Condensed Matter Seminars

Due to Jahn-Teller effect the systems with orbital degeneracy usually have an orbital ordering with corresponding lattice distortion. However this distortion weakens and finally disappears when the electrons become itinerant. We show that in the intermediate regime between localized and itinerant electrons there exist a new possibility: orbital degeneracy may be lifted not by Jahn-Teller distortion, but by charge ordering, or charge differentiation. We demonstrate, by theoretical calculations and experimentally, that this happens in particular in rare earths nickelates such as YNiO3 or LuNiO3. The same physics apparently operates also in several other systems, such as ferrates (e.g. CaFeO3) and some others. I will also discuss a possible role of small charge transfer gap and oxygen holes in this phenomenon.