- 07.09.2018 -
S3 SEMINAR Lucia Reining

**Date and Time:** Friday September 7, 2018 - 15.00

**Venue:** S3 Seminar Room, Third Floor, Physics Building, FIM Department

**Speaker:** Lucia Reining

(ETSF Centre for Users and Technology)

**Title:** Coupling of excitations: how to describe dynamical effects in electronic spectra?

**Abstract:**Density functional theory and many-body perturbation theory based on Green's functions are powerful approaches to describe many properties of materials. In particular, approximations to the Green's functions methods such as Hedin's GW approximation for the self-energy [1] are widely and successfully used to calculate quasi-particle band structures of a wide range of materials. However, electronic excitation spectra contain much more than just quasi-particle energies: there can be lifetime broadening, an incoherent background and additional distinct structures that are sometimes called satellites. These features are pure correlation effects and cannot be captured by any independent-particle approach, nor by any real and static approximation to the self-energy. Often even the frequency-dependent self-energy of the GW approximation and related approaches are not sufficient to obtain good satellite spectra, for example in the case of strong coupling where the quasi-particle picture is no longer adequate, but also in simple semiconductors with intense plasmon satellites.

In this seminar we will discuss two main directions to design frequency-dependent effective potentials and effective interactions that can overcome some of the problems of the state-of-the-art approximations.
One direction explores an alternative route to the calculation of interacting electron Green's functions. It is based on a set of functional differential equations relating the one-body Green’s function to its functional derivative with respect to an external perturbing potential [2]. This set of equations can be used to generate the perturbation series.
The present talk, instead, will show how one can work directly with these differential equations [3] to obtain new approximations.
The second strategy is to perform advanced calculations in a model system and to design a procedure that we call ``connector'', which allows us to use the results of the model in order to calculate spectra in real systems. We will discuss the principles and general properties of such an approach, and give illustrative examples for photoemission and inelastic x-ray scattering spectra [4]. Ultimately, the two approaches might be combined into a single powerful scheme.

[1] L. Hedin, Phys. Rev. 139, A796 (1965).
[2] L.P. Kadanoff and G. Baym, Quantum Statistical Mechanics (New York: Benjamin, 1962)
[3] G. Lani, P. Romaniello, and L. Reining, New J. Phys. 14, 013056 (2012); J.A. Berger et al., New J. Phys. 16, 113025 (2014); A. Stan, et al., New J. Phys. 17, 093045 (2015);
JS Zhou, et al., J. Chem. Phys. 143, 184109; JS Zhou, M Gatti, JJ Kas, JJ Rehr, L Reining, Phys. Rev. B 97, 035137 (2018).
[4] M Vanzini, L Reining, M Gatti, arXiv:1708.02450; M Panholzer, M Gatti, L Reining; Phys. Rev. Lett. 120, 166402 (2018).

**Host:** Elisa Molinari elisa.molinari@unimore.it