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

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