Mapping of the energetically lowest exciton in bulk 1T-HfS2

Carsten Habenicht (1) , Lorenzo Sponza (2) , Roman Schuster (1) , Martin Knupfer (1) , Bernd Büchner (1, 3)

1 IFW Dresden - Leibniz Institute for Solid State and Materials Research
2 LEM, UMR 104 CNRS-ONERA, Université Paris Saclay (COmUE) [Châtillon]
3 TU Dresden - Technische Universität Dresden = Dresden University of Technology

By combining electron energy-loss spectroscopy and state-of-the-art computational methods, we were able to provide an extensive picture of the excitonic processes in 1T-HfS2. The results differ significantly from the properties of the more scrutinized group VI semiconducting transition metal dichalcogenides such as MoS2 and WSe2. The measurements revealed a parabolic exciton dispersion for finite momentum q parallel to the ΓK direction which allowed the determination of the effective exciton mass. The dispersion decreases monotonically for momentum exchanges parallel to the ΓM high symmetry line. To gain further insight into the excitation mechanisms, we solved the ab-initio Bethe-Salpeter equation for the system. The results matched the experimental loss spectra closely, thereby confirming the excitonic nature of the observed transitions, and produced the momentum-dependent binding energies. The simulations also demonstrated that the excitonic transitions for q || ΓM occur exactly along that particular high symmetry line. For q || ΓK on the other hand, the excitations traverse the Brillouin zone crossing various high symmetry lines. A particular interesting aspect of our findings was that the calculation of the electron probability density revealed that the exciton assumes a six-pointed star-like shape along the real space crystal planes indicating a mixed Frenkel-Wannier character.

Domaines: Science des matériaux [cond-mat.mtrl-sci]

Carsten Habenicht, Lorenzo Sponza, Roman Schuster, Martin Knupfer, Bernd Büchner. Mapping of the energetically lowest exciton in bulk 1T-HfS2. Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2018, ⟨10.1103/PhysRevB.98.155204⟩. ⟨hal-01883557⟩