Exact dimer ground state of the two dimensional Heisenberg spin system SrCu2(BO3)(2)

The two dimensional Heisenberg model for SrCu2(BO3)(2) has an exact dimer g round state which was proven by Shastry and Sutherland almost twenty years ago. The critical value of the quantum phase transition from the dimer stat e to the Neel ordered state is determined. Analysis of the experimental dat a shows that SrCu2(BO3)(2) has the dimer ground state but its coupling cons tants are close to the transition point, which leads to an unusual temperat ure dependence of the susceptibility. The almost localized nature of the tr iplet excitations explains the plateaus observed in the magnetization curve . [S0031-9007(99)09090-0].