  We consider a generic time-reversal invariant model of fermions hopping
randomly on a square lattice. By means of the conventional replica-trick within
the fermionic path-integral formalism, the model is mapped onto a non-linear
sigma-model with fields spanning the coset U(4N)/Sp(2N), N->0. We determine the
proper scaling combinations of an infinite family of relevant operators which
control deviations from perfect two-sublattice symmetry. This allows us to
extract the low-energy behavior of the density of states, which agrees with
earlier results obtained in particular two-sublattice models with Dirac-like
single-particle dispersion. The agreement proves the efficacy of the
conventional fermionic-path-integral approach to disordered systems, which, in
spite of many controversial aspects, like the zero-replica limit, remains one
of the more versatile theoretical tool to deal with disordered electrons.
