My Research activity is mainly based on the theoretical study, via the state-of-the-art of numerical methods, of electronic properties of real materials. I am particularly interested in the study of the limits of actual approaches and approximations, in order to propose solutions to go towards a better comprehension and description of the physics of the studied system. Several research lines are currently investigated:
My group, whose activity mainly concerns theoretical and numerical developements of excitonic effects in varied spectroscopies, is today composed of 2 PhD students, Abdallah El-Sahili and Alam Osorio (in co-supervision with Lucia), and 1 post-doc, Laura Urquiza (in co-supervision with Matteo).
An important part of my activity is also devoted to code developments. I am the coordinator of the ab initio codes DP (linear response TDDFT code) and EXC (Bethe-Salpeter equation code).
This study presents an ab initio investigation of the XANES spectra at the aluminum K-edge for three compounds: Al2O3, AlF3, and AlCl3, where the Al atoms share the same oxidation state (III) and are coordinated in an octahedral symmetry. The XANES spectra calculated within the independent-particle approximation (IPA) reveal significant differences, including shifts in the spectrum onset, variations in the spectral shapes, and the presence of a prepeak in the case of AlCl3, all in correspondence with the behavior of the projected density of states of the absorbing atom in the different materials. The origin of those features can, therefore, be identified in the specific band structure of each compound. When electron-hole interactions are taken into account through the solution of the Bethe-Salpeter equation, a series of dark and bright excitons with large binding energies and Frenkel character is obtained. The strong excitonic effects lead to the suppression of the prepeak in AlCl3 and further accentuate the differences among the three Al K-edge spectra, which shows that drawing conclusions solely on the basis of IPA spectra may be misleading.
We present an ab initio study of neutral core and valence electronic excitations in alpha-Al2O3 by solving the Bethe-Salpeter equation (BSE) of many-body perturbation theory within an all-electron framework. Calculated spectra at the Al K and L1 edges are in remarkable agreement with available experiments from x-ray absorption (XAS) and x-ray Raman spectroscopy once excitonic effects are taken into account. The combination of the BSE spectra for the two techniques confirms the dipole-forbidden nature of the exciton prepeak as suggested by recent calculations based on density-functional theory. Moreover, we make predictions for resonant inelastic x-ray scattering (RIXS) spectra at K and L1 edges, which strikingly fully overlap also beyond an independent-particle picture. The RIXS calculations reveal two distinct regimes as a function of incoming photon energy. Below and at the XAS threshold, we observe Raman-like features, characterized by strong excitonic effects, which we directly compare to peaks in the loss function. Above the XAS threshold, instead, fluorescence features become predominant: RIXS spectra can be well described and analyzed within an independent-particle approximation showing similarity with the x-ray emission spectrum.
All articles available here.
Organizer of 21 international schools and tutorials. They can be divided in several different types of events:
Material for a more general audience.