Orientational symmetry-breaking correlations in square lattice t-J model

We study a tendency to orientational symmetry breaking of the square lattice t-J model by applying the exact diagonalization technique. We introduce a small external anisotropy to t and J, and calculate the induced anisotropy of hole-hole correlations as a response. It is found that the response is strongly enhanced for particular band parameters, meaning that the system has a strong tendency to orientational symmetry breaking. The analysis of the momentum distribution function indicates that the correlation develops when the Fermi surface is close to (pi,0) and (0,pi). Some properties of high-Tc cuprates previously attributed to stripes can also be understood from orientational symmetry breaking alone.