Dynamical responses and instabilities of the Shastry-Sutherland model

The Shastry-Sutherland model (SSM) is famous for holding an exact ground state solution for inter-dimer exchange J'/J≤0.7. In this exact ground state, spins form singlets on the dimers. Single triplon excitations have a rather flat dispersion that renders perturbative treatments of the triplon-triplon interactions simply inadequate. In addition, the multiple competing length scales of this model also limits the applicability of finite size numerical simulations.
We introduce an unbiased variational method to study static and dynamic T=0 properties of the SSM in the thermodynamic limit. The variational basis is generated by dressing up the triplon excitations via a systematic inclusion of quantum fluctuations. The accuracy of this method is controlled by the number of iterations, which controls the size of the variational space. By solving the model in the thermodynamic limit, we can obtain dynamical responses (relevant to neutron, Raman and inelastic X-ray scattering) with high momentum resolution. Moreover, we can unbiasedly predict instabilities near the exact dimer phase, induced by single or multi-triplon condensation. We will discuss several competing orders (magnetic and nematic) next to the dimer phase and reveal the resulting phase diagram.