  A two-dimensional lattice model for d-wave superconductor with chiral
symmetry is studied. The field theory at the band center is shown to be in the
universality class of U(2n)/O(2n) and U(2n) nonlinear sigma model for the
system with broken and unbroken time-reversal symmetry, respectively. Vanishing
of the beta function implies extended states at the band center. Density of
state vanishes as a cubic function of the energy at the band center for the
former case, while linear for the latter.
