Giant excitonic effects in vacancy-ordered double perovskites

Using first-principles GW plus Bethe-Salpeter equation calculations, we identify exceptionally strong excibinding energies of about 1 eV are found in these moderate-gap, inorganic bulk semiconductors, pushing the limit of our understanding of the electron-hole interaction and exciton formation in solids. Not only are the exciton binding energies extremely large compared with any other moderate-gap bulk semiconductors, but they are also larger than typical two-dimensional semiconductors with comparable quasiparticle gaps. Our calculated lowest bright exciton energies agree well with the measured optical band gaps. The low-energy excitons closely resemble the Frenkel excitons in molecular crystals, as they are highly localized in a single [MX6]2- octahedron and extended in the reciprocal space. The weak dielectric screening effects and the nearly flat frontier electronic bands, which are derived from the weakly coupled [MX6]2- units, together explain the significant excitonic effects. Spin-orbit coupling effects play a crucial role in redshifting the lowest bright exciton by mixing up spin-singlet and spin-triplet excitons, while exciton-phonon coupling effects have minor impacts on the calculated exciton binding energies.

Automated summary

Using first-principles calculations, we find extremely strong exciton binding energies of about 1 eV in moderate-gap, inorganic bulk semiconductors. These binding energies exceed those of other similar semiconductors and even two-dimensional semiconductors. The lowest bright exciton energies calculated are in good agreement with optical measurements. The excitons in these materials resemble Frenkel excitons in molecular crystals and exhibit weak dielectric screening effects and nearly flat frontier electronic bands derived from weakly coupled [MX6]2- units. Spin-orbit coupling effects play a crucial role in redshifting the lowest bright exciton, while exciton-phonon coupling effects are minor.