We consider the effects of disorder in a Dirac-like Hamiltonian. In order to use conformal field theory techniques, we argue that one should consider disorder in an imaginary vector potential. This affects significantly the signs of the lowest order $\beta$ functions. We present evidence for the existence of two distinct universality classes, depending on the relative strengths of the gauge field verses impurity disorder strengths. In one class all disorder is made irrelevant by the gauge field disorder.

• Received 17 May 1999

DOI:https://doi.org/10.1103/PhysRevLett.84.1292