Condensed-Matter Physics & Materials Science Seminar

The charge dynamics in cuprates is not comprehensively studied due to the limitation of experimental methods. Resonant inelastic x-ray scattering (RIXS) is a technique that is capable of coupling to various types of excitations and recently has been established as a new way to probe charge density response. Here we use RIXS technique to study two types of charge excitations, the plasmon excitations in electron-doped cuprates and the charge density wave (CDW) excitations in hole-doped cuprates. 1) We track the doping dependence of charge excitations in electron-doped cuprates La2-xCexCuO4. From the resonant energy dependence and the out-of-plane momentum dependence, the charge excitations are identified as three-dimensional plasmons, which reflect the nature of the electronic structure and Coulomb repulsion on both short and long length scale. With increasing electron doping, the plasmon excitations increase monotonically in energy and life time, which reflects the reduction of short-range electronic correlation. 2) We report a comprehensive RIXS study of La2−xSrxCuO4 finding that CDW effects persist up to a remarkably high doping level of x = 0.21 before disappearing at x = 0.25. The inelastic excitation spectra remain essentially unchanged with doping despite crossing a topological transition in the Fermi surface. This indicates that the spectra contain little or no direct coupling to electronic excitations near the Fermi surface, rather they are dominated by the resonant cross-section for phonons and CDW-induced phonon-softening. We interpret our results in terms of a CDW that is generated by strong correlations and a phonon response that is driven by the CDW-induced modification of the lattice.