Optical Spectrum of AntiferromagneticCr2O3

The sharp absorption spectrum of antiferromagnetic ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ is investigated. The molecular-field approximation to the exchange interaction is shown to be inapplicable to the ${}_{}{}^2{}_{}{}^{}E$ states. A modified exchange interaction is introduced via an effective Hamiltonian, and the resulting transitions are interpreted in terms of exciton theory. Reasonable agreement with experiment is found. With a magnetic field parallel to the $c$ axis, a new line not present in zero field is observed. This absorption is explained by the exciton theory. The 3.75-${\mathrm{cm}}^{-1\mathrm{}}$ separation between the satellite line and the line from which it borrows its intensity is a Davydov splitting. No electric field effect for fields up to 100 kV/cm is found. Finally, the low-temperature fluorescence spectrum is discussed.