Researchers from the University of Luxembourg, including Muralidhar Nalabothula, Davide Sangalli, Fulvio Paleari, Sven Reichardt, and Ludger Wirtz, have delved into the symmetrical properties of excitons, which are bound pairs of electrons and holes that play a crucial role in the optical properties of materials. Their work, published in the journal Physical Review B, offers a new framework for understanding and exploiting the symmetries of excitons, which could have practical applications in the energy sector, particularly in the development of optoelectronic devices and solar cells.
Excitons are responsible for strong optical resonances near the bandgap in low-dimensional materials and wide-bandgap insulators. While current methods can accurately determine exciton energies and states, their symmetries have been less explored. The researchers employed standard group-theory methods to analyze the transformation properties of excitonic states under crystal symmetry operations. They developed an approach to assign symmetry labels to excitonic states, providing a framework for analyzing their symmetries and selection rules, including exciton-phonon coupling.
The team introduced the concept of total crystal angular momentum for excitons in the presence of rotational symmetries, allowing the derivation of conservation laws. They demonstrated how these symmetry properties can enhance the computational efficiency of exciton calculations. The researchers applied their methodology to three prototypical systems to understand the role of symmetries in different contexts.
In the case of LiF, they presented a symmetry analysis of the entire excitonic dispersion and examined the selection rules for optical absorption. For monolayer MoSe2, they calculated resonant Raman spectra and showed how the conservation of total crystal angular momentum governs exciton-phonon interactions, leading to resonant enhancement. In bulk hexagonal boron nitride (hBN), they analyzed the role of symmetries for the coupling of finite-momentum excitons to finite-momentum phonons and their manifestation in phonon-assisted luminescence spectra.
The practical applications of this research for the energy sector include the development of more efficient solar cells and optoelectronic devices. By understanding the symmetry properties of excitons, researchers can better design materials that can effectively capture and convert light into energy. This work establishes a general and robust framework for understanding the symmetry properties of excitons in crystals, providing a foundation for future studies in the field.
This article is based on research available at arXiv.