Quantum Phase Transitions in Two-Dimensional Spin Systems with Ladder, Plaquette and Mixed-Spin Structures

Quantum phase transitions between the magnetically ordered and disordered states are studied for the two-dimensional antiferromagnetic quantum spin systems with ladder, plaquette, and mixed-spin structures. Starting with properly chosen singlet-cluster configurations, we perform the series expansion for the staggered magnetic susceptibility. The phase boundary is determined by applying the Dlog and biased Pad\'e approximants to the staggered susceptibility thus obtained. The resulting phase diagram allows us to discuss the quantum phase transitions quantitatively, which agrees fairly well with the quantum Monte Carlo results for several cases previously studied.