Resonant Inelastic X-Ray Scattering Study of Electron-Exciton Coupling in High-Tc Cuprates

Explaining the mechanism of superconductivity in the high-Tccuprates requires an understanding of what causes electrons to form Cooper pairs. Pairing can be mediated by phonons, the screened Coulomb force, spin or charge fluctuations, excitons, or by a combination of these. An excitonic pairing mechanism has been postulated, but experimental evidence for coupling between conduction electrons and excitons in the cuprates is sporadic. Here we use resonant inelastic x-ray scattering to monitor the temperature dependence of thed_dexciton spectrum ofBi2Sr2CaCu2O8−xcrystals with different charge carrier concentrations. We observe a significant change of thed_dexciton spectra when the materials pass from the normal state into the superconductor state. Our observations show that thed_dexcitons start to shift up (down) in the overdoped (underdoped) sample when the material enters the superconducting phase. We attribute the superconductivity-induced effect and its sign reversal from underdoped to overdoped to the exchange coupling of the site of thed_dexciton to the surrounding copper spins.