| In this work, we apply the technique of time-resolved THz spectroscopy to study the photoinduced depletion and subsequent re-formation of the condensate of Cooper pairs in the high-Tc superconductor Bi2Sr2CaCu2O8+delta (BSCCO) and in the conventional metallic superconductor NbN. This technique allows a THz probe pulse to measure the nonequilibrium, frequency-dependent complex conductivity over a broad spectrum at low energies (∼3--10 meV) after photoexcitation by a near-infrared pump pulse. Probing on this energy scale provides access to the condensate and quasiparticle densities in BSCCO, and our spectrum spans the superconducting energy gap of NbN.; While the mechanism behind the formation of a superconducting condensate in the cuprates remains unresolved, understanding the dynamics of the condensate's low-energy excitations can reveal information about intrinsic quasiparticle interactions. Photoexcitation in the superconducting state of BSCCO gives rise to quasi-instantaneous changes in the complex conductivity spectra Deltasigma(o) ≡Deltasigma 1(o) + iDeltasigma2(o), which reflect a photoinduced depletion of the superconducting condensate density and concomitant increase in quasiparticle dissipation. This photoinduced transfer of spectral weight from the condensate delta-function to finite frequencies corresponds to a reduction in the sample's kinetic inductance, as indicated by a reduction in the imaginary part sigma2(o). The two-fluid model is consistent with the observed spectra. The reduction and subsequent recovery of the imaginary part, then, give access to the depletion and reformation of the superconducting condensate.; We first investigate the recovery dynamics of the superconducting condensate. In NbN, we show that the recovery takes place over hundreds of picoseconds, is essentially independent of pump intensity, and follows nearly single-exponential behavior with a time constant that depends on film thickness. This presents the first direct evidence from the condensate density that recombination is limited in conventional metallic superconductors by the so-called phonon bottleneck. In constrast, the recovery of the condensate in BSCCO proceeds as if the quasiparticles were decoupled from the phonon system. (Abstract shortened by UMI.)... |
