Coupled cluster treatment of the Shastry-Sutherland antiferromagnet

Abstract

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 (N\'eel, 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 ${J}_{2}^{d}∕{J}_{1}\ensuremath{\sim}1.477$. For the critical point ${J}_{2}^{c}∕{J}_{1}$ where the semiclassical N\'eel 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 N\'eel 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.