Plasmonic contribution to the van der Waals energy in strongly interacting bilayers

We investigate the van der Waals or interaction energy due to the plasmon modes in bilayer fermion and boson systems for several layer separation and coupling strength values. Interaction effects are studied within the random-phase approximation (RPA), the quasilocalized charge approximation (QLCA), and the Singwi, Tosi, Land, and Sjoumllander (STLS) models of the dielectric function formalism. We find that the interaction becomes repulsive at short separation distances for strongly coupled systems described by the QLCA and STLS approaches in contrast to attractive behavior predicted by the RPA. At larger separation distances, the interaction energy changes sign within the QLCA and STLS models leading to an attractive interaction. The evident relation between our calculations and the Casimir effect is emphasized.