Andrea Marini
My major research interest is the application of Many-Body techniques to Condensed matter physics and nanoscience. In particular the description and prediction of the ground-state, electronic and spectroscopic properties of solids and nanomaterials. I am especially interested in studying the fundamentals of many body perturbation theory and static as well as time-dependent Density Functional Theory (DFT and TDDFT).
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Highlights
Within the Tamm−Dancoff approximation, ab initio approaches describe excitons as packets of electron−hole pairs propagating only forward in time. However, we show that in nanoscale materials excitons and plasmons hybridize, creating exciton-plasmon states where the electron−hole pairs oscillate back and forth in time. Then, as exemplified by the trans-azobenzene molecule and the carbon nanotubes, the Tamm−Dancoff approximation yields errors larger than the accuracy claimed in ab initio calculations. Instead, we propose a general and efficient approach that avoids the Tamm−Dancoff approximation, correctly describes excitons, plasmons, and exciton-plasmon states, and provides a good agreement with experimental results.
An exciton projected on a h-BN layer made pulsating by the external temperature.
The Ab-Initio description of the excitonic states, obtained by solving the Bethe-Salpeter (BS) equation of Many-Body Perturbation Theory, constitutes a well-established approach to interpret the photoexcited properties of bulk materials, surfaces, nanostructures and organic/bio-molecules. Although absorption and photoluminescence experiments are usually performed at room temperature, in the standard approach the BS equation is solved assuming the atoms frozen in their crystallographic positions, thus neglecting the effect of lattice vibrations. As a consequence excitons turn out to be insensitive to the temperature T and to have an infinite lifetime, in stark contrast with the experimental results. Moreover, in bulk semiconductors, it is a well known fact that the absorption line position, width, and intensity show a clear T dependence. In the frozen-atom BS equation this dependence is not described at all. Even in the T→0 limit, where atoms vibrate to fulfill the uncertainty principle (zero-point vibrations), the calculated absorption spectra is commonly convoluted with some artificial, ad-hoc numerical broadening function chosen to yield the best agreement with the experiment. In this work I have shown how to solve, in a fully Ab-Initio manner, the Bethe-Salpeter equation including the coupling with the lattice vibrations. The picture of the excitons obtained within a frozen-atom approximation turns out to be deeply modified, both at zero and finite temperature. Excitons acquire a non-radiative lifetime, otherwise infinite in the frozen-atom approximation. The thermal properties of the excitonic states are explained in terms of a weak and a strong exciton-phonon coupling. In the weak regime the lattice vibrations affect only the electron-hole substrate of the excitonic states, while in the strong case, they participate actively in the excitons build-up.
Axial polarizability of the molecular H2 chain.
The real-space excitonic wavefunction for selected chains
is also shown.
Excitonic confinement is identified as the main driving force for the saturation in one-dimensional molecular chains (i.e. polyacetylene and H2) of the chain polarizability as a function of the number of molecular units. This conclusion is based on first principles time-dependent density functional theory calculations using a recently developed exchange-correlation kernel that accounts for excitonic effects. The failure of simple local and semi-local functionals is shown to be linked to the lack of memory effects, spatial ultra-non-locality, and self-interaction corrections. These effects get smaller as the gap reduces, in which case such simple approximations do perform better.