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

TL;DR

This study uses time-resolved reflectivity to investigate quasiparticle dynamics in spin-density-wave states of specific antiferromagnets, revealing that the presence of a SDW gap depends on the modulation vector and challenging existing assumptions.

Contribution

It demonstrates that SDW phases do not always have a gap at the Fermi level, depending on the modulation vector, which is a novel insight into SDW states.

Findings

Relaxation time increases at T_N in UNiGa$_{5}$, indicating a SDW gap opening.

No change in relaxation time at T_N in UPtGa$_{5}$, suggesting no SDW gap.

SDW gap presence depends on the modulation vector Q, not just the SDW phase.

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.