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

• Published in: Physical review. X 2022-06, Vol.12 (2), Article 021068
• Main Authors: Barantani, F., Tran, M. K., Madan, I., Kapon, I., Bachar, N., Asmara, T. C., Paris, E., Tseng, Y., Zhang, W., Hu, Y., Giannini, E., Gu, G., Devereaux, T. P., Berthod, C., Carbone, F., Schmitt, T., van der Marel, D.
• Format: Article
• Language: English
• Subjects: CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; excitons; pairing mechanisms; resonant inelastic x-ray scattering; superconductivity
• ISSN: 2160-3308; 2160-3308
• DOI: 10.1103/PhysRevX.12.021068

Explaining the mechanism of superconductivity in the high-Tc cuprates 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 the $\underline {dd}$ exciton spectrum of Bi2Sr2CaCu2O8–x crystals with different charge carrier concentrations. We observe a significant change of the $\underline {dd}$ exciton spectra when the materials pass from the normal state into the superconductor state. Our observations show that the $\underline {dd}$ excitons 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 the $\underline {dd}$ exciton to the surrounding copper spins.