Weakly first-order melting of the 1/3 plateau in the Shastry-Sutherland model

We investigate the thermal properties of the 13 plateau in the Shastry-Sutherland model with infinite projected entangled-pair states by performing the imaginary-time evolution of the infinite-temperature density matrix. We show that both the Z2 and Z3 broken symmetries of the ground state are restored at a unique temperature where the correlation length has a peak, and that the melting of the plateau occurs via a single weakly first-order transition. We focus on the experimentally relevant coupling constants deep into the 13 plateau phase at h=1 and J′/J=0.63, which was estimated to describe the SrCu2(BO3)2 compound. By computing the free energy we are able to locate the transition temperature around Tc≃4.8K, well above the temperature T=2K at which the 13 plateau was observed in experiments. The investigation is supplemented by adding a bias term to the Hamiltonian and studying the induced crossover. We further map the transition line in the field-temperature phase diagram.