Topological exciton Fermi surfaces in two-component fractional quantized Hall insulators

A wide variety of two-dimensional electron systems (2DES) allow for independent control of the total andrelative charge density of two-component fractional quantum Hall (FQH) states. In particular, a recent experimenton bilayer graphene (BLG) observed a continuous transition between a compressible and incompressiblephase at total filling νT =12as charge is transferred between the layers, with the remarkable property that theincompressible phase has a finite interlayer polarizability. We argue that this occurs because the topologicalorder of νT =12systems supports a novel type of interlayer exciton that carries Fermi statistics. If the fermionicexcitons are lower in energy than the conventional bosonic excitons (i.e., electron-hole pairs), they can form anemergent neutral Fermi surface, providing a possible explanation of an incompressible yet polarizable state atνT =12. We perform exact diagonalization studies which demonstrate that fermionic excitons are indeed lowerin energy than bosonic excitons. This suggests that a “topological exciton metal” hidden inside a FQH insulatormay have been realized experimentally in BLG. We discuss several detection schemes by which the topologicalexciton metal can be experimentally probed.