Disordered Ground States of Ergodic Quantum Spin Systems

TL;DR

This paper proves that disordered quantum spin systems with random local interactions always have disordered ground states in the thermodynamic limit, and shows the spectrum of the associated Hamiltonian is deterministic despite the disorder.

Contribution

It introduces a formal framework for disordered ground states in quantum spin systems and establishes their existence using a novel disordered Lieb-Robinson bound.

Findings

Disordered ground states exist in the thermodynamic limit.

The spectrum of the GNS Hamiltonian is deterministic.

A weak-$\ast$ Riesz-Markov-Kakutani theorem is proved for random states.

Abstract

In this letter, we fill a hole in the existing literature about disordered quantum spin systems generated by a random local interaction $\{\mathfrak{h}(Z)\}_{Z\Subset \mathbb{Z}^\nu}$ satisfying a statistical version of translation invariance. We show such systems always have disordered ground states in the thermodynamic limit with the same symmetry. A key tool we use is a disordered version of the Lieb-Robinson bounds, which hold almost surely under mild conditions on $\mathfrak{h}$. Along the way, we formalize the notion of a random state on a $C^*$-algebra and prove a weak-$\ast$ version of the Riesz-Markov-Kakutani theorem, which seems not to have been recorded in the vector measures literature. As a consequence of the existence of the aforementioned disordered ground states, we show that the spectrum of the GNS Hamiltonain associated to the bulk dynamics is deterministic with respect to the disorder.