Theory of crystal field states for heavy rare-earth impurities in MgB2

For isolated rare-earth impurities substituting for Mg atoms in the superconductor MgB2 the crystal field parameters are calculated by the ab. initio density functional electron theory with constraints for the 4f charge and spin density. The crystal field parameter-A(6)(6) is extremely small due to the structure and bonding properties of MgB2, and therefore the crystal field levels are nearly exclusively determined by one magnetic quantum number M. Implications for the pair-breaking mechanism of the superonductivity in MgB2 are discussed.