Many-body phenomena driven by valley and orbital isospins in bilayer graphene

Owing to its spin, valley, and orbital degrees of freedom, the eight E=0 Landau levels of Bernal bilayer graphene offer a multi-component 2D platform with strong e-e interactions and competing anisotropies to form novel many-body ground states and collective excitations in the integer and fractional quantum effect of this clean 2D electron gas. In this talk, I will describe our efforts in understanding novel phenomena that emerged from this rich interaction landscape, focusing on the role played by the valley/layer isospin (+, -) and the orbital (N=0, 1) isospin. Unlike traditional homo-orbital two-component states, we show that bilayer graphene supports a new type of hetero-orbital two-component fractional quantum Hall effect at filling factors 7/5, 10/7, where the two components are |+-0> and |-+1>. The underlying SU(2) interaction matrix elements are anisotropic and this leads to two-component fractional states with very different properties. The richness of this system manifests in the phase diagram of 7/5, which supports four different fractional quantum Hall phases. Gap measurements at broken-symmetry integer quantum Hall states reveal fascinating low-energy charge excitations. As we tune the D-field to approach vanishing valley Zeeman energy of E_v /E_c ~ 10^-3, we find that valley skyrmions are not stabilized at either ν=1 or ν=2. Instead, the ν=1 state in clean samples exhibits a very large exchange-enhanced orbital splitting, in contrast to a small gap obtained in earlier measurements. Excitations of the nu=2 appears to involve spin in a non-trivial way. Scattering experiments show that edge state equilibration in bilayer graphene obeys valley selection rule very well. Finally timing permitting I will discuss our recent discovery of a topological phase transition between Chern insulator states of different Chern numbers, which is driven by the E₁₀ orbital splitting in a bilayer graphene/h-BN Moire superlattice.

In collaboration with Ke Huang, Chengqi Guo, Hailong Fu, Ajit Balram, Jainendra Jain, Takashi Taniguchi, and Kenji Watanabe