We introduce a model of Dirac fermions in $2+1$ dimensions with a semimetallic, a quantum spin-Hall insulating (QSHI), and an $s$-wave superconducting (SSC) phase. The phase diagram features a multicritical point at which all three phases meet as well as a QSHI-SSC deconfined critical point. The QSHI and SSC orders correspond to mutually anticommuting mass terms of the Dirac Hamiltonian. Based on this algebraic property, SO(5) symmetric field theories have been put forward to describe both types of critical points. Using quantum Monte Carlo simulations, we directly study the operator that rotates between QSHI and SSC states. The results suggest that it commutes with the low-energy effective Hamiltonian at criticality but has a gap in the ordered phases. This implies an emergent SO(5) symmetry at both the multicritical and the deconfined critical points.

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• Dirac fermions
• Quantum criticality
• Quantum field theory
• Quantum phase transitions
• Topological field theories
• Lattice models in condensed matter
• Quantum Monte Carlo