Out-of-phase plasmon excitations in the trilayer cuprate Bi2Sr2Ca2Cu3 O10+δ

Abstract

Within a homologous series of cuprate superconductors, variations in the stacking of CuO2 layers influence the collective charge dynamics through the long-range Coulomb interactions. We use O K-edge resonant inelastic x-ray scattering to reveal plasmon excitations in the optimally doped trilayer Bi2Sr2Ca2Cu3O10+δ. The observed plasmon exhibits nearly qz-independent dispersion and a large excitation gap of approximately 300 meV. This mode is primarily ascribed to the ω- mode, where the charge density on the outer CuO2 sheets oscillates out of phase while the density in the inner sheet remains unaltered at qz=0. The intensity of the acoustic ω3 mode is relatively weak and becomes vanishingly small near (qx,qy)=(0,0). This result highlights a qualitative change in the eigenmode of the dominant low-energy plasmon with the number of CuO2 layers.