Fermionization of Bosons in a Flat Band

TL;DR

This paper demonstrates that strongly interacting bosons in a flat band Kagome lattice favor a topologically ordered chiral spin-liquid state below a certain filling, preventing Bose-Einstein condensation and revealing novel quantum phases.

Contribution

It introduces a fermionization approach for hard-core bosons on non-Bravais lattices, showing the emergence of a chiral spin-liquid state with topological order and edge modes at low filling fractions.

Findings

Identifies a topologically ordered chiral spin-liquid state below filling fraction 1/3.

Shows the ground state energy matches the flat band energy, preventing Bose condensation.

Proposes experimental methods to distinguish the spin-liquid from a condensate.

Abstract

Strongly interacting bosons that live in a lattice with degeneracy in its lowest energy band experience frustration that can prevent the formation of a Bose-Einstein condensate. Such systems form an ideal playground to investigate spin-liquid behavior. We use the variational principle and the Chern-Simons technique of fermionization of hard-core bosons on Kagome lattice to find that below lattice filling fraction $\nu=1/3$ the system favors a topologically ordered chiral spin-liquid state that is gapped in bulk, spontaneously breaks Time-Reversal Symmetry, and supports massless chiral bosonic edge mode. We construct the many-body variational wave function of the state and show that the corresponding energy coincides with the energy of the flat band. This result proves that the ground state of the system cannot stabilize a Bose condensate below $\nu=1/3$. The fermionization and variational scheme we outline apply to any non-Bravais lattice. We distinguish between the roles played by the Chern-Simons gauge field in lattices with a flat band and those exhibiting a moat-like dispersion (which is degenerate along a closed contour in the reciprocal space). We also suggest experimental probes to differentiate the proposed ground state from a condensate.