Photons and magnetic fields: The use of synchrotron sources to study pairbreaking in superconductors

Pair-breaking effects in metallic superconductors have been studied by both linear and nonlinear spectroscopy using infrared synchrotron radiation. The measurements were performed at the National Synchrotron Light Source, Brookhaven National Laboratory, in magnetic fields up to 10 T. The optical conductivity of thin-film superconductors in applied magnetic fields has been estimated from the results of far-infrared transmission and reflection measurements. The combined measurements have been analyzed to give the real and imaginary parts of the conductivity. In turn, these quantities allow the magnetic-field dependence of the superconducting energy gap, pairbreaking parameter, and superfluid density to be estimated. Photons also may break Cooper pairs. Pump-probe studies of excess quasiparticle relaxation in the these superconducting films show a relaxation rate proportional to the excess quasiparticle number density, as expected for bimolecular recombination driven by a large excess quasiparticle population. Application of a magnetic field parallel to the sample surface is found to slow significantly the quasiparticle recombination process.