Daniel Khomskii: Orbital-selective effects in transition metal compounds: 'molecules' in solids against magnetism

Abstract:

Close to Mott transition several novel states can appear. In particular, “molecular clusters” can be formed in the solid, such as dimers, trimers, etc. In my talk I will illustrate such phenomena, especially the 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. Such transitions can occur in case of strongly non-uniform hopping amplitudes for different orbitals and small intra-atomic Hund's rule coupling J_H . For odd number of electrons per dimer, the dimer formation competes with the double exchange mechanism for ferromagnetism. I discuss some implications of these phenomena, and consider examples of realistic materials, in which orbital-selective phases seem to be realized.