Excitation spectrum and ground - state properties of the S 5 12 Heisenberg ladder with staggered dimerization

We study the excitation spectrum and ground-state properties of the two-leg S5 1 2 quantum spin ladder with staggered dimerization. Two massive phases separated by a critical line are found, as predicted by previous analysis, based on the nonlinear sigma model. We have used dimer series expansions, exact diagonalization of small clusters, and diagrammatic analysis of an effective interacting Bose gas Hamiltonian, obtained by using the bond operator representation for spins. We compute the excitation spectrum in the phase, dominated by strong rungs in the parameter regimes far away and close to the point of instability. The exact location of the phase boundary is determined. @S0163-1829~99!06917-9#