Critical structure and emergent symmetry of Dirac fermion systems

Emergent symmetry in Dirac system means that the system acquires an enlargement of two basic symmetries at some special critical point. The continuous quantum criticality between the two symmetry broken phases can be described within the framework of Gross-Neveu-Yukawa (GNY) model. Using the first-orderεexpansion in4-ϵdimensions, we study the critical structure and emergent symmetry of theO(N)-GNY model withNflavors of four-component Dirac fermions coupled strongly to anO(N) scalar field under a smallO(N)-symmetry breaking perturbation. After determining the stable fixed point, we calculate the inverse correlation length exponent and the anomalous dimensions (bosonic and fermionic) for generalNandN. Further, we discuss the emergent-symmetry and the emergent supersymmetric critical point forN⩾4on the basis ofO(N)-GNY model. It turns out that theO(N)-GNY universality class is physically meaningful if and only ifN<2Nf+4. On this premise, the smallO(N)-symmetry breaking perturbation is always irrelevant in theO(N)-GNY universality class. Our studies show that the emergent symmetry in Dirac systems has an upper limitO(2Nf+3), depending on the flavor numbersN. As a result, the emergent-O(4) andO(5) symmetries are possible to be found in Dirac systems with fermion flavorNf=1, and the emergent-O(4),O(5),O(6) andO(7) symmetries are expected to be found in the systems with fermion flavorNf=2. Our result suggests some richer transitions with emergent-Z2×O(2)×O(3)symmetry, and so on. Interestingly, in theO(4)-GNY universality class, we find that there is a new supersymmetric critical point which is expected to be found in Dirac systems with fermion flavorNf=1.