Theoretical descriptions of quantum phase transitions have indicated the existence of critical points with higher symmetry than those of the underlying Hamiltonian. Points of emergent symmetry have not been expected at discontinuous (first-order) transitions, however. I will discuss such an example [1], where phase coexistence at a first-order transition takes the form of an enhanced rotational symmetry in a space of two order parameters. Using quantum Monte Carlo simulations to study a 2D S=1/2 quantum magnet hosting the antiferromagnetic (AFM) and plaquette singlet (PS) states recently detected in SrCu2(BO3)2, we observe that the O(3) symmetric AFM order and the Z2 symmetric PS order form an O(4) vector at the transition. The control parameter (a coupling ratio) rotates the vector from the AFM sector to the PS sector, with the length of the combined order parameter vector always remaining non-zero. This phenomenon should be observable in neutron scattering experiments on SrCu2(BO3)2.
[1] B. Zhao, P. Weinberg, A. W. Sandvik (in preparation)