Energy gap in the bilayer: Understanding the failure of the Singwi-Tosi-Land-Sjolander approximation

We demonstrate that the conflicting predictions of the STLS (Singwi-Tosi-Land-Sjolander) and the quasilocalized charge approximations concerning the lack of, or the existence of, an energy gap, respectively, in the collective excitation spectrum of an electronic bilayer can be traced to a formal defect of the STLS scheme which renders it inappropriate for the analysis of the k-->0 behavior of collective modes in binary systems. We conclude that in addition to the cogent physical reasons that mitigate in favor of its existence, the structural isomorphism between the bilayer and binary systems also mandates an energy gap in the former. [S0163-1829(98)04312-4].