Temperature-dependent electronic structure and magnetic behavior of Mott insulators

We investigate the temperature-dependent electronic and magnetic properties of transition-metal monoxides by use of a theoretical model that takes into account the 3d subbands of the transition metal and the oxygen 2p subbands. Characteristic properties of these Mott insulators are caused by strong 3d-electron correlations, described by intra-atomic Coulomb interactions and characterized by three different coupling constants: U, U¯, and J. The intraband coupling U turns out to be decisive for the appearance of spontaneous antiferromagnetism. U¯ is a direct interband Coulomb interaction responsible for a nonuniform occupation of the 3d subbands, and therewith decisive for the insulator properties of these materials. The interband exchange J describes an effective electron-magnon interaction and polarizes magnetically inactive subbands. The self-consistent solution of our model yields a quasiparticle density of states that contains three nonconnected energy regions with predominantly 3d character: two below, and one above, the chemical potential μ. The insulator gap is practically temperature independent, persisting for arbitrary T≫${\mathit{T}}_{\mathit{N}}$. A necessary condition for the insulating behavior is an integral number ${\mathit{n}}_{3\mathit{d}}$≥5 of 3d electrons. The antiferromagnetism is caused by 10-${\mathit{n}}_{3\mathit{d}}$ exactly half-filled 3d subbands, while ${\mathit{n}}_{3\mathit{d}}$-5 subbands are completely filled. The magnetic moment remains stable for arbitrarily high temperatures. The temperature dependence of the sublattice magnetization is caused by a ‘‘kinetic Heisenberg exchange.’’ The 2p-3d hybridization plays only a minor role with respect to the magnetic and insulating properties of Mott insulators, but becomes decisive for the nature of the band gap and the interpretation of photoemission data. Our model calculation correctly predicts MnO, FeO, CoO, NiO, and CuO to be antiferromagnetic insulators, while VO turns out to be a nonmagnetic metal.