Phase diagram of the Shastry-Sutherland antiferromagnet

The Shastry-Sutherland model, which provides a representation of the magnetic properties of SrCu₂(BO₃)₂, has been studied by series-expansion methods at $T=0.$ Our results support the existence of an intermediate phase between the N\'eel long-range ordered phase and the short-range dimer phase. They provide strong evidence against the existence of helical order in the intermediate phase, and somewhat less strong evidence against a plaquette-singlet phase. The nature of the intermediate phase thus remains elusive. It appears to be gapless, or nearly so.