Charge collective modes in an incommensurately modulated cuprate

TL;DR

This study measures collective charge modes in an insulating cuprate using inelastic resonant x-ray scattering, revealing dispersive ladder modes and contrasting behavior with 2D cuprates, highlighting the utility of momentum-tunable inelastic scattering.

Contribution

First measurement of charge collective modes in Sr14Cu24O41, demonstrating dispersive ladder modes and their interpretation in a strong coupling framework.

Findings

Ladder modes are dispersive along legs but localized along rungs.

Charge spectrum resembles 1D Mott insulators.

Momentum-tunable scattering helps resolve mode contributions.

Abstract

We report the first 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 the ladder-containing planar substructures. The observed ladder modes (E vs. Q) are found to be dispersive for momentum transfers along the "legs" but nearly localized along the "rungs". Dispersion and peakwidth characteristics are similar to the charge spectrum of 1D Mott insulators, and we show that our results can be understood in the strong coupling limit (U >> t_{ladder}> t_{chain}). The observed behavior is in marked contrast to the charge spectrum seen 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 incommensurate systems.