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 (Néel, 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 J₂^d∕J₁∼1.477. For the critical point J₂^c∕J₁ where the semiclassical Néel order disappears we obtain a significantly lower value than J₂^d∕J₁, namely, J₂^c∕J₁ in the range 1.14–1.39. We therefore conclude that an intermediate phase exists between the Néel 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 J₂. The intermediate phase is therefore magnetically disordered but may exhibit plaquette or columnar dimer ordering.