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-neighbor 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 Néel 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 ′ c increases with growing s according to J ′ c ∝ s(s + 1). Influence of the spin quantum number on zero-temperature phase transition 2