Acoustic plasmons and conducting carriers in hole-doped cuprate superconductors

The layered crystal structures of cuprates enable collective charge excitations fundamentally different from those of three-dimensional metals. Acoustic plasmons have been observed in electron-doped cuprates by resonant inelastic x-ray scattering (RIXS); in contrast, the characteristics of acoustic plasmons in hole-doped cuprates are under debate, despite extensive measurements. This contrast led us to investigate the charge excitations of hole-doped cuprate ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_x{\mathrm{CuO}}₄$. Here we present incident-energy-dependent RIXS measurements and calculations of collective charge response via the loss function to reconcile the issues above. Our results provide evidence for the acoustic plasmons of Zhang-Rice singlet (ZRS), which has a character of the Cu $3{d}_{{x}²\ensuremath{-}{y}²}$ strongly hybridized with the O $2p$ orbitals; the metallic behavior is implied to result from the movement of ZRS rather than the simple hopping of O $2p$ holes.