Quasi-one-dimensional band in the t-J model

We study the two-dimensional (2d) t-J model in a mean filed approximation. Assuming uniform charge distribution, we find that the 2d t-J model has an instability toward forming a quasi-one-dimensional (q-1d) Fermi surface. Th is q-1d state competes with a d-wave pairing state, which overcomes the q-1 d state for a realistic parameter region. However, we can show that the q-1 d state coexists with the d-wave pairing state when we introduce a small (a bout 3%) spatial anisotropy in t and J in the original t-J model.