Symmetry breaking and fermionic fractional Chern insulator in topologically trivial bands*

We describe a mechanism by which fermions in topologically trivial bands can form correlated states exhibiting a fractional quantum Hall (FQH) effect upon introduction of strong repulsive interactions. These states are solid-liquid composites, in which a FQH liquid is induced by the formation of charge order (CO), following a recently proposed paradigm of symmetry-breaking topological (SBT) order [Phys. Rev. Lett. 113, 216404 (2014)]. We devise a spinless fermion model on a triangular lattice, featuring a topologically trivial phase when interactions are omitted. Adding strong short-range repulsion, we first establish a repulsion-driven CO phase at density ρ_CO = 2/3 particles per site, then dope the model to higher densities ρ = ρ_CO + ν/6. At ν=1/3,2/5 (ρ=13/18,11/15), we observe definitive signatures of both CO and the FQH effect --- sharply peaked static structure factor, gapped and degenerate energy spectrum and fractionally quantized Hall conductivity σ_H = 1/3,2/5 in units of e²/h --- over a range of all model parameters. We thus obtain the first strong evidence for fermionic SBT order in topologically trivial bands.