Sub-ballistic operator growth in spin chains with heavy-tailed random fields

Abstract

We rigorously prove that in nearly arbitrary quantum spin chains with power-law-distributed random fields, namely such that the probability of a field exceeding $h$ scales as $h^{-\hat{I}\pm }$, it is impossible for any operator evolving in the Heisenberg picture to spread with dynamical exponent less than $1/\hat{I}\pm$. In particular, ballistic growth is impossible for $\hat{I}\pm <1$, diffusive growth is impossible for $\hat{I}\pm <1/2$, and any finite dynamical exponent becomes impossible for sufficiently small $\hat{I}\pm$. This result thus establishes a wide family of models in which the disorder provably prevents conventional transport. We express the result as a tightening of Lieb-Robinson bounds due to random fields -- the proof modifies the standard derivation such that strong fields appear as effective weak interactions, and then makes use of analogous recent results for random-bond spin chains.

Published version (very minor updates following comments from colleagues and reviewers)