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

Abstract

Explaining the mechanism of superconductivity in the high-$T_c$ 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 (RIXS) to monitor the temperature dependence of the $\underline{d}d$ exciton spectrum of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8-x}$ (Bi-2212) crystals with different charge carrier concentrations. We observe a significant change of the $\underline{d}d$ exciton spectra when the materials pass from the normal state into the superconductor state. Our observations show that the $\underline{d}d$ 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 $\underline{d}d$ exciton to the surrounding copper spins.

13 pages, 10 figures

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Cited by in corpus (5)

• Reconciling scaling of the optical conductivity of cuprate superconductors with Planckian resistivity and specific heat
• Collective nature of orbital excitations in layered cuprates in the absence of apical oxygens
• Dynamics of non-thermal states in optimally-doped $B{i}_{2}S{r}_{2}C{a}_{0.92}{Y}_{0.08}C{u}_2{O}_{8+\delta }$ revealed by mid-infrared three-pulse spectroscopy
• Ligand Field Exciton Annihilation in Bulk CrCl3
• Evolution of orbital excitations from insulating to superconducting MgTi${}_2$O${}_4$ films