Renormalization group analysis of phase transitions in the two dimensional Majorana-Hubbard model

TL;DR

This paper investigates phase transitions in a 2D Majorana-Hubbard model using renormalization group analysis, confirming previous predictions about universality classes and the nature of symmetry breaking.

Contribution

It applies a modified epsilon-expansion to analyze phase transitions in the Majorana-Hubbard model, validating earlier mean field and RG results about universality classes.

Findings

Confirmed supersymmetric universality class for attractive interactions with emergent U(1) symmetry.

Identified XY universality class for transitions with broken time reversal symmetry.

Linked repulsive interaction transitions to the Gross-Neveu universality class.

Abstract

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.