| The magnetic properties of two thin-film type-II superconductors are investigated by optical spectroscopy. We first performed conventional Fourier-transform infrared spectroscopy on the Nb0.5Ti0.5 N and NbN samples in a magnetic field. When the field is parallel to the film surface, it breaks the time-reversal symmetry of the Cooper pairs in the thin sample and causes spatial variation of the order parameter in the thick sample, both resulting in pair-breaking effects. The extracted optical conductivity is consistent with the pair-breaking theory. When the field is perpendicular to the film surface, it creates vortices in both samples and weakens the strength of superconductivity. The optical conductivity data of the two samples are consistent with the Maxwell-Garnett theory. We also demonstrated that the pair-breaking effects should be included for an accurate description of the effective electrodynamic response in the mixed state. After elucidating these magnetic-field-dependent equilibrium state properties, we performed time-resolved infrared spectroscopy to study the charge dynamics in these superconducting thin films after photo-excitation. We found that the quasiparticle recombination process is significantly slowed by a parallel magnetic field. The effective recombination rate scales linearly with the photo-induced excess quasiparticle signal, with the slope showing strong field dependence. Our recombination model explains the field dependence though the field-induced pair breaking. |
