Saddle point anomaly of Landau levels in graphenelike structures

Abstract: Studying the tight binding model in an applied rational magnetic field (H) we show that in graphene there are very unusual Landau levels situated in the immediate vicinity of the saddle point (M-point) energy epsilon_M. Landau levels around $\epsilon_M$ are broadened into minibands (even in relatively weak magnetic fields ~40-53 T) with the maximal width reaching 0.4-0.5 of the energy separation between two neighboring Landau levels though at all other energies the width of Landau levels is practically zero. In terms of the semiclassical approach a broad Landau level or magnetic miniband at epsilon_M is a manifestation of the so called self-intersecting orbit signifying an abrupt transition from the semiclassical trajectories enclosing the $\Gamma$ point to the trajectories enclosing the K point in the momentum space. Remarkably, the saddle point virtually does not affect the diamagnetic response of graphene, which is caused mostly by electron states in the vicinity of the Fermi energy \epsilon_F. Experimentally, the effect of the broading of Landau levels can possibly be observed in twisted graphene where two saddle point singularities can be brought close to the Fermi energy.