One-dimensional spin-liquid without magnon excitations

We show that the Coulomb interaction between magnetic chains is essential even in the insulating phase introducing frustration and pushing the system towards dimerization. The dimerized phase of the spin-1/2 Heisenberg spin ladder is thermodynamically undistinguishable from the Haldane phase, but has dramatically different correlation functions. Its dynamical magnetic susceptibility does not have a sharp single magnon peak near $q = \pi$, but shows only a two-particle threshold separated from the ground state by a gap.