Ab initio theory of excitons and optical properties for spin-polarized systems:: Application to antiferromagnetic MnO

We study quasiparticle and electron-hole pair excitations in the presence of spin polarization within the framework of many-body perturbation theory. The influence of magnetic ordering on linear optical response is examined in detail for collinear spins. Whereas Hedin's expression for the exchange-correlation self-energy in the GW approximation remains diagonal with respect to the one-particle spin quantum number, the Bethe-Salpeter equation becomes more complex: The separation between triplet and singlet excitons no longer holds. The submatrices of the two-particle Hamiltonian are coupled by the matrix elements of the bare Coulomb potential which account for local-field effects. Therefore, the rank of the corresponding eigenvalue problem doubles. Numerical results are computed from first principles for the antiferromagnetic insulator MnO starting from electronic states obtained with nonlocal exchange-correlation potentials. The calculated electronic structure and optical absorption spectra including excitonic and local-field effects are compared with experimental data. The dominance of interatomic transitions between Mn atoms in the interband region of the optical absorption is revealed.