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Dynamical effects on neutral excitations

Periodic Reporting for period 1 - DENE (Dynamical effects on neutral excitations)

Reporting period: 2015-06-01 to 2017-05-31

One of the main objectives of today’s ongoing research is the design of new materials with specific properties for technological applications. A successful material modeling requires the microscopic understanding of the elementary excitations induced by external perturbations (e.g. light, electronic currents, etc.). The aim of DENE is the development of a theoretical tool in the framework of many body perturbation theory (MBPT) for the description of neutral excitations that are directly probed in absorption, electron energy loss and inelastic X-Ray scattering experiments. It is focused on the description of dynamical effects beyond the quasi particle picture that are not taken into account in the present theories with application to challenging problems namely: (i) the correct description of the spectrum in systems where dynamical effects are not negligible as metals and strongly correlated insulators, (ii) the description of multiple excitations that are not included in a quasi particle picture, (iii) the understanding of the role played by electronic correlations on the lifetime of neutral excitations.
In state-of-the-art MBPT neutral excitations are described through an approximate version of the Bethe-Salpeter equation (BSE) that completely neglects dynamical effects. As a consequence, in this approach finite lifetime effects are completely neglected. Moreover we are not able to describe multiple excitations that are strictly related to dynamical effects. The aim of DENE is the development of a theoretical tool for the description of dynamical effects beyond the static BSE. The project, carried out at the Laboratoire ded Solides Irradies (LSI), Ecole Polytechnique under the supervision of Dr. Lucia Reining, has been developed in three main steps: The first two (WP1 and WP2) concern the development and the implementation in a numerical code of the theory in order to include dynamical effects beyond the BSE. The last step (WP3) Is focused on the application of the theory in real materials where dynamical effects are expected to be important.

1) Dynamical effects beyond the state-of the-art: theory (WP1) and coding (WP2)

Starting from the recent results obtained from the host group in the evaluation of the single particle Green’s function through a “cumulant” approach, the aim of this part of the project was to do a further step along that direction: the generalization of the theory to the two particles Green’s function that contains all the informations about neutral excitations. To this end, I derived an integro-differential equation for the two particles Green’s function and I demonstrated that in the case of a two level system coupled with a boson its solution has a cumulant structure in analogy with what happens for the single particle Green's function where all dynamical effects are included through a frequency dependent coefficient (cumulant coefficient). In a second step I generalized the theory to real systems involving infinite crystals as well as finite systems as atoms and molecules demonstrating that also in this case it is possible to find an approximate expression for the two particle Green's function with a cumulant structure. In particular I derived an expression for the cumulant coefficient in terms of an effective exciton self-energy that describes the coupling between excitons and other neutral excitations namely other excitons and plasmons. The exciton self-energy is expressed in terms of the eigenstates and eigenvalues of the excitonic hamiltonian that is diagonalized by standard numerical codes used to solve the static BSE. This make possible the implementation of the cumulant as a post processing of the static BSE. In collaboration with other members of the LSI (Dr. Matteo Gatti and Dr. Francesco Sottile) I am implementing the evaluation of the cumulanf coefficient in the EXC code, a GPL computational code that is freely available to the whole community (http://dp-code.org).

2) Scientific cases (WP3)

At the beginning the method was applied to a model system characterized by two bands coupled with a boson where the excitonic effect has been treated in two limits: strong localization (Frenkel exciton) and weak localization (Wannier exciton). The study allowed inferring a general picture of the dynamical effects and how this latter are related to the properties of the exciton as the exciton Bohr radius and exciton dispersion. Moreover I identified some features that could enhance dynamical effects as the presence of localized states and charge transfer excitations.

The application of the cumulant to model systems suggests that dynamical effects are important in systems characterized by localized states. For this reason I decided to apply the new method to the study of the optical properties of 2D and layered systems.
In particular I did standard BSE calculations on single layer and bulk hBN investigating the properties of the excitons and their dispersion as a function of the momentum transfer. I demonstrated that, due to the confinement of the electronic charge on a layer, at
The inclusion of dynamical effects in the theory not only represents a further step towards the understanding and interpretation of experimental spectra but also open the path towards new physical phenomena namely multiple excitations, damping of neutral excitations, electron-hole coupling in photoemission spectra etc., that have never been investigated using ab-initio methods and that, thanks to the recent improvements in experimental techniques, can be directly observed. On the other hand, the study of double excitations in NiO and CoO (a part of the project still in progress) will give an important contribution in the understanding of the electronic properties of strongly correlated materials.
Finally, DENE has been highly beneficial in facilitating my career developments at different levels:
-It supported my research at the forefront of the development of new methods in theoretical spectroscopy and their use in challenging applications of relevance for both fundamental science and technology;
-It enlarged my network of international collaborations;
-It fostered the development of an original theory-experiment synergy in the field of spectroscopy
of correlated materials.
At the same time DENE opened new collaboration opportunities for the host organisation. In fact,
I established effective international collaborations with experimental groups
performing EELS as Martin Knupfer at IFW Dresden (Germany) and IXS as Simo Huotari (University of Helsinki).
Multiple excitation