Quantum diffusion in two-dimensional random systems with particle-hole   symmetry
  We study the scattering dynamics of an $n$-component spinor wavefunction in a random environment on a two-dimensional lattice. In the presence of particle-hole symmetry we find diffusion on large scales. The latter is described by a non-interacting Grassmann field, indicating a special kind of asymptotic freedom in $d=2$. 
K. Ziegler
cond-mat.dis-nn quant-ph