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 ${\Delta }_2$ terms. We compute numerically dependence of spin conductivity $\sigma (\omega )$ on the anisotropy $\Delta$ and the ratio $r={\Delta }_2/\Delta$, in the large $\Delta$ regime. We find that, when $0<r<1$, the low-frequency conductivity assumes an anomalous form $\sigma (\omega )\approx a{\omega }^2+b{\Delta }^{-2}$. In particular, when $\Delta \to \infty$ 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.

10 pages, 7 figures