Ground state properties of two spin models with exactly known ground   states on the square lattice
  We introduce a new two-dimensional model with diagonal four spin exchange and an exactly knownground-state. Using variational ansaetze and exact diagonalisation we calculate upper and lower bounds for the critical coupling of the model. Both for this model and for the Shastry-Sutherland model we study periodic systems up to system size 6x6. 
A. Sindermann, U. L\"ow, J. Zittartz