Frenkel versus charge-transfer exciton dispersion in molecular crystals

By solving the many-body Bethe-Salpeter equation at finite momentum transfer, we characterize the exciton dispersion in two prototypical molecular crystals, picene and pentacene, in which localized Frenkel excitons compete with delocalized charge-transfer excitons. We explain the exciton dispersion on the basis of the interplay between electron and hole hopping and electron-hole exchange interaction, unraveling a simple microscopic description to distinguish Frenkel and charge-transfer excitons. This analysis is general and can be applied to other systems in which the electron wave functions are strongly localized, as in strongly correlated insulators.
The authors acknowledge financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374), Spanish grants (2010-21282-C02-01 and PIB2010US-00652), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13), European Commission projects CRONOS (Grant No. 280879-2 CRONOS CP-FP7), and the Maison de la simulation for technical support. Computational time was granted by GENCI (Project No. 544) and by the CNANO-SIMULEE-2009 project. This work was carried out under the HPC-EUROPA2 project, with the support of the European Community–Research Infrastructure Action of the FP7