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

Theoretical descriptions of quantum phase transitions have indicated the existence of critical points with higher symmetries than those of the underlying Hamiltonian. Here we present an example of such an emergent symmetry at a first-order transition, where coexistence of two ordered phases takes the form of higher rotational symmetry in the space of the two order parameters. Using quantum Monte Carlo simulations, we study a two-dimensional (2D) 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 (a coupling ratio) rotates the vector from the AFM sector to the PSS sector, with the length of the combined order parameter vector always remaining non-zero. This phenomenon should be observable in SrCu₂(BO₃)₂.