On the dynamics of coupled S=1/2 antiferromagnetic zigzag chains

We investigate the elementary excitations of quasi-one-dimensional S = 1/2 systems built up from zigzag chains with general isotropic exchange constan ts, using exact (Lanczos) diagonalization for 24 spins and series expansion s starting from the decoupled dimer limit. For the ideal one-dimensional zi gzag chain we discuss the systematic variation of the basic (magnon) triple t excitation with general exchange parameters and in particular the presenc e of practically flat dispersions in certain regions of phase space. We ext end the dimer expansion in order to include the effects of three-dimensiona l interactions on the spectra of weakly interacting zigzag chains. In an ap plication to KCuCl3 we show that this approach allows us to determine the e xchange interactions between individual pairs of spins from the spectra as determined in recent neutron scattering experiments.