DIFFERENTIAL OPERATOR TECHNIQUE FOR HIGHER SPIN PROBLEMS

A new effective-field theory for the Blume-Capel model with a high spin value S is developed by making use of exact spin identities and taking advantage of the differential operator technique. The general formulation for evaluating the transition line and the tricritical point is derived. In particular, the phase diagrams are examined for S = 3/2 and S = 2. Our results show that the tricritical behavior does not exist in the spin-3/2 Blume-Capel model but does exist in the spin-2 Blume-Capel model. The tricritical point in the S = 2 systems is found at D/zJ congruent-to -0.498, where z is the coordination number, D the crystal-field constant and J the exchange interaction.