Magnetic Davydov Splittings in the Optical Absorption Spectrum ofCr2O3

Five sharp lines in the optical absorption spectrum of antiferromagnetic ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ are investigated, and the assignments of four of the lines as Frenkel exciton absorptions are discussed in detail. A previous exciton assignment of these lines is shown to be inconsistent with the results of a detailed group-theory analysis of the manner in which interion interactions govern the spectrum, and of the effect of an externally applied magnetic field. All difficulties of the previous assignment are removed by new assignments in which the lines are pairs of Davydov-split Frenkel excitons and none of the energy separations in the spectrum are single-ion exchange splittings. The Davydov splittings are very large, and the mechanism for the large splittings is found, in a first-principles treatment of the interion interactions, to be interion exchange. This result is not predicted by the usually assumed, phenomenological, total-spin Heisenberg exchange interaction, which is an inadequate representation of the exchange interaction between two ions in states of different total spin. New experimental data on the circular polarizations of the split components of the lines in an axial Zeeman experiment, and on the temperature dependence of the positions of the lines, confirm the new assignments. In addition, new data on the temperature dependence of the integrated intensities of the lines and on their stress dependence are presented. The assignment of the fifth line as a magnon sideband is discussed briefly.