Fig. S3.

Group velocities of the low-lying excitations around the steady state after the quench from the Néel state in the $XXZ$ chain. The maximum velocity in each string sector (different symbols used in the figure) is plotted against the anisotropy $\mathrm{\Delta }$. The dashed–dotted lines are the analytical results of Eq. S14 in the large $\mathrm{\Delta }$ limit. The horizontal dotted line is the (low-energy) spin-wave velocity $v_{sw}=\pi /2$ at $\mathrm{\Delta }=1$.