Strong-coupling theory of bilayer plasmon excitations

Abstract

Recently plasmon excitations in bilayer lattice systems were studied extensively in the weak-coupling regime. Unlike single-layer systems, these bilayers exhibit two distinct modes, 𝜔±, which show characteristic dependences upon the momentum and hopping integrals along the 𝑧 direction. To apply them to cuprates, strong correlation effects should be considered, but a comprehensive analysis has not yet been investigated. In this work, we present a strong-coupling theory to analyze the charge dynamics of a bilayer system, utilizing the 𝑡−𝐽−𝑉 model, which includes the long-range Coulomb interaction, 𝑉, on a lattice. Although our theoretical framework is fundamentally different from the weak-coupling approach, we find that resulting plasmon excitations are similar to those of a weak-coupling theory. A key distinction is that our strong-coupling framework reveals a noticeable suppression of particle-hole excitations, which allows the plasmon modes to remain well defined over a wider region of momentum. We suggest that the experimentally reported plasmon excitations in Y-based cuprates can be described by the 𝜔− mode, although we call for more systematic experiments to verify this.