Length-resolved Operator Growth and Path-Entropy Obstructions to Many-Body Localization

Abstract

For the disordered Ising chain with transverse and longitudinal fields, where couplings and fields are drawn from strictly positive distributions, Cao~\cite{Cao} has shown that the moments ${\mu }_{2k}=\Vert [H,{\sigma }_0^z{]}^{(k)}{\Vert }_2^2$ grow almost factorially, ${\mu }_{2k}^{1/(2k)}\sim k/\ln k$, and thus asymptotically at the maximal allowed rate. We generalize this result by resolving the operator norm in support length and show that the weight at length ${\ell }_k\sim k/\ln k$ already exhibits almost factorial growth, $\Vert [H,{\sigma }_0^z{]}_{{\ell }_k}^{(k)}{\Vert }_2\gtrsim (k/\ln k{)}^k$. This implies maximal spatial delocalization of local operators and, in particular, rules out dynamical locality---the strongest form of many-body localization---at any disorder strength. We further establish rigorously a finite-size crossover scale $L\sim (W/J{)}^2$, where $W$ is the disorder and $J$ the coupling strength. For $L\lesssim (W/J{)}^2$ numerical studies only access a pre-asymptotic regime. Finally, we identify a structural path-entropy obstruction to perturbative LIOM constructions, based on the almost factorial branching of operator content and independent of resonance effects; the same mechanism strongly suggests ballistic real-time operator spreading, so sub-ballistic or localized dynamics would require a presently unidentified cancellation principle acting on almost factorially many disorder-dependent paths with random amplitudes.

The path-entropy obstruction theorem concerning the perturbative construction of LIOMs has been rewritten and significantly sharpened. Also, parts of theorems 3 and 5 have been reformulated in more general terms, and no longer rely on the asymptotic operator growth formula