Weak universal critical behaviour of the mixed spin-(1/2, S) Ising model on the union jack (centered square) lattice: integer versus half-odd-integer spin-S case

The mixed spin-(1/2, S) Ising model on the union jack (centered square) lattice is investigated by establishing the mapping relationship with its corresponding eight-vertex model. An interplay between the nearest-neighbour interaction, the competing next-nearest-neighbour interaction and the single-ion anisotropy gives rise to a rather complex critical behaviour displayed in the reentrant phase transitions, the weak universal critical behaviour, as well as, a presence of first- and second-order phase transitions. The most interesting finding to emerge from the present study relates to a variation of the weak-universal critical exponents along the line of bicritical points, which is being twice as large for the mixed spin-(1/2, S) systems with the integer spin-S atoms as for the ones with the half-odd-integer spin-S atoms.