Effect of exchange interactions onΓexcitons in transition-metal halides

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Abstract
Γ interband excitons in transition-metal halides are discussed on the basis of the energy-band structure. The influence of the electron-hole exchange and the exchange of the electron spin with the localized metal-ion spins is taken into account. In the paramagnetic temperature region this exchange interaction with the randomly oriented metal-ion spins results in an autolocalization of the Γ exciton, with the electron residing on a single metal ion, and the hole located on the six neighboring anions. The exchange splitting of the Γ exciton states is calculated with this model. The calculations give a satisfactory explanation of the complicated fine structure, observed for Γ excitons in the chlorides, bromides, and iodides of Fe, Co, and Ni.