Ground state phase diagram of a spin-2 antiferromagnet on the square lattice

We use the vertex state model approach to construct optimum ground states f or a large class of quantum spin-2 antiferromagnets on the square lattice. Optimum ground states are exact ground states of the model which minimize a ll local interaction operators. The ground state contains two continuous pa rameters and exhibits a second order phase transition from a disordered pha se with exponentially decaying correlation functions to a Neel ordered phas e. The behaviour is very similar to that of the corresponding ground state of a quantum spin-3/2 model on the hexagonal lattice, which has been invest igated in an earlier paper.