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

TL;DR

This paper proves that in quantum spin chains with heavy-tailed random fields, operator growth cannot be slower than a certain rate, showing disorder can prevent typical transport behaviors like ballistic and diffusive spreading.

Contribution

It establishes rigorous bounds on operator growth in spin chains with power-law-distributed random fields, linking disorder strength to dynamical exponents.

Findings

Operator growth is at least ballistic for $ ext{exponent} eq 1$

Disorder prevents diffusive transport for $ ext{exponent} < 1/2$

Heavy-tailed disorder can suppress conventional transport mechanisms.

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