Phase Diagram for Two-Dimensional Spin Systems with Ladder, Plaquette and Mixed-Spin Structures

Abstract

By using the cluster expansion techniques, we study the excitation spectrum for the two-dimensional quantum spin systems with ladder, plaquette and mixed-spin structures. We thus determine the phase boundary between the spin-gap phase and the magnetically ordered phase rather precisely. The phase diagram obtained is consistent with the one previously obtained via the staggered susceptibility, and is further improved for several cases.