Excitation Spectrum for Quantum Spin Systems with Ladder, Plaquette and Mixed-Spin Structures

By using the series expansion techniques, we study the excitation spectrum for the two-dimensional quantum spin systems with ladder, plaquette and mixed-spin structures. We calculate the spin excitation gap and thus determine the phase boundary between the spin-gap phase and the magnetically ordered phase rather precisely. It is found that the phase diagram obtained improves fairly well the one previously obtained via the ground-state susceptibility.