Bilayer Graphene: Interaction-Induced Quantum Hall States and Unusual Excitations

Recently, new interaction-induced quantum Hall (QH) states were observed\footnote{B. Feldman et al., Nature Physics, doi:10.1038/nphys1406 (2009); Y. Zhao et al., arXiv:0910.0217 (2009).} in bilayer graphene (BG). In this talk we address the nature of these QH states, as well as their properties\footnote{D. Abanin et al., Phys. Rev. Lett. 103, 076802 (2009), and to be published.}. We focus on the ferromagnetic (FM) states at even filling factors, which, in the leading approximation, result from the spontaneous breaking of valley/spin $SU(4)$ symmetry. Calculating microscopic anisotropies of the QHFM, we find the order in which Landau level (LL) degeneracies are lifted. Furthermore, we discuss the phase diagram of the system as a function of perpendicular electric field and parallel magnetic field, and find that they can be used to drive transitions between different QH states. We show that, as a result of unusual LL structure of BG, some of the QHFM states support new type of excitations -- spin/valley textures (skyrmions) that carry charge two, which provides a unique example of pairing of charges in a system with purely repulsive interactions. We propose several experiments to test our findings.