Acoustic plasmons and conducting carriers in hole-doped cuprate superconductors
Amol Singh, H. Y. Huang, Christopher Lane, J. H. Li, J. Okamoto, S. Komiya, R. S. Markiewicz, Arun Bansil, T. K. Lee, A. Fujimori, C. T. Chen, D. J. Huang
Abstract
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.