GCI Group seminar: 

Dr. Andrea Marini (Consiglio Nazionale delle Ricerche - Istituto di Struttura della Materia)

SEMINAR TITLE:

A Many-Body approach to the equilibrium and out-of-equilibrium properties of complex materials.

ABSTRACT:

Ultra--fast optical spectroscopy is a powerful tool for the observation of dynamical processes in several kind of materials. The basic time--resolved optical experiment is the so-called “pump-probe”: a first light pulse, the “pump”, resonantly triggers a photo-induced process. The subsequent system evolution can be monitored, for example, by the time--dependent transmission changes of a delayed “probe” pulse. The pump pulse photon energy, spectral width and peak intensity creates a certain density of electron-hole pairs in a more or less localized region of space. After the creation of the initial carrier density the time evolution of the single--particle and many--particle excitations is now governed by a non-trivial interplay between electron--electron scatterings and energy relaxation. De-phasing will be driven by different phenomena. One of the most important is the energy transfer to the atomic motion in form of phonon excitations. In this talk I will first review the basic concepts of the merging of Density Functional Theory with Many-Body Perturbation Theory in the case of equilibrium phenomena. I will discuss merits and drawbacks also discussing the key aspects of the competition between electronic--mediated and phononic--mediated scattering processes. I will particularly concentrate on the less known electron-phonon interaction treated in a fully Ab-Initio manner. Then I will present a novel approach based on the merging of Non-Equilibrium Green's function theory and Density Functional Theory to investigate the carrier dynamics following a pump excitation. The case of bulk Silicon, a paradigmatic indirect gap semiconductor, is studied by using the Baym--Kadanoff equations. Both the electron--electron\,(e--e) and electron--phonon\,(e--p) self--energies are calculated fully {\em Ab--Initio} by using a semi--static $GW$ approximation in the e--e case and a Fan self--energy in the e--p case. By using the generalized Baym--Kadanoff ansatz the two--time evolution is replaced by the only dynamics on the macroscopic time axis. The enormous numerical difficulties connected with a real--time simulation of realistic systems is overcomed by using a completed collision approximation that further simplifies the memory effects connected to the time evolution. The carrier dynamics is shown to reduce in such a way to have stringent connections to the well--known equilibrium electron--electron and electron--phonon self--energies. This link allows to use general arguments to motivate the relative balance between the e--e and e--p scattering channels on the basis of the carrier energies.