# Loss Spectroscopy and Excitons

The Team Loss Spectroscopy and Excitons (lead by F.Sottile) deals, unsurprisingly, with low-loss spectroscopies, like those measurable with Electron-Energy Loss (EELS) or by non-resonant Inelastic X-ray Scattering (NIXS). Both EELS and IXS are modern techniques to investigate the properties of matters, through the measurement of the inverse dielectric function $\varepsilon^{-1}(\mathbf{q},\omega)$. The inverse dielectric function is the main result of a linear response calculation that can be carried out using Time Dependent Density Functional Theory (TDDFT) or in the many-body framework, via the Bethe-Salpeter Equation (BSE). In particular the latter, permits one to explicitely consider the excitonic effects, that can have a very important effects on semi-conductors, insulators and in general in low-dimensional systems.

• TDDFT calculations for the Dynamical Structure factor (DSF) of semiconductors: Adiabatic Local Density Approximation (ALDA) within TDDFT has shown to work very well in the description of DSF, for example in bulk Silicon or Sodium, with excellent agreement with experiments.
• The case of wide-gap insulators is instead different. Presenting strong excitonic effects, LiF or Argon, for instance, present sharp excitonic features in the Loss Spectra, requiring the use of the BSE.
• Exciton Dispersion has become a new topic in the group. The possibility to evaluate the DSF at finite $\mathbf{q}$, within the BSE, permits us to study dd excitations, exciton dispersion, well beyond models.
• Links with high-quality experiment are today possible, thanks to the high spatial and energy resolution of STEM electron microscope, or to the high momentum-energy resolution of the new generation synchrotron sources (ESRF, SOLEIL, etc.)

## Selected Bibliography

1. Low-energy electronic excitations and band-gap renormalization in CuO (2017)
2. Low-energy electronic excitations and band-gap renormalization in CuO (2017)
3. Exciton Band Structure in Two-Dimensional Materials (2016)
4. Interpretation of monoclinic hafnia valence electron energy-loss spectra by time-dependent density functional theory (2016)
5. Exciton dispersion in molecular solids (2015)