Synchrotron studies of non-equilibrium effects in the classic superconductors

Synchrotron storage rings are powerful and flexible light sources. They are known to be the brightest sources of ultraviolet and x-ray radiation, but they also are good sources of far-infrared light. This talk describes the use of an infrared beamline at the National Synchrotron Light Source (Brookhaven, NY) to study pairbreaking effects in metallic superconductors. The experiments employed both linear and nonlinear spectroscopic techniques that used infrared synchrotron radiation in conjunction with magnetic fields up to 10 Tesla. The optical conductivity of the thin-film superconductors was obtained from the results of far-infrared transmission and reflection measurements. The conductivity allows measurements of the superconducting energy gap and of magnetic-field- induced pairbreaking. Photons also may break Cooper pairs. Pump-probe studies of quasiparticle recombination in these superconducting films show a relaxation rate proportional to the excess quasiparticle number density, as expected for bimolecular recombination processes. Application of a magnetic field parallel to the sample surface is found to slow significantly the quasiparticle recombination process. These results can be compared with models of recombination that consider two candidate magnetic field effects: quasiparticle spin polarization (paramagnetism) and field-induced pairbreaking.