Infinite temperature transport in the strong coupling regime of a nonintegrable quantum spin chain

Abstract

We study spin transport of the XXZ model with next-nearest neighbor $Δ_2$ terms. We compute numerically dependence of spin conductivity $σ(ω)$ on the anisotropy $Δ$ and the ratio $r= Δ_2/Δ$, in the large $Δ$ regime. We find that, when $0<r<1$, the low-frequency conductivity assumes an anomalous form $σ(ω)\approx a ω^2 + b Δ^{-2} $. In particular, when $Δ\to\infty$ the model becomes dynamically constrained and most states are localized. We show, microscopically, existence of magnon bound states in the strong coupling regime, which behave as self-generated disorders for single magnons. Based on this quasiparticle picture, we obtain analytical scalings, which match well with the numerical results.