Mixed spin transverse Ising models; a cluster theory

We investigate the critical properties of a mixed spin-1/2 and spin-S (S>1/2) transverse Ising model, changing the value of S. In this study, the new type of cluster theory is introduced on the basis of the differential operator technique and Ising spin identities. It is found that the theory shows the statistical accuracy almost equivalent to the Bethe–Peierls approximation.