Effective theory of fractional topological insulators in two spatial dimensions

Electrons subjected to a strong spin-orbit coupling in two spatial dimensions could form fractional incompressible quantum liquids without violating the time-reversal symmetry. Here we construct a Lagrangian description of such fractional topological insulators by combining the available experimental information on potential host materials and the fundamental principles of quantum field theory. This Lagrangian is a Landau-Ginzburg theory of spinor fields, enhanced by a topological term that implements a state-dependent fractional statistics of excitations whenever both particles and vortices are incompressible. The spin-orbit coupling is captured by an external static SU(2) gauge field. The presence of spin conservation or emergent U(1) symmetries would reduce the topological term to the Chern-Simons effective theory tailored to the ensuing quantum Hall state. However, the Rashba spin-orbit coupling in solid-state materials does not conserve spin. We predict that it can nevertheless produce incompressible quantum liquids with topological order but without a quantized Hall conductivity. We discuss two examples of such liquids whose description requires a generalization of the Chern-Simons theory. One is an Abelian Laughlin-like state, while the other has a new kind of non-Abelian many-body entanglement. Their quasiparticles exhibit fractional spin-dependent exchange statistics, and have fractional quantum numbers derived from the electron's charge and spin according to their transformations under time-reversal. In addition to conventional phases of matter, the proposed topological Lagrangian can capture a broad class of hierarchical Abelian and non-Abelian topological states, involving particles with arbitrary spin or general emergent SU(N) charges.

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