The interplay between antiferromagnetism (AF) and superconductivity (SC) in cuprate is studied for the two-dimensional Hubbard model with a diagonal transfer t′, using a variational Monte Carlo method. Optimizing an improved wave-function for strongly correlated values of U/t, we construct a phase diagram. It is found that the stable state is sensitive to the value of model parameters: For t′/t ≳ -0.15 a coexisting state is realized, 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 the area of pure AF shrinks as U/t increases.
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