Z₂ Topological Order and the Quantum Spin Hall Effect

The quantum spin Hall (QSH) phase is a time reversal invariant electronic state with a bulk electronic band gap that supports the transport of charge and spin in gapless edge states. We show that this phase is associated with a novel Z₂ topological invariant, which distinguishes it from an ordinary insulator. The Z₂ classification, which is defined for time reversal invariant Hamiltonians, is analogous to the Chern number classification of the quantum Hall effect. We establish the Z₂ order of the QSH phase in the two band model of graphene and propose a generalization of the formalism applicable to multiband and interacting systems.