Cluster-expansion method for the infinite-range quantum transverse Ising spin-glass model

Abstract
The infinite-range quantum Ising spin-glass model in a transverse field Γ is studied within the cluster-expansion method formulated originally for spin systems by Morita and Tanaka. With use of the pair approximation, the mean-field equation and condition for critical values of Γc and Tc are obtained including the case Tc→0.