Network Models in Class C on Arbitrary Graphs

TL;DR

This paper introduces a quantum localization network model on arbitrary graphs, linking quantum properties to classical random walks, and provides a rigorous supersymmetric path integral analysis of the model.

Contribution

It extends existing models to arbitrary graphs, relating quantum localization to classical random walks, and offers a rigorous supersymmetric path integral formulation.

Findings

Mean density of states can be computed via classical random walk observables.

Mean conductance relates to a history-dependent random walk on the graph.

Transition weights are explicitly connected to S-matrix elements.

Abstract

We consider network models of quantum localisation in which a particle with a two-component wave function propagates through the nodes and along the edges of an arbitrary directed graph, subject to a random SU(2) rotation on each edge it traverses. The propagation through each node is specified by an arbitrary but fixed S-matrix. Such networks model localisation problems in class C of the classification of Altland and Zirnbauer, and, on suitable graphs, they model the spin quantum Hall transition. We extend the analyses of Gruzberg, Ludwig and Read and of Beamond, Cardy and Chalker to show that, on an arbitrary graph, the mean density of states and the mean conductance may be calculated in terms of observables of a classical history-dependent random walk on the same graph. The transition weights for this process are explicitly related to the elements of the S-matrices. They are correctly normalised but, on graphs with nodes of degree greater than 4, not necessarily non-negative (and therefore interpretable as probabilities) unless a sufficient number of them happen to vanish. Our methods use a supersymmetric path integral formulation of the problem which is completely finite and rigorous.