Quasiparticle Relaxation Across a Spin Gap in the Itinerant   Antiferromagnet UNiGa5
  Ultrafast time-resolved photoinduced reflectivity is measured for the itinerant antiferromagnet UNiGa$_{5}$ ($T_{N} \approx$85 K) from room temperature to 10 K. The relaxation time $\tau$ shows a sharp increase at $T_{N}$ consistent with the opening of a spin gap. In addition, the temperature dependence of $\tau$ below $T_{N}$ is consistent with the opening of a spin gap leading to a quasiparticle recombination bottleneck as revealed by the Rothwarf-Taylor model. This contrasts with canonical heavy fermions such as CeCoIn$_{5}$ where the recombination bottleneck arises from the hybridization gap. 
Elbert E. M. Chia, H. J. Lee, Namjung Hur, E. D. Bauer, T.   Durakiewicz, R. D. Averitt, J. L. Sarrao, and A. J. Taylor