Possible realization of the exciton Bose liquid phase in a hard-core boson model with ring-only exchange interactions

We investigate a hard-core boson model with ring-only exchanges on a square lattice, where a K-1 term acts on 1 x 1 plaquettes and a K-2 term acts on 1 x 2 and 2 x 1 plaquettes, with a goal of realizing a novel exciton Bose liquid (EBL) phase first proposed by Paramekanti et al. [Phys. Rev. B 66, 054526 (2002)]. We construct Jastrow-type variational wave functions for the EBL, study their formal properties, and then use them as seeds for a projective quantum Monte Carlo study. Using the Green's function Monte Carlo approach, we obtain an unbiased phase diagram that at half-filling reveals a charge density wave for small K-2, a valence bond solid for intermediate K-2, and possibly for large K-2 the EBL phase. Away from half-filling, the EBL phase is present for intermediate K-2 and remains stable for a range of densities below 1/2 before phase separation occurs at lower densities.