# Niko Säkkinen - Application of Time-Dependent Many-Body Perturbation Theory to Excitation Spectra of Selected Finite Model Systems

### MPSD Seminar

- Date: Feb 15, 2017
- Time: 14:00 - 15:00
- Speaker: Niko Säkkinen
- University of Jyväskylä, Finland
- Location: CFEL (Bldg. 99)
- Room: Seminar Room III, EG.080
- Host: Angel Rubio

In this talk, by considering only small, finite lattice systems, I instead summarize some of the properties of simple, but self-consistent, many-body approximations in obtaining excitation spectra [1, 3]. In particular, I focus on density response functions from which, e.g., absorption spectra are calculated. Our method of choice for evaluating these functions is time-dependent many-body perturbation theory. What makes this an interesting application is that the used method is equivalent to the traditional approach based on the Bethe-Salpeter Equation (BSE) but the accessible approximations are diagrammatically more sophisticated.

In addition, I summarize the main steps enabling one to take vibrational effects into account, e.g., in such applications. I do this by presenting the Kadanoff-Baym Equations (KBE) for systems of interacting electrons and phonons for which the initial preparation to an equilibrium state is taken into account [2, 3]. The correlated many-body approximations to be discussed in this context are the partially and fully self-consistent Born approximations.

[1] Säkkinen N, Manninen M, and van Leeuwen R, NJP, 14:013032 (2012)

[2] Säkkinen N, Peng Y, Appel H, and van Leeuwen R, JCP, 143:234101 (2015)

[3] Säkkinen N, Peng Y, Appel H, and van Leeuwen R, JCP, 143:234102 (2015)