Orbital Competition in Bilayer Graphene's Fractional Quantum Hall Effect

The lowest Landau level of bilayer graphene has an octet of internal degrees of freedom, composed from spin, valley, and orbital two-level systems. Dominance of n=0 orbitals over n=1 orbitals in low energy quantum fluctuations leads to distinct fractional quantum Hall characteristics compared dominance of n=1 over n=0. The competition between n=0 and n=1 orbitals depends sensitively on particle-hole asymmetry in the single-particle Hamiltonian and on Lamb shifts due to exchange interactions with the negative energy sea, which must be accounted for simultaneously in assessing the orbital competition. We identify the circumstances under which n=1, which supports strong even-denominator fractional quantum Hall states with non-Abelian quasiparticles, emerges robustly as the low-energy Landau level.