Spin Waves and Three Dimensionality in the High-Pressure Antiferromagnetic Phase of SrCu_{2}(BO_{3})_{2}

Quantum magnetic materials can provide explicit realizations of paradigm models in quantum many-body physics. In this context, SrCu_{2}(BO_{3})_{2} is a faithful realization of the Shastry-Sutherland model for ideally frustrated spin dimers, even displaying several of its quantum magnetic phases as a function of pressure. We perform inelastic neutron scattering measurements on SrCu_{2}(BO_{3})_{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 Shastry-Sutherland model with an interlayer interaction, which is determined accurately as J_{c}=0.053(3)  meV. The presence of J_{c} indicates the need to account for the three-dimensional nature of SrCu_{2}(BO_{3})_{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 inelastic neutron scattering experiments on quantum magnetic materials.