Electronic structure of biased alternating-twist multilayer graphene

We theoretically study the energy and optical absorption spectra of alternating-twist multilayer graphene (ATMG) under a perpendicular electric field. We obtain analytically the low-energy effective Hamiltonian of ATMG up to pentalayer in the presence of the interlayer bias by means of first-order degenerate-state perturbation theory, and present general rules for constructing the effective Hamiltonian for an arbitrary number of layers. Our analytical results agree to an excellent degree of accuracy with the numerical calculations for twist angles $\ensuremath{\theta}\ensuremath{\gtrsim}2.{2}^{\ensuremath{\circ}}$ that are larger than the typical range of magic angles. We also calculate the optical conductivity of ATMG and determine its characteristic optical spectrum, which is tunable by the interlayer bias. When the interlayer potential difference is applied between consecutive layers of ATMG, the Dirac cones at the two moir\'e Brillouin zone corners $\overline{K}$ and ${\overline{K}}^{\ensuremath{'}}$ acquire different Fermi velocities, generally smaller than that of monolayer graphene, and the cones split proportionally in energy resulting in a steplike feature in the optical conductivity.