Excitation spectrum and ground-state properties of the S=1/2 Heisenberg ladder with staggered dimerization

We study the excitation spectrum and ground-state properties of the two-leg S=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, e xact diagonalization of small clusters, and diagrammatic analysis of an eff ective interacting Bose gas Hamiltonian, obtained by using the bond operato r 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 deter mined. [S0163-1829(99)06917-9].