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 $\mu_{2k} = \|[H,\sigma^z_0]^{(k)}\|_2^2$ 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, $\|[H,\sigma^z_0]^{(k)}_{\ell_k}\|_2 \gtrsim (k/\ln k)^k$. 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.