Spectroscopic techniques are an essential tool in probing electronic, optical and structural properties of materials. Besides those techniques, numerical simulations of spectroscopic properties help in the interpretation of the experimental results, and may even guide and motivate new experiments. In this context, we develop theoretical and numerical approaches, as well as computational tools, within the framework of many-body perturbation theory and density functional theory. Many-body perturbation theory allows to systematically capture correlation effects responsible for emergent collective behaviours in an electronic system. Combining many-body perturbation theory with density-functional theory results in universal, parameter-free numerical approaches that allows to quantitatively address spectroscopic properties of materials, given the atomic positions as the only input. Indeed, those techniques are nowadays the state-of-the-art to simulate e.g. optical absorption, electron energy loss and photo emission spectra for a broad range of materials.
Notwithstanding this success, active development of those approaches, and the associated computational tools is still needed. Two of the major challenges are:
a. The application of the above-mentioned approaches to large and complex systems, as those that come into play in problems of technological interest (e.g. nanostructures, interfaces, defects).
a. The extension of those approaches to address real-time, femto-second spectroscopic techniques, that provides information on the photo-induced electron dynamics and changes in the structures and properties of an electronic system.
Following those challenges, our research proceeds along two main lines:
1. Designing of strategies and efficient numerical algorithms to treat systems with a large number of degrees of freedom.
2. Development of theoretical and numerical approaches within non-equilibrium Green’s function theory to simulate real-time femto-second spectroscopy experiments.
Theoretical and numerical progress from both lines are implemented in Yambo, a code that allows the calculation of excited-state properties of materials. The code results from the collaboration of several researchers and being publicly available (and interfaced with two of the most used, publicly available density-functional codes) it is used by researchers all over the world.
Exciton-Plasmon States in Nanoscale Materials: Breakdown of the Tamm-Dancoff Approximation
Grüning, M; Marini, A; Gonze, X, Nano Letters 9 (2009) 2820
Real-time approach to the optical properties of solids and nanostructures: Time-dependent Bethe-Salpeter equation
Attaccalite, C.; Grüning, M.; Marini, A. Physical Review B 84 (2011) 245110
Yambo: An ab initio tool for excited state calculations
Marini, A; Hogan, C; Grüning, M; Varsano D. Computer Physics Communications, 180 (2009) 1392
Quasiparticle calculations of the electronic properties of ZrO2 and HfO2 polymorphs and their interface with Si
Grüning, M; Shaltaf, R; Rignanese, GM Physical Review B 81 (2010) 035330
Local-field and excitonic effects in the optical response of alpha-alumina
Marinopoulos, AG; Gruning, M
Physical Review B 83, (2011) 19529