  To understand the interplay of d-wave superconductivity and
antiferromagnetism, we consider a two-dimensional extended Hubbard model with
nearest neighbor attractive interaction. The Hamiltonian is solved in the
mean-field approximation on a finite lattice. In the impurity-free case, the
minimum energy solutions show phase separation as predicted previously based on
free energy argument. The phase separation tendency implies that the system can
be easily rendered inhomogeneous by a small external perturbation. Explicit
solutions of a model including weak impurity potentials are indeed
inhomogeneous in the spin-density-wave and d-wave pairing order parameters.
Relevance of the results to the inhomogeneous cuprate superconductors is
discussed.
