Quasiparticle energies and excitonic effects of chromium trichloride: From two dimensions to bulk

Two-dimensional (2D) van der Waals magnetic structures provide precious opportunities to study excited-state properties of correlated materials under quantum confinement. This paper reveals enhanced excitonic effects in 2D and bulk CrCl${}₃$, in which the electron-hole (e-h) binding energy reaches more than 2 eV. The calculated exciton positions agree with measurements. Particularly, the e-h binding energy is more sensitive to quantum confinement than the self-energy correction. As a result, an unusual red shift of the ``optical gap'' is observed in thinner samples, which is opposite to the widely observed blue shift trend.