We report inelastic neutron scattering (INS) measurements on the magnetically frustrated S=1/2 sawtooth-chain compound atacamite Cu₂Cl (OH) ₃ featuring inequivalent Cu (1) and Cu (2) sites. Transverse to the sawtooth chains, INS reveals two dispersive spin-wave modes and a gap of at least 0. 75 meV. This behavior is rationalized within a zigzag-chain model of Cu (2) spins in an effective magnetic field of Cu (1) spins. The model is compatible with first-principles calculations and accounts for INS dispersions within linear spin-wave theory calculations. Our results reveal a unique case of an effective separation of energy scales between two differently oriented one-dimensional chains, with the zigzag-chain model being essential to fully characterize the low-energy magnetism of atacamite.
Allen et al. (Thu,) studied this question.