The interplay between antiferromagnetism (AF) and superconductivity (SC) in cuprates is studied for the two-dimensional Hubbard model with a diagonal transfer t', using a variational Monte Carlo method. Optimizing an improved function for strongly correlated values of U/t, we construct phase diagrams in the d (doping rate)-t'/t space. It is found that the stable state is sensitive to the value of model parameters: For the extremely large values of U/t, a coexisting state is realized for t'/t ≳ -0.15, whose range of doping rate extends as t'/t increases. In contrast, for t'/t= -0.3, AF and SC states are mutually exclusive, and a coexisting state does not appear. As U/t decreases, the area of pure AF extends, and that of coexisting state shrinks. As a result, the coexisting state disappears for t'/t= -0.15 and U/t= 12, probable values for holedoped cuprates. Compared with the t-J model, the Hubbard model has richer phases.
- Hubbard model
- Variational Monte Carlo method