Correlated phases of moat-band excitons in two-dimensional systems

TL;DR

This paper investigates how moat-shaped dispersions in two-dimensional excitonic systems lead to novel phases like chiral spin liquids, inhomogeneous condensates, and supersolidity, with implications for experimental realization.

Contribution

It introduces a comprehensive analysis of exciton phases in moat-band systems, highlighting the emergence of supersolidity and the importance of proper interaction renormalization.

Findings

Low-density excitons form chiral spin liquids.

Higher densities favor inhomogeneous condensates and supersolidity.

Moat dispersion enhances the likelihood of supersolid phases at weak coupling.

Abstract

We study two-dimensional systems of interacting excitons with a moat dispersion, for which the ground-state energy manifold presents a ring of discrete or continuously degenerate minima around a single point in momentum space. At low densities, the excitons undergo statistical transmutation and stabilize a chiral spin liquid. At higher densities, the moat dispersion favors Bose-Einstein condensation into states occupying multiple momenta, leading to inhomogeneous condensate phases and potentially supersolidity. We discuss the impact of band-structure warping present in realistic systems, which may lower the formation threshold of Bose-Einstein condensate phases. We analyze the superfluid response of the latter, which is unconventional due to the moat band. We also demonstrate that a proper renormalization of the exciton-exciton interaction is essential for describing these phases, and show that even purely repulsive interactions can favor inhomogeneous condensates. To further explore inhomogeneous condensate phases, we employ a Gross-Pitaevskii framework with a pseudopotential approximation and map out the resulting phase diagram. We show that the presence of degenerate dispersion minima can drive supersolidity already at weak coupling, in contrast to systems with a standard parabolic dispersion. Finally, we discuss our results in the context of real excitonic systems and argue that moat-band-induced supersolidity can be within experimental reach for realistic values of the model parameters.