Quantum melting of charge order due to frustration in two-dimensional quarter-filled systems

The effect of geometrical frustration in a two-dimensional $1∕4$-filled strongly correlated electron system is studied theoretically, motivated by layered organic molecular crystals. An extended Hubbard model on the square lattice is considered, with competing nearest-neighbor Coulomb interaction $V$, and that of next-nearest neighbor along one of the diagonals ${V}^{\ensuremath{'}}$, which favor different charge ordered states. Based on exact diagonalization calculations, we find a metallic phase stabilized over a broad window at ${V}^{\ensuremath{'}}\ensuremath{\sim}V$ even for large Coulomb repulsion strengths as a result of frustrating the charge ordered states. Slightly modifying the lattice geometry relevant to the actual organic compounds does not alter the results, suggesting that this ``quantum melting'' of charge order is a robust feature of frustrated strongly correlated $1∕4$-filled systems.