Orbital-selective effects in transition metal compounds: “molecules” in solids against magnetism

Close to Mott transition several novel states can appear. In particular, “molecular clusters” can be formed in the solid, such as dimers, trimers, etc. This talk will illustrate such phenomena, especially dimer formation, on many examples (pyroxenes; La4Ru2O10; Li2RuO3; M1 and M2 phases of VO2; V and Cr hollandites). The concept of orbital-selective Peierls transitions will be proposed and justified. In systems containing structural metal dimers there may exist in the presence of different orbitals a special state with partial formation of singlets by electrons on one orbital, while others are effectively decoupled and may give e.g. long-range magnetic order or stay paramagnetic. Similar situation can be realized in dimers spontaneously formed at structural phase transitions, which can be called orbital-selective Peierls transition. This can occur in case of strongly nonuniform hopping integrals for different orbitals and small intra-atomic Hund's rule coupling JH. Yet another consequence of this picture is that for odd number of electrons per dimer there exist competition between double exchange mechanism of ferromagnetism, and the formation of singlet dimer by electron on one orbital, with remaining electrons giving a net spin of a dimer. The first case is realized for strong Hund's rule coupling, typical for 3d compounds, whereas the second is more plausible for 4d-5d compounds. We discuss some implications of these phenomena, and consider examples of real systems, in which orbital-selective phase seems to be realized.