Rényi entropies after releasing the Néel state in the XXZ spin-chain

Abstract: We study the R\'enyi entropies in the spin-$1/2$ anisotropic Heisenberg chain after a quantum quench starting from the N\'eel state. The quench action method allows us to obtain the stationary R\'enyi entropies for arbitrary values of the index $\alpha$ as generalised free energies evaluated over a calculable thermodynamic macrostate depending on $\alpha$. We work out this macrostate for several values of $\alpha$ and of the anisotropy $\Delta$ by solving the thermodynamic Bethe ansatz equations. By varying $\alpha$ different regions of the Hamiltonian spectrum are accessed. The two extremes are $\alpha\to\infty$ for which the thermodynamic macrostate is either the ground state or a low-lying excited state (depending on $\Delta$) and $\alpha=0$ when the macrostate is the infinite temperature state. The R\'enyi entropies are easily obtained from the macrostate as function of $\alpha$ and a few interesting limits are analytically characterised. We provide robust numerical evidence to confirm our results using exact diagonalisation and a stochastic numerical implementation of Bethe ansatz. Finally, using tDMRG we calculate the time evolution of the R\'enyi entanglement entropies. For large subsystems and for any $\alpha$, their density turns out to be compatible with that of the thermodynamic R\'enyi entropies