Davydov Splitting in the Optical Absorption Spectra of theCr+3E2State in Some Rare-Earth Orthochromites

Abstract

We report here the $R$-line optical absorption spectra of ${\mathrm{Cr}}^{+3\mathrm{}}$ in the rare-earth orthochromites ErCr${\mathrm{O}}_3$, HoCr${\mathrm{O}}_3$, and YCr${\mathrm{O}}_3$ as a function of an applied magnetic field. All three materials exhibit Davydov splittings. Their polarized spectra are found to be dissimilar. It is shown that these differences arise from the different types of spin structures which these materials exhibit in their magnetically ordered phases, and that from the polarization characteristics the magnetic factor group may be determined. ErCr${\mathrm{O}}_3$ is found to undergo a spin reorientation at 9 °K. The $\overrightarrow{\mathrm{k}}=0\mathrm{}$ exciton problem is solved for the different spin structures and the transfer-of-energy matrix elements responsible for the Davydov splittings are determined. They are 5 to 100 times smaller than those of the corresponding excited state of ${\mathrm{Cr}}^{+3\mathrm{}}$ in ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$. The differences are shown to arise from a combination of the different crystal structures of the two materials and the spin selection rules on the transfer-of-energy matrix elements.