Spectrum of elementary and collective excitations in the dimerized S=1/2 Heisenberg chain with frustration

We have studied the low-energy excitation spectrum of a dimerized and frust rated antiferromagnetic Heisenberg chain. We use an analytic approach, base d on a description of the excitations as triplets above a strong-coupling s inglet ground state. The quasiparticle spectrum is calculated by treating t he excitations as a dilute Bose gas with infinite on-site repulsion. Additi onal singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of the elementary triplets. We have also calculated the contribution s of the elementary and collective excitations into the spin structure fact or. Our results are in excellent agreement with exact diagonalizations and dimer series expansions data as long as the dimerization parameter Sis not too small (delta > 0.1), i.e., while the elementary triplets can be treated as localized objects. [S0163-1829(99)02329-2].