Z2 fractional topological insulators in two dimensions

We propose a simple microscopic model to numerically investigate the stability of a two-dimensional fractional topological insulator (FTI). The simplest example of an FTI consists of two decoupled copies of a Laughlin state with opposite chiralities, or double-semion phase. We focus on bosons at half filling. We study the stability of the FTI phase upon addition of two coupling terms of different nature: an interspin interaction term, and an inversionsymmetry- breaking term that couples the copies at the single-particle level. Using exact-diagonalization and entanglement spectra, we numerically show that the FTI phase is stable against both perturbations. We compare our system to a similar bilayer fractional Chern insulator. We show evidence that the time-reversal-invariant system survives the introduction of interaction coupling on a larger scale than the time-reversal-symmetry-breaking one, stressing the importance of time-reversal symmetry in the FTI phase stability. We also discuss possible fractional phases beyond nu = 1/2.