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

TL;DR

This study uses resonant inelastic x-ray scattering to observe how electron-exciton coupling varies with temperature in high-Tc cuprates, revealing a link between excitons and superconductivity that depends on doping levels.

Contribution

It provides experimental evidence of temperature-dependent electron-exciton interactions in cuprates, supporting the excitonic pairing mechanism hypothesis.

Findings

Significant shifts in $ ext{d}d$ exciton spectra across the superconducting transition.

Doping-dependent sign reversal of exciton energy shifts.

Evidence of exchange coupling between excitons and copper spins.

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.