The generalized Kadanoff-Baym ansatz. Computing nonlinear response properties of finite systems

TL;DR

This paper uses the generalized Kadanoff-Baym ansatz and second Born approximation to analyze nonlinear density-response in finite Hubbard-type systems, revealing correlation effects and double excitation phenomena.

Contribution

It demonstrates the application of GKBA and 2B approximation to nonlinear response, highlighting correlation features and the U-dependence of double excitations.

Findings

Strong correlation features in response spectra

Connection to double excitation processes

Anomalous U-dependence of double excitation energy

Abstract

For a minimal Hubbard-type system at different interaction strengths U, we investigate the density-response for an excitation beyond the linear regime using the generalized Kadanoff-Baym ansatz (GKBA) and the second Born (2B) approximation. We find strong correlation features in the response spectra and establish the connection to an involved double excitation process. By comparing approximate and exact Green's function results, we also observe an anomalous U-dependence of the energy of this double excitation in 2B+GKBA. This is in accordance with earlier findings [K. Balzer et al., EPL 98, 67002 (2012)] on double excitations in quantum wells.