  Layered van der Waals (vdW) magnetic materials have attracted significant
research interest to date. In this work, we employ the first-principles
many-body perturbation theory to calculate excited-state properties of a
prototype vdW magnet, chromium trichloride (CrCl3), covering monolayer,
bilayer, and bulk structures. Unlike usual non-magnetic vdW semiconductors, in
which many-electron interactions and excited states are sensitive to
dimensionality, many-electron interactions are always enhanced and dominate
quasiparticle energies and optical responses of both two-dimensional and bulk
CrCl3. The electron-hole (e-h) binding energy can reach 3 eV in monolayer and
remains as high as 2 eV in bulk. Because of the cancellation effect between
self-energy corrections and e-h binding energies, the lowest-energy exciton
(optical gap) is almost not affected by the change of dimensionality. Besides,
for the excitons with similar e-h binding energies, their dipole oscillator
strength can differ by a few orders of magnitude.Our analysis shows that such a
big difference is from a unique interference effect between complex exciton
wavefunctions and interband transitions. Finally, we find that the interlayer
stacking sequence and magnetic coupling barely change quasiparticle band gaps
and optical absorption spectra of CrCl3. Our calculated low-energy exciton peak
positions agree with available measurements. These findings give insight into
the understanding of many-electron interactions and the interplay between
magnetic orders and optical excitations in vdW magnetic materials.
