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 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.
@article{arxiv.cond-mat/0107019,
title = {The Phase Diagram of the Shastry-Sutherland Antiferromagnet},
author = {Zheng Weihong and J. Oitmaa and C. J. Hamer},
journal= {arXiv preprint arXiv:cond-mat/0107019},
year = {2007}
}