Nonequilibrium conductivity dynamics in YBa₂Cu₃O₇

Summary form only given. High temperature superconductors (HTSC) remain an area of active research in large part due to the incomplete understanding of the electronic properties in both the superconducting and normal states. These materials are also potential candidates for a variety of optoelectronic applications, and thus it is important to understand the nonequilibrium. properties as well. Optical-pump THz-probe experiments measure the evolution of the real and imaginary conductivity, resulting in a direct probe of the recovery of the superconducting state after optical excitation. In previous work, we showed that in optimally doped films, the recovery time for long-range phase-coherent pairing increases with temperature, consistent with the closing of the superconducting gap. For underdoped films, the measured recovery time was temperature independent. Improvements in our films (as determined from low-temperature impurity dominated scattering time) and the signal-to-noise ratio of our experiment have allowed us to look more precisely at the picosecond conductivity dynamics in the superconducting state. The new results allow for more precise determination of the superconducting pair recovery time at fluences that do not destroy the superconductivity. Importantly, we have also observed a frequency dependent lifetime for the imaginary part of the conductivity. Near T_c, the lifetime decreases from 3.7 ps at 1.0 THz to 3.0 ps at 1.5 THz. This maybe associated with the interplay between the quasiparticles and superconducting pairs.