Fig. 1: Landau level spectroscopy of band-gapped bilayer graphene (BLG).

a Illustration of the trigonal-warped BLG band structure with a bandgap Δ in the K valley. b Evolution of Landau level energies calculated by a continuum model with Δ = 80 meV. We use red and green colors to label K and K′ valleys, respectively, throughout the paper. Solid and dashed arrows illustrate inter-LL transitions obeying the usual selection rule of Δ|N| = ± 1. c Photocurrent spectra with a displacement field of D = 0.874 V/nm at B = 0, 2, and 4 T. The 0 T spectrum features two exciton peaks near the bandgap and a flat spectrum above the bandgap. Oscillations corresponding to inter-LL transitions emerge at 2 T with an alternated high-low pattern. The contrast between high and low peaks increases at 4 T, where splittings of peaks also appear. d A 2D color plot of photocurrent spectra as the magnetic field is continuously tuned. Dashed curves trace high peaks in c, which grow stronger towards higher magnetic fields and we label them by branch A-H (only branch A-D are shown in c). These strong transitions obey the usual optical selection rule of Δ |N| = ± 1. The low peaks in c evolve into weak transitions between dashed lines, whose optical selection rules will be discussed later.