Strong-coupling theory of bilayer plasmon excitations

TL;DR

This paper develops a strong-coupling theoretical framework using the $t$-$J$-$V$ model to analyze bilayer plasmon excitations, revealing similarities to weak-coupling results but with enhanced mode stability and suppression of particle-hole excitations.

Contribution

It introduces a strong-coupling approach to bilayer plasmon excitations, extending understanding beyond the weak-coupling regime with implications for cuprate materials.

Findings

Plasmon modes are similar in strong- and weak-coupling theories.

Strong-coupling suppresses particle-hole excitations, stabilizing plasmons.

Y-based cuprate plasmons may correspond to the $oldsymbol{ ext{ω}}_{-}$ mode.

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, $\omega_{\pm}$, 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 $\omega_{-}$ mode, although we call for more systematic experiments to verify this.