₂Cu₃O_7-δ

We have measured, using optical-pump terahertz-probe spectroscopy, the ultrafast (ab)-plane conductivity dynamics of quasiparticles and superconducting pairs in YBa₂Cu₃O_7-δ (δ=0, 0.5) thin films from 4 K to Tc. In optimally doped films, the recovery time for long-range phase-coherent pairing increases from ∼1.5 ps at 4 K to ∼3.5 ps near Tc, consistent with the closing of the superconducting gap. For underdoped films, the measured recovery time is temperature independent (3.5 ps) in accordance with the presence of a pseudogap. These results are compared to all optical pump-probe measurements at 1.5 eV.