Fractional quantum anomalous Hall effect in a semimetal

In the search of fractional quantum anomalous Hall (FQAH) effects, the conventional wisdom is to start from a flat Chern band isolated from the rest of the Hilbert space by bandgap, so that many-body interaction can be projected to a landscape that mimics a Landau level. Here we report the finding of FQAH in a 2D semimetal. Described by a 2π-flux dice lattice, the model features a flat band in touching with a dispersive lower band, where the band touching is symmetry-protected from being gapped by electron interaction. At 1/3 and 2/3 filling of the gapless flat band, FQAH phases are found using density matrix renormalisation group calculations taking into accounts all bands. Symmetry breaking to gap the band touching can turn the semimetal into a Chern insulator while keeping the Chern band nearly flat, but counter-intuitively this suppresses the FQAH, as the gap opening introduces strong inhomogeneity to the quantum geometry. We show an optical scheme to realize the 2π-flux dice lattice for cold atoms. Our finding uncovers a new arena for the exploration of fractional quantum Hall physics in addition to the Landau levels and Chern insulators.