Fig. 1.

Third-order spin Drude weight ${\mathcal{D}}^{\left(3\right)}$ in the easy-axis regime of the XXZ spin chain with $\beta h=1$. Main plot shows comparison between GHD and ED results for fixed $\mathrm{\Delta }$. The lower (upper) boundaries of the shaded region correspond to extrapolations of finite-size ED results with a degree 1 (degree 2) polynomial in $1/L$. Inset shows ${\mathcal{D}}^{\left(3\right)}$ as a function of $\eta ={\cosh }^{-1}\mathrm{\Delta }$.