Ground state splitting of ^8S rare earth ions in semiconductors

We propose a new mechanism leading to the ground state splitting for the rare earth 8 S ions in semiconductor crystals. The resulting splitting is due to three effects, the first is the intra atomic 4 f − 5 d spin-spin interaction, the second one is the spin - orbit interaction for 5 d electrons and the third one is their hybridization with the valence band states of semiconductors host. The resulting splitting significantly depends on the relative position of 5 d level with respect to semiconductor host band structure. We also discuss different model, already known in the literature, which is also based on ion - band states hybridization. For both models, as an example, we present results of numerical calculations for rare earth ion in IV-VI semiconductor PbTe.