Dimerized Phase and Transitions in a Spatially Anisotropic Square Lattice Antiferromagnet

We investigate the spatially anisotropic square lattice quantum antiferromagnet. The model describes isotropic spin-1/2 Heisenberg chains (exchange constant J) coupled antiferromagnetically in the transverse (J_⊥) and diagonal (J_×), with respect to the chain, directions. Classically, the model admits two ordered ground states–with antiferromagnetic and ferromagnetic interchain spin correlations–separated by a first-order phase transition at J_⊥=2J_×. We show that in the quantum model this transition splits into two, revealing an intermediate quantum-disordered columnar dimer phase, both in two dimensions and in a simpler two-leg ladder version. We describe quantum-critical points separating this spontaneously dimerized phase from classical ones.