Dispersive collective charge modes in an incommensurately modulated cuprate Mott insulator

We report measurement of collective charge modes of insulating Sr₁₄Cu₂₄O₄₁ using inelastic resonant x-ray scattering over the complete Brillouin zone. Our results show that the intense excitation modes at the charge gap edge predominantly originate from ladder-containing planar substructures. The observed modes are found to be dispersive for momentum transfers along the “legs” (ℏQ⃗‖ĉ) but nearly localized along the “rungs” (ℏQ⃗‖â) . We show that the dispersion and peak width characteristics of the modes can be understood in the strong-coupling quantum limit (Hubbard U≫t_ladder>t_chain , where t is the hopping parameter). Quite generally, we demonstrate that the momentum tunability ( Q⃗ resolution) of inelastic x-ray scattering can be utilized to resolve mode contributions in multicomponent incommensurate quantum electron systems.