Two-dimensional S=1/2 antiferromagnet on a plaquette lattice
  We consider a simplified model of the magnetic structure of the two-dimensional compound $CaV_4O_9$ in terms of interacting square plaquettes of spins with two distinct antiferromagnetic exchange constants. We analyze the competition between two types of singlet ground states and the Neel ordered one in terms of respectively, numerical cluster expansion and nonlinear spin wave theory. The resulting phase diagram agrees well with known Quantum Monte Carlo results and suggests a first order transition between ordered and singlet ground states as a function of the exchange constants. 
S.V. Meshkov (Montpellier, France) and D. Foerster (Bordeaux, France)