Exploring the dynamics of superconductors by time-resolved far-infrared spectroscopy

We have examined the recombination of excess quasiparticles in superconducting Pb by time-resolved far-infrared spectroscopy using a pulsed synchrotron source. The energy gap shift calculated by Owen and Scalapino [Phys. Rev. Lett. 28, 1559 (1972)] is directly observed, as is the associated reduction in the Cooper pair density. The relaxation process involves a two-component decay; the faster (similar to 200 ps) is associated with the actual (effective) recombination process, while the slower (similar to 10 to 100 ns) is due to heat transport across the film/substrate interface. The temperature dependence of the recombination process between 0.5T(c) and 0.85T(c) is in good agreement with theory.