Spin waves and three-dimensionality in the high-pressure antiferromagnetic phase of SrCu₂(BO₃)₂

Quantum magnetic materials can provide explicit realizations of paradigm models in quantum many-body physics. In this context, SrCu₂(BO₃)₂ is a faithful realization of the Shastry-Sutherland model (SSM) for ideally frustrated spin dimers, even displaying several of its quantum magnetic phases as a function of pressure. We perform inelastic neutron scattering (INS) measurements on SrCu₂(BO₃)₂ at 5.5 GPa and 4.5 K, observing spin waves that characterize the high-pressure antiferromagnetic phase. The experimental spectra are well described by linear spin-wave calculations on a SSM with an inter-layer interaction, which is determined accurately as Jc = 0.053(3) meV. The presence of Jc indicates the need to account for the three-dimensional nature of SrCu₂(BO₃)₂ in theoretical models, also at lower pressures. We find that the ratio between in-plane interactions, $J'/J = 1.8(2)$, undergoes a dramatic change compared to lower pressures that we deduce is driven by a sharp drop in the dimer coupling, J. Our results underline the wide horizons opened by high-pressure INS experiments on quantum magnetic materials.