  Flat bands near M points in the Brillouin zone are key features of honeycomb
symmetry in artificial graphene (AG) where electrons may condense into novel
correlated phases. Here we report the observation of van Hove singularity
doublet of AG in GaAs quantum well transistors, which presents the evidence of
flat bands in semiconductor AG. Two emerging peaks in photoluminescence spectra
tuned by backgate voltages probe the singularity doublet of AG flat bands, and
demonstrate their accessibility to the Fermi level. As the Fermi level crosses
the doublet, the spectra display dramatic stability against electron density,
indicating interplays between electron-electron interactions and honeycomb
symmetry. Our results provide a new flexible platform to explore intriguing
flat band physics.
