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Photons, magnetic fields, Cooper pairs, and quasparticles: 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 and magnetic fields up to 10 T. The optical conductivity of thin-film superconductors in applied magnetic fields has been measured, allowing the magnetic-field dependence of the superconducting energy gap, pairbreaking parameter, and  superfluid density to be estimated. The field leads to Cooper pair breaking, in accord with the theory of by Abrikosov and Gor'kov. Photons (from a short-pulsed laser also break Cooper pairs). Laser pump/synchrotron probe studies of excess quasiparticle relaxation show a relaxation rate proportional to the excess quasiparticle number density, as expected for bimolecular recombination. Application of a magnetic field parallel to the sample surface is found to slow significantly the quasiparticle recombination process. We consider two candidate magnetic field effects: quasiparticle spin polarization (paramagnetism) and finite pair lifetime effects. Both tend to slow the recombination, as observed.