Dispersive collective charge modes in an incommensurately modulated cuprate Mott insulator

We report measurement of collective charge modes of insulating Sr14 Cu24 O41 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" (Q c) but nearly localized along the "rungs" (Q a). We show that the dispersion and peak width characteristics of the modes can be understood in the strong-coupling quantum limit (Hubbard U tladder > tchain, where t is the hopping parameter). Quite generally, we demonstrate that the momentum tunability (Q resolution) of inelastic x-ray scattering can be utilized to resolve mode contributions in multicomponent incommensurate quantum electron systems.