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

Understand

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 SrCu$_2$(BO$_3$)$_2$.
• We observe that the O(3) symmetric AFM order and the Z$_2$ symmetric PSS order form an O(4) vector at the transition.