FisMat2017 - Submission - View

Abstract's title: Zero point motion and direct/indirect bandgap crossover in layered transition-metal dichalcogenides
Submitting author: Luciano Ortenzi
Affiliation: Institute for Complex Systems (ISC) CNR
Affiliation Address: P.le Aldo Moro 5, 00185
Country: Italy
Oral presentation/Poster (Author's request): Oral presentation
Other authors and affiliations: Emmanuele Cappelluti (Institute for Complex Systems (ISC) CNR) Luciano Pietronero (Physics Departement "Sapienza" University of Rome )

The presence of many entangled degrees of freedom very sensitive to lattice deformations and thickness, and the intrinsic band gap being located in the visible range,  makes  transition-metal dichalcogenides (TMD) very exciting both from the fundamental point of view and for technological applications. The crucial role of the in plane strain in tuning the electronic and optical properties of TMD was already largely explore at the static level. In this presentation I show that the band structure of TMDs is highly sensitive to the intra-layer/inter-plane distance h revealing how this internal vertical lattice parameter alone can drive possible direct/indirect gap crossover in single-layer compounds and a crossover between different indirect gaps in multilayer systems. The relevance of this scenario is furthermore analysed with respect to the role of the zero point motion fluctuations induced by the quantum lattice dynamics. Within this context I show how the quantum lattice fluctuations associated with lattice parameter h can probe at a dynamical level both direct and indirect bandgap configurations, so that the direct/indirect bandgap character cannot be captured by the analysis of the band-structure of the perfect crystal. In this situation the basic assumptions underlying the adiabatic Born-Oppenheimer principle are no more fulfilled and the very idea a well-defined direct/indirect bandgap character is questionable. The consequences of this scenario are discussed.