Alternating spin chains with singlet ground states

We investigate low-energy properties of the alternating-spin-chain model composed of spins s(1) and s(2) with a singlet ground state. After examining the spin-wave spectrum in detail, we map low-energy spin excitations to the O(3) nonlinear sigma model in order to take into account quantum fluctuations. Analyzing the topological term in the resulting sigma model, we discuss how the massless or massive excitations are developed, especially according to the topological nature of the alternating spin system.