Quantum phase transitions in two-dimensional spin systems with ladder, plaquette and mixed-spin structures

Quantum phase transitions between the magnetically ordered and disordered s tates are studied for the two-dimensional antiferromagnetic quantum spin sy stems with ladder, plaquette, and mixed-spin structures. Starting with prop erly chosen singlet-cluster configurations, we perform the series expansion for the staggered magnetic susceptibility. The phase boundary is determine d by applying the Dlog and biased Pade approximants to the staggered suscep tibility thus obtained. The resulting phase diagram allows us to discuss th e quantum phase transitions quantitatively, which agrees fairly well with t he quantum Monte Carlo results for several cases previously studied.