Symmetry enhanced first-order phase transition in a two-dimensional quantum magnet

Theoretical studies of quantum phase transitions have suggested critical points with higher symmetries than those of the underlying Hamiltonian. Here we demonstrate a surprising emergent symmetry of the coexistence state at a strongly discontinuous phase transition between two ordered ground states. We present a quantum Monte Carlo study of a two-dimensional S=1/2 quantum magnet hosting the antiferromagnetic (AFM) and plaquette-singlet solid (PSS) states recently detected in SrCu2(BO3)2. We observe that the O(3) symmetric AFM order and the Z2 symmetric PSS order form an O(4) vector at the transition. The control parameter g (a coupling ratio) rotates the vector between the AFM and PSS sectors and there are no energy barriers between the two at the transition point gc. This phenomenon may be observable in SrCu2(BO3)2.First author draf