Magnetic field dependence of quasiparticle recombination times in superconducting NbTiN at low laser fluence*

We use a pump-probe technique to study the temperature and field dependent quasiparticle recombination process in superconducting Nb$_{0.5}$Ti$_{0.5}$N in the low laser fluence regime. The sample was excited by a picosecond near-infrared Ti:Sapphire laser pulses and probed with subnanosecond infrared synchrotron pulses at the National Synchrotron Light Source, Brookhaven National Laboratory. Time-resolved photoinduced transmission data were obtained as a function of temperature (2.2-10 K) and magnetic field (0-10 T), from which we calculated the quasiparticle effective lifetime. Both temperature and field dependence were observed, with the effective lifetime increasing as the field became stronger. A model of this phenomenon will be discussed.