Microwave spectroscopic evidence of a moir'e-commensurate electron crystal in monolayer graphene

We probe the conductivity of the lowest Landau level of a moiré-aligned monolayer graphene device by broadband microwave transmission through a coplanar waveguide (about 50 mK, 15.4--18 T), isolating the longitudinal channel through a low-minus-high frequency band difference D(ν). We resolve a series of minima in D(ν) that drift with field along lines of fixed carrier density rather than fixed Landau filling, the hallmark of moiré-commensurate electron crystals. The strongest minimum is present at every field and matches no fractional quantum Hall or integer Hofstadter state. It lies at band filling s=1/3, one electron for every three moiré cells---the generalized Wigner crystal that DaSilva, Jung, and MacDonald predict as the most favorable fixed-density (t=0) commensuration. The weaker minima fall near neighboring commensurate fillings.