Classification of massive and gapless phases in bilayer graphene

Abstract

We here classify of all the fully gapped \emph{massive} and \emph{gapless} phases in bilayer graphene. The effective low energy theory in bilayer graphene is constructed, and various discrete and continuous symmetries of the non-interacting system is analyzed. Spinless fermions, placed in a quantizing magnetic field is considered. The quantum anomalous Hall insulator is properly defined. Constructing a particle-hole doubled 16 component Nambu-Dirac spinor, we recognize all the possible fully gapped, and the gapless states, which, on the other hand, split the parabolic dispersion into two anisotropic Dirac like conical ones. A thorough symmetry analysis of all the ordered states is performed. Altogether there are 8 insulating and 4 superconducting phases in bilayer graphene, that can lead to fully gapped spectrum. Among the gapped superconductors, \emph{three} are spin-singlet, which include uniform s-wave and two spatially inhomogeneous, translational symmetry breaking Kekule superconductors. The triplet pairing exhibits an f-wave symmetry. Besides the gapped phases, there are 8 semimetallic and 8 gapless superconducting states in total, available for fermions to condense into. We also find novel gapless superconducting states, which break the translational symmetry, dubbed as \emph{gapless-Fulde-Farrell-Larkin-Ovchinikov} superconductors. We also discuss the role of Coulomb interaction, and propose various experimental tools to determine the underlying ordered states.

14 pages; no figure, Published version: Slightly modified Title, Revised abstract, Sharpen terminology, New section on experimental signatures of broken symmetry phases, New references, typos corrected