We have studied the single-band Hubbard model on a two-dimensional square lattice slightly below half filling. Using the canonical transformation and a form of variational wave function that allows an expansion in terms of hole concentration $\delta$ with respect to half filling, we find that a nonzero $\delta$ results in a frustration of the antiferromagnetic (AF) order favored by the superexchange interaction. Use of a variational Monte Carlo calculation enables us to construct a phase diagram in the $\frac{t}{U-\delta }$ plane in which the AF order disappears at $\delta \approx 0.06$ for a typical value of $\frac{t}{U}=\frac{1}{4}$.

• Received 19 April 1988

DOI:https://doi.org/10.1103/PhysRevB.38.6631