Negative differential conductance near Landau Levels in small-angle twisted bilayer graphene

Negative differential conductance (NDC), described by decreasing current with increasing voltage, is an uncommon property arising due to nonlinear electronic response. Systems displaying NDC are of great interest because they are essential to the operation of electronic components such as amplifiers, switches, oscillators and rectifiers. In the tunneling spectroscopy of two-dimensional electron systems, NDC is revealed by resonant tunneling with extremely sharp features in the density of states. Here we report the observation of NDC in the tunneling spectroscopy of small-angle (0.7 °) twisted bilayer graphene. In the presence of perpendicular magnetic fields above 2T, NDC is observed near sharp and pronounced Landau level peaks. The gate and bias-voltage region of the NDC can be controlled by tuning the Landau level energy with a magnetic field. The differential conductance in the NDC regime is not simply proportional to the local density of states, but that it strongly depends on the transmission coefficient. This effect is especially pronounced in flat bands where the kinetic energy is suppressed as is the case of Landau levels or the flat band emerging at charge neutrality in twisted bilayer graphene.