Observation of flat bands and singularity collapse in back-gated semiconductor artificial graphene*

Electronic flat bands are key predicted features in tunable artificial graphene (AG) near M points in the Brillouin zone. Such dispersionless electron flat bands could support unconventional superconducting states. Here, we report the first direct observation of flat bands in semiconductor AG transistors where the electron density is tuned by applied voltage. Doublets observed in low temperature photoluminescence (PL) display a density dependence that is linked to the van-Hove singularity doublet near M points. We find that the doublets in PL persist for a large range of electron densities, suggesting a pinning of the Fermi energy in a flat band. Interestingly, as the temperature is lowered to below 10K, the lineshape of the doublet in PL spectra shows a dramatic temperature dependence in which the higher energy peak quickly collapses. This unexpected temperature dependence suggests possible formation of electron liquid states near the flat band at low temperature.