2012/04/19 by Pina Romaniello, Friedhelm Bechstedt, Lucia Reining · 1 citation
Physics and Astronomy · #cond-mat.str-el
paper · pdf · doi:10.1103/physrevb.85.155131
published as Phys. Rev. B 85, 155131 (2012) · 15 pages, 7 figures
arxiv created 2012/04/19 · arxiv updated 2012/04/23
In many-body perturbation theory (MBPT) the self-energy Σ=iGWΓplays the key role since it contains all the many body effects of the system. The exact self-energy is not known; as first approximation one can set the vertex function Γto unity which leads to the GW approximation. The latter properly describes the high-density regime, where screening is important; in the low-density regime, instead, other approximations are proposed, such as the T matrix, which describes multiple scattering between two particles. Here we combine the two approaches. Starting from the fundamental equations of MBPT we show how one can derive the T-matrix approximation to the self-energy in a common framework with GW. This allows us to elucidate several aspects of this formulation, including the origin of, and link between, the electron-hole and the particle-particle T matrix, the derivation of a screened T matrix, and the conversion of the T matrix into a vertex correction. The exactly solvable Hubbard molecule is used for illustration.