Electronic correlations in oxides : the role of Hund coupling

The last lecture (June 23) focused on some effects linked to orbital interactions in oxides. After an introduction recalling the work of Slater andRacah on atomic multiplets, situations were presented in which the crystal field (which tends to polarize orbital populations) and Hund coupling (which tends, on the contrary, to compensate these populations) are in competition. It has been suggested that the physics of many 4d-layer transition metals is dominated by Hund coupling, and therefore exhibits strong correlation effects, whereas Mott-Hubbard effects are less sensitive in these materials due to the lower values of the screened Coulomb interaction. In a seminar entitled "Spin-orbit physics in the Mott regime", Leon Balents (University of California, Santa Barbara) showed how, for 5d-layer transition metal oxides, it is spin-orbit coupling that is responsible for strong electronic correlations.