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

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 $h^{-\alpha }$, it is impossible for any operator evolving in the Heisenberg picture to spread with dynamical exponent less than $1\mathrm{/}\alpha$. In particular, ballistic growth is impossible for $\alpha <1$, diffusive growth is impossible for $\alpha <1\mathrm{/}2$, and any finite dynamical exponent becomes impossible for sufficiently small $\alpha$. 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.