Ground state stability, symmetry, and degeneracy in Mott insulators with long-range interactions

Recently, models with long-range interactions - known as Hatsugai-Kohmoto (HK) models - have emerged as a promising tool to study the emergence of superconductivity and topology in strongly correlated systems. Two obstacles, however, have made it difficult to understand the applicability of these models, especially to topological features: they have thermodynamically large ground state degeneracies and they tacitly assume spin conservation. We show that neither are essential to HK models and that both can be avoided by introducing interactions between tight-binding states in the orbital basis rather than between energy eigenstates. To solve these orbital models, we introduce a general technique for solving HK models and show that previous models appear as special cases. We illustrate our method by exactly solving graphene and the Kane-Mele model with HK interactions. Both realize Mott insulating phases with finite magnetic susceptibility; the graphene model has a fourfold degenerate ground state while the Kane-Mele model has a nondegenerate ground state in the presence of interactions. Our technique then allows us to study the effect of strong interactions on symmetry-enforced degeneracy in spin-orbit coupled double-Dirac semimetals. We show that adding HK interactions to a double Dirac semimetal leads to a Mott-insulating, spin-liquid phase. We then use a Schrieffer-Wolff transformation to express the low-energy Hamiltonian in terms of the spin degrees of freedom, making the spin-charge separation explicit. Finally, we enumerate a broader class of symmetry-preserving HK interactions and show how they can violate insulating filling constraints derived from space-group symmetries. This suggest that additional care is needed to study topological order in the presence of long-range interactions of the HK type.