Topological Phase Separation in a Two-dimensional Quantum Lattice Bose-Hubbard System Away from Half-filling

Abstract

We suppose that the doping of the two-dimensional (2D) hard-core (hc) boson system away from half-filling may result in the formation of a multicenter topological inhomogeneity (defect), such as charge order (CO) bubble domain(s) with Bose superfluid (BS) and extra bosons both localized in domain wall(s), or else in a topological CO + BS phase separation rather than a uniform mixed CO + BS supersolid phase. Starting from the classical model, we predict the properties of the respective quantum system. The long-wavelength behavior of the system is believed to be reminiscent of that of granular superconductors, charge-density wave materials, Wigner crystals, and multi-Skyrmion systems akin to the quantum Hall ferromagnetic state of a 2D electron gas. To elucidate the role played by quantum effects and that of the lattice discreteness, we have addressed the simplest nanoscopic counterpart of the bubble domain in the checkerboard CO phase of a 2D hc Bose-Hubbard (BH) square lattice. It is shown that the relative magnitude and symmetry of a multicomponent order parameter are mainly determined by the sign of the nearest-neighbor and next-nearest-neighbor transfer integrals. In general, the topologically inhomogeneous phase of the hc-BH system away from the half-filling can exhibit the signatures of the s, d, and p symmetries of the off-diagonal order.