We show that in transition metal compounds containing structural metal dimersthere may exist in the presence of different orbitals a special state withpartial formation of singlets by electrons on one orbital, while others areeffectively decoupled and may give e.g. long-range magnetic order or stayparamagnetic. Similar situation can be realized in dimers spontaneously formedat structural phase transitions, which can be called orbital-selective Peierlstransition. This can occur in case of strongly nonuniform hopping integrals fordifferent orbitals and small intra-atomic Hund's rule coupling JH. Yet anotherconsequence of this picture is that for odd number of electrons per dimer thereexist competition between double exchange mechanism of ferromagnetism, and theformation of singlet dimer by electron on one orbital, with remaining electronsgiving a net spin of a dimer. The first case is realized for strong Hund's rulecoupling, typical for 3d compounds, whereas the second is more plausible for4d-5d compounds. We discuss some implications of these phenomena, and considerexamples of real systems, in which orbital-selective phase seems to berealized.
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