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 J2d/J1∼1.477. For the critical point J2c/J1 where the semiclassical Néel order disappears we obtain a significantly lower value than J2d/J1, namely, J2c/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. © 2005 The American Physical Society.