Two-dimensional S = 1/2 antiferromagnet on a plaquette lattice

Abstract

We consider a simplified model of the magnetic structure of the two-dimensional compound in terms of interacting square plaquettes of spins with two distinct antiferromagnetic exchange constants. We analyse 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.