Effective mass and tricritical point for lattice fermions localized by a random mass

This is a numerical study of quasiparticle localization in symmetry class BD (realized, for example, in chiral p-wave superconductors), by means of a staggered-fermion lattice model for two-dimensional Dirac fermions with a random mass. For sufficiently weak disorder, the system size dependence of the average (thermal) conductivity sigma is well described by an effective mass M-eff, dependent on the first two moments of the random mass M (r). The effective mass vanishes linearly when the average mass M (M) over bar -> 0, reproducing the known insulator-insulator phase boundary with a scale invariant dimensionless conductivity sigma(c)=1/pi and critical exponent nu=1. For strong disorder a transition to a metallic phase appears, with larger sigma(c) but the same nu. The intersection of the metal-insulator and insulator-insulator phase boundaries is identified as a repulsive tricritical point.