Electron correlations play a dominant role in the charge dynamics of the cuprates. We use resonant inelastic X-ray scattering (RIXS) to track the doping dependence of the collective charge excitations in electron doped La\({}_{2-x}\)Ce\({}_{x}\)CuO\({}_{4}\) (LCCO). From the resonant energy dependence and the out-of-plane momentum dependence, the charge excitations are identified as three-dimensional (3D) plasmons, which reflect the nature of the electronic structure and Coulomb repulsion on both short and long lengthscales. With increasing electron doping, the plasmon excitations increase monotonically in energy, a consequence of the electron correlation effect on electron structure near the Fermi surface (FS). Importantly, the plasmon excitations evolve from a broad feature into a well-defined peak with much increased life time, revealing the evolution of the electrons from incoherent states to coherent quasi-particles near the FS. Such evolution marks the reduction of the short-range electronic correlation, and thus the softening of the Mottness of the system with increasing electron doping.

电子相关性在铜酸盐的电荷动力学中起主要作用。我们使用共振非弹性X射线散射（RIXS）来跟踪电子掺杂La \（{} _ {2-x} \） Ce \ {{} _ {x} \） CuO \中集体电荷激发的掺杂依赖性。（{} _ {4} \）（LCCO）。从共振能量依赖性和平面外动量依赖性，电荷激发被识别为三维（3D）等离子体激元，它们在长短尺度上均反映了电子结构和库仑排斥的性质。随着电子掺杂的增加，等离激元激发的能量单调增加，这是电子相关效应对费米表面（FS）附近的电子结构的影响。重要的是，等离激元激发从宽泛的特征演变为具有延长的寿命的明确定义的峰，揭示了电子从FS附近的非相干态演化为相干的准粒子。这种演变标志着短程电子相关性的降低，并因此随着电子掺杂的增加而使系统的Mottness软化。