Edge States of an Excitonic Insulator with the Spin-Orbit Interaction

The effect of the spin-orbit and Coulomb interactions on the symmetry and topological properties of the s, p, d, and s + d phases of an excitonic insulator has been considered by example of the inverted two-band structure of the HgTe quantum well. It has been found that only the p phase has a nontrivial topology, but it is metastable. It has been shown that the exchange interaction between fermions induces the ground state with the s + d symmetry of the excitonic order parameter. This phase of the excitonic insulator has zero Chern number; nevertheless, edge states occur in the case of open boundaries.