Coupled cluster treatment of the Shastry-Sutherland antiferromagnet

We consider the zero-temperature properties of the spin-half two-dimensional Shastry-Sutherland antiferromagnet by using a high-order coupled cluster method treatment. We find that this model demonstrates various ground-state phases (Neel, magnetically disordered, orthogonal dimer), and we make predictions for the positions of the phase transition points. In particular, we find that the orthogonal-dimer state becomes the ground state at J(2)(d)/J(1)similar to 1.477. For the critical point J(2)(c)/J(1) where the semiclassical Neel order disappears we obtain a significantly lower value than J(2)(d)/J(1), namely, J(2)(c)/J(1) in the range 1.14-1.39. We therefore conclude that an intermediate phase exists between the Neel and the dimer phases. An analysis of the energy of a competing spiral phase yields clear evidence that the spiral phase does not become the ground state for any value of J(2). The intermediate phase is therefore magnetically disordered but may exhibit plaquette or columnar dimer ordering.