发表机构
Lawrence Berkeley National Laboratory; University of California, Berkeley; SLAC National Accelerator Laboratory; Stanford University; Seoul National University; National Synchrotron Radiation Research Center; Central Research Institute of Electric Power Industry（劳伦斯伯克利国家实验室; 加州大学伯克利分校; 斯坦福直线加速器中心; 斯坦福大学; 首尔国立大学; 台湾国家同步辐射研究中心; 电力中央研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
通过共振非弹性X射线散射发现La2-xSrxCuO4中偶数阶声子的选择性耦合，揭示超出弗兰克-康登图像的异常电子-声子耦合，可能暗示绝缘铜氧化物中存在局域配对准粒子。
AI 中文摘要
在电荷载流子与原子振动之间具有强耦合的晶体和分子中，可以观察到振动激发的高次谐波，其强度遵循弗兰克-康登包络线。在此，我们利用共振非弹性X射线散射揭示了La2-xSrxCuO4中电子-声子耦合的一个新机制。在未掺杂化合物中，我们观察到在约85±5 meV、180±7 meV和330±5 meV处有尖锐峰，且无可观测的色散，同时在该能量范围内拉曼光谱中观测到的其他模式没有明显的强度。180 meV和330 meV激发能量与面内Cu-O键伸缩模式的双声子和四声子能量大致一致，表明共振激发的价电子与该模式强耦合。这些激发表现出显著的掺杂依赖性，在10%空穴掺杂时最终变得无法分辨，此时低能电荷激发由色散等离激元主导。对偶数阶声子的选择性耦合以及与拉曼光谱的鲜明对比表明，存在超出弗兰克-康登图像的异常电子-声子耦合。一个有趣的可能性是绝缘铜氧化物中存在局域配对的准粒子。
英文摘要
In crystals and molecules with strong coupling between the charge carriers and atomic vibrations, high order harmonics of the vibrational excitations can be observed with intensity following the Franck-Condon envelope. Here, we uncover a new regime of electron-phonon coupling in La2-xSrxCuO4 using resonant inelastic X-ray scattering. In the undoped compound, we find sharp peaks at approximately 85$\pm$5 meV, 180$\pm$7 meV, and 330$\pm$5 meV with no observable dispersion, and an absence of appreciable intensity from other modes observed in Raman spectroscopy in this energy range. The 180 meV and 330 meV excitations have energies approximately consistent with two-phonon and four-phonon in-plane Cu-O bond stretching mode, suggesting that the resonantly excited valence electrons strongly couple to this mode. These excitations exhibit significant doping dependence, eventually becoming unresolvable at 10% hole doping, where the low energy charge excitations are dominated by dispersive plasmons. The observation of selective coupling to even-ordered phonons and stark contrast with the Raman spectra indicate anomalous electron-phonon coupling beyond the Franck-Condon picture. One intriguing possibility is the existence of locally paired quasiparticles in insulating cuprates.
