Real- and Momentum-Space Description of the Excitons in Bulk and Monolayer Chromium Tri-Halides: Article No. 33

Swagata Acharya, Dimitar Pashov, Alexander Rudenko, Malte Rosner, Mark van Schilfgaarde, Mikhail Katsnelson

Research output: Contribution to journalArticlepeer-review

15 Scopus Citations

Abstract

Excitons with large binding energies ~2–3 eV in CrX3 have been characterized as being localized (Frenkel) excitons that emerge from the atomic d − d transitions between the Cr-3d-t2g and eg orbitals. The argument has gathered strength in recent years as the excitons in recently made monolayers are found at almost the same energies as the bulk. The Laporte rule, which restricts such parity forbidden atomic transitions, can relax if a symmetry-breaking mechanism is present. While what can be classified as a purely Frenkel exciton is a matter of definition, we show using an advanced first principles parameter-free approach that these excitons in CrX3, in both its bulk and monolayer variants, have band origin and it is the dp hybridization between Cr and X that primarily acts as the symmetry-breaking mechanism that relaxes the Laporte rule. We show that the character of these excitons is mostly determined by the Cr-d orbital manifold, nevertheless, the fractions of the spectral weight shared with the ligand halogen states increases as the dp hybridization enhances. The hybridization enhances as the halogen atom becomes heavier, bringing the X-p states closer to the Cr-d states in the sequence Cl → Br → I, with an attendant increase in exciton intensity and a decrease in binding energy. By applying a range of different kinds of perturbations that qualitatively mimics the effects originating from the missing vertex in self-energy, we show that moderate changes to the two-particle Hamiltonian that essentially modifies the Cr-d-X-p hybridization, can alter both the intensities and positions of the exciton peaks. A detailed analysis of several deep-lying excitons, with and without strain, elucidates the fact that the exciton is most Frenkel-like in CrCl3 and CrBr3 and acquires mixed Frenkel–Wannier character in CrI3, making the excitons in CrI3 most susceptible to environmental screening and spin–orbit coupling.

Original languageAmerican English
Article number33
Number of pages10
Journalnpj 2D Materials and Applications
Volume6
Issue number1
DOIs
StatePublished - 2022

Bibliographical note

Publisher Copyright:
© 2022, The Author(s).

NREL Publication Number

  • NREL/JA-5F00-82163

Keywords

  • excitons
  • Frenkel
  • layered chromium tri-halides
  • Wannier-Mott

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