On the nature of the absolute maximal observable magneto-optical Kerr rotation of CeSb

We report first-principles calculations of the polar magneto-optical (MO) Kerr spectra of the Ce compounds CeSb, CeSb 0.75 Te 0.25 , CeTe, and CeSe, using the local-density approximation (LDA) of density-functional theory as well as its generalization which contains an explicit Coulomb interaction U (LDA+ U ). Although the LDA predicts a large Kerr rotation of about 20° in CeSb, we find that the LDA+ U gives a better description of the Kerr spectra of all four compounds. For CeSb the LDA+ U predicts a colossal Kerr rotation of 45° up to 60°, in agreement with the recent discovery of the maximal observable rotation of 90° in CeSb by Pittini et al. ( Phys. Rev. Lett. , 77 (1996) 944). The origin of the colossal Kerr rotation realized in CeSb is examined.