The Transition from an Ordered Antiferromagnet to a Quantum Disordered Spin Liquid in a Solvable Bilayer Model

We present a spin-(1/2) bilayer model for the quantum order-disorder transition which i) can be solved by mean-field theory for bulk quantities, ii) becomes critical at the transition, and iii) allows to include intra-layer frustration. We present numerical data (for systems with up to 240 sites) and analytical results for the critical coupling strength, ground-state energy, order parameter and for the gap. We show that the critical coupling decreases linearly with frustration.