Quantum Phase Transition in a Heisenberg Antiferromagnet on a Square   Lattice with Strong Plaquette Interactions
  We present numerical results for an $S=1/2$ Heisenberg antiferromagnet on a inhomogeneous square lattice with tunable interaction between spins belonging to different plaquettes. Employing Quantum Monte Carlo, we significantly improve on previous results for the the critical point separating singlet-disordered and N\'{e}el-ordered phases, and obtain an estimate for the critical exponent $\nu$ consistent with the three-dimensional classical Heisenberg universality class. Additionally, we show that a fairly accurate result for the critical point can be obtained from a Contractor Renormalization (CORE) expansion by applying a surprisingly simple analysis to the effective Hamiltonian. 
A. Fabricio Albuquerque, Matthias Troyer, Jaan Oitmaa