Operator Growth in Disordered Spin Chains: Indications for the Absence of Many-Body Localization

• URL: http://arxiv.org/abs/2401.08031v3
• Date: Sat, 13 Jul 2024 04:40:29 GMT
• Title: Operator Growth in Disordered Spin Chains: Indications for the Absence of Many-Body Localization
• Authors: A. Weisse, R. Gerstner, J. Sirker,
• Abstract summary: We derive general bounds for the operator norm of this commutator in free and interacting fermionic systems. We show, in particular, that in a localized system the norm does grow at most exponentially.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: We consider the growth of a local operator $A$ in one-dimensional many-body systems with Hamiltonian $H$ by calculating the $k$-fold commutator $[H,[H,[...,[H,A]]]]$. We derive general bounds for the operator norm of this commutator in free and interacting fermionic systems with and without disorder thus directly connecting the {\it operator growth hypothesis} with questions of localization. We show, in particular, that in a localized system the norm does grow at most exponentially and that the contributions of operators to the total norm are exponentially suppressed with their length. We support our general results by considering one specific example, the XXZ chain with random magnetic fields. We solve the operator spreading in the XX case without disorder exactly. For the Anderson and Aubry-Andr\'e models we provide strict upper bounds. We support our results by symbolic calculations of the commutator up to high orders. For the XXX case with random magnetic fields, these symbolic calculations show a growth of the operator norm faster than exponential and consistent with the general bound for an ergodic system. Also, there is no exponential decay of the contribution of operators as function of their length. We conclude that there is no indication for a many-body localization transition. Finally, we also discuss the differences between the interacting and non-interacting cases when trying to perturbatively transform the microscopic to an effective Hamiltonian of local conserved charges by consecutive Schrieffer-Wolff transformations. We find that such an approach is not well-defined in the interacting case because the transformation generates $\sim 4^\ell$ terms connecting sites a distance $\ell$ apart which can overwhelm the exponential decay with $\ell$ of the amplitude of each individual term.

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