Quantum Disordered Phase near the Mott Transition in the Staggered-Flux Hubbard Model on a Square Lattice

We investigate ground state properties of the half-filled staggered-flux Hubbard model on a square lattice. Energy gaps to charge and spin excitations and magnetic as well as dimer orders are calculated as a function of interaction strength U/t by means of a constrained-path quantum Monte Carlo method. It is found that the system is a semimetal at U/t less than or similar to 5.6 and a Mott insulator, with long-range antiferromagnetic order, at U/t greater than or similar to 6.6. In the range 5.6 less than or similar to U/t less than or similar to 6.6, the ground state is a correlated insulator where both magnetic and dimer orders are absent. Furthermore, spin excitation in the intermediate phase appears to be gapless, and the measured spin-spin correlation function exhibits power-law decaying behavior. The data suggest that the nonmagnetic ground state is a possible candidate for the putative algebraic spin liquid.