Biased bilayer graphene: semiconductor with a gap tunable by electric field effect

A graphene bilayer with an electrostatic potential difference between layers~--~biased bilayer~--~has been experimentally realized recently. Using a tight binding description we demonstrate that the externally applied gate bias effectively controls the electronic gap between the valence and the conduction bands of bilayer graphene. Applying the theory to the description of magneto-transport data (Shubnikov-de Haas measurements of the cyclotron mass) we extract the value of the gap as a function of the electronic density. We show that the gap can be tuned between zero and mid-infrared energies using fields still below the electric breakdown of SiO$_2$. The opening of a gap is clearly seen in the quantum Hall regime, where the zero-energy double step characteristic to the anomalous quantum Hall effect in unbiased bilayer graphene, splits into two, giving rise to an additional plateau at zero Hall conductivity, besides the standard quantum Hall sequence.