Resonant Inelastic X-ray Scattering Evidence for Electron-Exciton Coupling in Bi2Sr2CaCu2O8 High-Tc Cuprates

We use resonant inelastic x-ray scattering (RIXS) to monitor the temperature dependence of the dd exciton spectrum in Bi2Sr2CaCu2O8 (Bi-2212) high-Tc superconductors with different charge carrier concentrations. We observe a significant change of the dd exciton spectra when the materials pass from the normal state into the superconductor state, namely the exciton shifts to higher (lower) energy when the overdoped (underdoped) material enters the superconducting phase. These effects can be understood from the exchange coupling of the dd exciton to the surrounding copper spins in the different parts of the doping/temperature phase diagram and points toward strong electron-exciton coupling. The proposal of electron-exciton coupling dates back to the pioneering work by William Little in the field of organic superconductors, and of Vitaly Ginzburg and Allender, Bray and Bardeen on sandwich structures some 50 years ago. Interest was revived in the context of high-Tc cuprates by Little and Holcomb, who obtained indications for electron-exciton coupling from optical experiments.