Measurements of the quasiparticle recombination process in a type II superconductor with field-suppressed energy gap*

Our pump-probe studies of excess quasiparticle relaxation in the type II superconductor NbTiN 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. We compare our results with a model of recombination that considers two candidate magnetic field effects: quasiparticle spin polarization (paramagnetism) and finite pair lifetime effects, with the latter appearing to be the more important.