Symmetry classes of disordered fermions

TL;DR

This paper classifies disordered fermion systems based on their symmetries, linking them to symmetric spaces, and confirms a long-standing conjecture about their correspondence.

Contribution

It provides a comprehensive classification of disordered fermion systems using symmetry and symmetric space theory, extending Dyson's threefold way.

Findings

Classification of fermion systems via symmetry groups

Establishment of a one-to-one correspondence with symmetric spaces

Validation of the symmetry classes conjecture

Abstract

Building upon Dyson's fundamental 1962 article known in random-matrix theory as 'the threefold way', we classify disordered fermion systems with quadratic Hamiltonians by their unitary and antiunitary symmetries. Important examples are afforded by noninteracting quasiparticles in disordered metals and superconductors, and by relativistic fermions in random gauge field backgrounds. The primary data of the classification are a Nambu space of fermionic field operators which carry a representation of some symmetry group. Our approach is to eliminate all of the unitary symmetries from the picture by transferring to an irreducible block of equivariant homomorphisms. After reduction, the block data specifying a linear space of symmetry-compatible Hamiltonians consist of a basic vector space V, a space of endomorphisms in End(V+V*), a bilinear form on V+V* which is either symmetric or alternating, and one or two antiunitary symmetries that may mix V with V*. Every such set of block data is shown to determine an irreducible classical compact symmetric space. Conversely, every irreducible classical compact symmetric space occurs in this way. This proves the correspondence between symmetry classes and symmetric spaces conjectured some time ago.