  A lattice of interacting Majorana modes can occur in a superconducting film
on a topological insulator in a magnetic field. The phase diagram as a function
of interaction strength for the square lattice was analyzed recently using a
combination of mean field theory and renormalization group methods, and was
found to include second order phase transitions. One of these corresponds to
spontaneous breaking of an emergent U(1) symmetry, for attractive interactions.
Despite the fact that the U(1) symmetry is not exact, this transition was
claimed to be in a supersymmetric universality class when time reversal
symmetry is present and in the conventional XY universality class otherwise.
Another second order transition was predicted for repulsive interactions with
time reversal symmetry to be in the same universality class as the transition
occurring in the Gross-Neveu model, despite the fact that the U(1) symmetry is
not exact in the Majorana model. We analyze these phase transitions using a
modified $\epsilon$-expansion, confirming the previous conclusions.
