Gapped plasmon excitations and interlayer hopping in cuprate superconductors

Acoustic plasmons emerge in layered systems with conducting planes and poorly screened interlayer Coulomb interaction. Here, we use resonant inelastic x-ray scattering (RIXS) to probe the dispersion of plasmons in the electron-doped cuprate superconductor Sr_0.9La_0.1CuO₂ (SLCO). We detect a plasmon gap of ∼120 meV at the two-dimensional Brillouin zone center, indicating that low-energy plasmons in SLCO are not strictly acoustic. Our t-J-V model calculations accurately capture the plasmon gap and establish that its size is directly related to the magnitude of the interlayer hopping t_z—a parameter that is of key importance for realistic theories, yet notoriously difficult to assess with other techniques. Our work signifies the three-dimensionality of the charge dynamics in layered cuprates and provides a new method to determine t_z.