Dynamics of many-body localized systems: logarithmic lightcones and $\log \, t$-law of $\alpha$-R\'enyi entropies

In the context of the Many-Body-Localization phenomenology we consider arbitrarily large one-dimensional spin systems. The XXZ model with disorder is a prototypical example. Without assuming the existence of exponentially localized integrals of motion (LIOMs), but assuming instead a logarithmic lightcone we rigorously evaluate the dynamical generation of $ \alpha$-R\'enyi entropies, $ 0< \alpha<1 $ close to one, obtaining a $\log \, t$-law. Assuming the existence of LIOMs we prove that the Lieb-Robinson (L-R) bound of the system's dynamics has a logarithmic lightcone and show that the dynamical generation of the von Neumann entropy, from a generic initial product state, has for large times a $ \log \, t$-shape. L-R bounds, that quantify the dynamical spreading of local operators, may be easier to measure in experiments in comparison to global quantities such as entanglement.
Comment: 12 pages plus references