Z2 Topological Order and the Quantum Spin Hall Effect

We study two-dimensional electron systems where the spin-orbit coupling leads to a quantum spin Hall effect. We introduce a Z2 topological order parameter that distinguishes between a conventional insulator and the quantum spin Hall phase. We show that this topological distinction is maintained in the presence of Rashba coupling, which does not conserve the spin, and demonstrate the robustness of helical edge states against time-reversal-symmetric perturbations.