Time-Resolved Quasiparticle Dynamics in the Spin-Density-Wave State

Abstract

Time-resolved photoinduced reflectivity is measured in the spin-density-wave (SDW) phase using itinerant antiferromagnets UMGa${}_5$ (M=Ni, Pt). For UNiGa${}_5$ [$T_N$=85 K, $Q$=($\pi$,$\pi$,$\pi$)], the relaxation time $\tau$ shows a sharp increase at $T_N$ consistent with the opening of a SDW gap. For UPtGa${}_5$ [$T_N$=26 K, $Q$=(0,0,$\pi$)], no change in $\tau$ is observed at $T_N$ or at the lowest temperatures. We attribute this to the absence of the SDW gap at the Fermi level, due to a different modulation vector $Q$, which leads to a gapless quasiparticle spectrum. Our results challenge the conventional wisdom that a SDW phase necessarily implies a SDW gap at the Fermi level.

5 pages, 4 figures