Charge dynamics in an incommensurate layered cuprate $Sr_{14}Cu_{24}O_{41}$ : A momentum-resolved study

We report the first \textit{momentum resolved} charge mode spectrum of insulating $Sr_{14}Cu_{24}O_{41}$ using inelastic resonant x-ray scattering. Our results show that the intense excitation modes at the charge gap edge predominantly originate from the ladder-containing two-dimensional planes. The observed modes (E vs. Q) are found to be dispersive for momentum transfers along the ``legs'' ($\vec{Q}$ $\parallel$ $\hat{c}$) but nearly localized along the ``rungs'' ($\vec{Q}$ $\parallel$ $\hat{a}$). Dispersion and peakwidth characteristics are strongly similar to the low energy charge spectrum of quasi-one dimensional $SrCuO_2$, and we suggest a qualitative explanation in terms of a model in the strongly correlated limit ($U\gg t$). This behavior is in marked contrast to the charge spectrum observed in most two dimensional cuprates. Quite generally, our results also show that momentum-tunability of inelastic scattering can be used to resolve mode contributions in multi-component correlated systems.