Influence of the spin quantum number s on the zero-temperature phase transition in the square-lattice J-J′ model

We investigate the phase diagram of the Heisenberg antiferromagnet on the square lattice with two different nearest-neighbour bonds J and J' (J-J' model) at zero temperature. The model exhibits a quantum phase transition at a critical value J(c)' > J between a semi-classically ordered Neel and a magnetically disordered quantum paramagnetic phase of valence-bond type, which is driven by local singlet formation on J' bonds. We study the influence of spin quantum number s on this phase transition by means of a variational mean-field approach, the coupled cluster method and the Lanczos exact-diagonalization technique. We present evidence that the critical value J,,increases with growing s according to J(c)' proportional to s(s + 1).