Slowest local operators in quantum spin chains

TL;DR

This paper numerically constructs and analyzes local operators in quantum spin chains that relax more slowly than typical hydrodynamic modes, revealing potential generic slow relaxation mechanisms due to locality and unitarity.

Contribution

It introduces a numerical method to identify slowly relaxing local operators in nonintegrable and Floquet spin chains, highlighting their generic presence beyond conserved quantities.

Findings

Operators with relaxation times longer than hydrodynamic modes found

Slow operators exist even in systems without conservation laws

Slow relaxation linked to locality and unitarity principles

Abstract

We numerically construct slowly relaxing local operators in a nonintegrable spin-1/2 chain. Restricting the support of the operator to $M$ consecutive spins along the chain, we exhaustively search for the operator that minimizes the Frobenius norm of the commutator with the Hamiltonian. We first show that the Frobenius norm bounds the time scale of relaxation of the operator at high temperatures. We find operators with significantly slower relaxation than the slowest simple "hydrodynamic" mode due to energy diffusion. Then, we examine some properties of the nontrivial slow operators. Using both exhaustive search and tensor network techniques, we find similar slowly relaxing operators for a Floquet spin chain; this system is hydrodynamically "trivial", with no conservation laws restricting their dynamics. We argue that such slow relaxation may be a generic feature following from locality and unitarity.