Theoretical perspectives on charge dynamics in high-temperature cuprate superconductors

We review recent theoretical progress on the charge dynamics of doped carriers in high-temperature cuprate superconductors. Advances in this field have clarified that doped charges in cuprates exhibit remarkably rich collective behavior, governed by the combined effects of strong electronic correlations, the intrinsic layered crystal structure, and long-range Coulomb interaction. First, the emergence of acousticlike plasmons has been firmly established through quantitative analyses of resonant inelastic x-ray scattering (RIXS) spectra based on the<i>t</i>-<i>J</i>-<i>V</i>model-an extension of the conventional<i>t</i>-<i>J</i>model that incorporates the layered crystal structure and the long-range Coulomb interaction<i>V</i>. These acousticlike plasmons arise near the in-plane momentumq∥=(0,0)and possess characteristic energies far below the well-known ∼1 eV optical plasmon. This behavior is found to be universal across both hole- and electron-doped cuprates, including multilayer systems. Second, in electron-doped cuprates, a pronounced tendency toward<i>d</i>-wave bond-charge order develops nearq∥=(0.5π,0), as revealed by resonant x-ray scattering and RIXS. As a result, the charge dynamics acquires a dual structure, in which low-energy bond-charge excitations coexist with relatively high-energy plasmons. Third, analogous signatures of charge-order tendency have also been reported in hole-doped cuprates. However, a direct application of the<i>d</i>-wave bond-charge-order framework fails to account for experimental observations. Similarly, the charge-stripe order in La-based cuprates remains unresolved within existing theoretical approaches.