Topologically nontrivial and nearly flat bands in multi-orbital models

Full text: For topologically nontrivial and very narrow bands, Coulomb repulsion between electrons has been predicted to give rise to a spontaneous fractional quantum-Hall (FQH) state in absence of magnetic fields. We show that orbital degrees of freedom in frustrated lattice systems lead to a narrowing of topologically nontrivial bands for robust parameter ranges. We nd that a strongly correlated t2g system on a triangular lattice can support a spin-chiral magnetic ordering pattern with such topologically nontrivial and flat bands. In order to go beyond mean field and to study the impact of longer-range interactions, we map the low-energy electronic states onto an effective one-band model. Exact diagonalization is then used to establish signatures of a spontaneous FQH state. (author)