Symmetry-enhanced discontinuous phase transition in a two-dimensional quantum magnet

In a quantum phase transition, the ground state and low-temperature properties of a system change drastically as some parameter controlling zero-point quantum fluctuations is tuned to a critical value. Like classical phase transitions driven by thermal fluctuations, a ground-state transition can be discontinuous (first order) or continuous. Theoretical studies have suggested exotic continuous transitions where a system develops higher symmetries than those of the underlying Hamiltonian. Here, we demonstrate an unconventional discontinuous transition between two ordered ground states of a quantum magnet, with an emergent symmetry of its coexistence state. We present a Monte Carlo study of a two-dimensional S = 1/2 spin system hosting an antiferromagnetic state and a plaquette-singlet solid state of the kind recently detected in SrCu2(BO3)2. We show that the O(3) symmetric antiferromagnetic order and the scalar plaquette-singlet solid order form an O(4) vector at the transition. Unlike conventional first-order transitions, there are no energy barriers between the two coexisting phases, as the O(4) order parameter can be rotated at constant energy. Away from the transition, the O(4) surface is uniaxially deformed by the control parameter (a coupling ratio). This phenomenon may be observable in SrCu2(BO3)2.