| The study of spin-polarized systems constitutes much of solid state physics. Though first-principles calculations enjoy considerable success explaining the ground state and excited state properties of a wide variety of bulk solids, magnetic systems still provide plenty of challenge. The present work tries to address this problem. We have developed and applied a formalism that can be used to compute the quasiparticle energies and wave functions of spin-polarized systems. The optical and electronic properties of solids are calculated using ab initio many-body methods. The quasiparticle excitation spectrum of antiferromagnetic solids is studied by constructing the self-energy operator within the GW approximation. This method can also be applied to a large supercell containing defects. Furthermore, in conjunction with a model Hamiltonian, the spin density-functional approach sheds light on more subtle qualitative features of the magnetic impurity problem. Several applications are presented here: (1) The metal-insulator transition of a model atomic hydrogen solid is studied, and a spin-polarized GW approximation is developed to compute the electronic properties of this system for varying electron densities. Comparison is made to other theoretical and experimental studies in the low-electron density atomic limit. (2) NiO, which has become a model system for strongly correlated transition metal oxides, is investigated, taking proper account of the antiferromagnetic ordering and its localized d-bands. Our quasiparticle energy calculation explains both the optical absorption edge and the energy gap found in XPS/BIS measurements. (3) The electronic properties of three group-III nitride systems are studied. The calculated quasiparticle-energy bands of InN are supported by a recent new measurement. For BN and AlN, native point defects are constructed using a 32-atom supercell. We find that a proper treatment of unpaired electrons in vacancy defects requires spin-polarized calculations. Theoretical energetic locations of defect levels are compared with available experimental data. (4) In a recent scanning tunneling microscope study, the electronic properties of two Ni atoms on gold surface were measured. We qualitatively explain the observed STM spectrum by a combination of a spin-½ s-d model and density-functional calculations. The energetic shifting of molecular d-orbitals is calculated, and we find a charge transfer between the host substrate and the Ni dimer. |
