Novel two-component ground state at 1/3 + 1/3 filling in strongly coupled graphene double-layer*

In the composite fermion (CF) picture, fractional quantum Hall effect (FQHE) states can be reinterpreted as effective integer quantum Hall effect (IQHE) for CFs. This provides a natural framework for interpreting the relative hierarchy of observable FQHE states, and leads to the prediction and observation of a Fermi surface of CFs at half filled LL. It is recently demonstrated that quantum Hall ground states in a strongly coupled graphene double-layer could be described by an expanded CF picture with two-component correlation that accounts for both interlayer and intralyer interaction. Apart from a novel sequence of two-component FQHE states that emerge as effective IQHE of CFs, this new construction implies a Fermi sea of CFs at 1/3 filling in each graphene layer, which could host incompressible ground state resulting from pairing instability. Here we report experimental observation of incompressible state 1/3 + 1/3 filling. Multiple phase transitions are observed as a function of magnetic field and density imbalance, and possible ground state order will be discussed.