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 to monitor the temperature dependence of the $\underset{\_}{d}d$ exciton spectrum of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8-x}$ crystals with different charge carrier concentrations. We observe a significant change of the $\underset{\_}{d}d$ exciton spectra when the materials pass from the normal state into the superconductor state. Our observations show that the $\underset{\_}{d}d$ 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 $\underset{\_}{d}d$ exciton to the surrounding copper spins.

中文翻译：

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高 TcCuprates 中电子-激子耦合的共振非弹性 X 射线散射研究

解释超导的机制在高${吨}_C$铜酸盐需要了解导致电子形成库珀对的原因。配对可以通过声子、屏蔽库仑力、自旋或电荷波动、激子或这些的组合来介导。已经假设了激子配对机制，但铜酸盐中传导电子和激子之间耦合的实验证据是零星的。在这里，我们使用共振非弹性 X 射线散射来监测温度依赖性$\underset{\_}{d}d$的激子谱${双}_2{锶}_2{\mathrm{钙铜}}_2{\mathrm{○}}_{8-X}$具有不同电荷载体浓度的晶体。我们观察到显着的变化$\underset{\_}{d}d$当材料从正常状态进入超导体状态时的激子光谱。我们的观察表明，$\underset{\_}{d}d$当材料进入超导相时，过掺杂（欠掺杂）样品中的激子开始上移（下移）。我们将超导引起的效应及其从欠掺杂到过掺杂的符号反转归因于该位点的交换耦合。$\underset{\_}{d}d$激子对周围的铜自旋。