Orbital Ordering in Paramagnetic LaMnO3 and KCuF3

Abstract

{\it Ab-initio} studies of the stability of orbital ordering, its coupling to magnetic structure and its possible origins (electron-phonon and/or electron-electron interactions) are reported for two perovskite systems, LaMnO${}_3$ and KCuF${}_3$. We present a new Average Spin State (ASS) calculational scheme that allowed us to treat a paramagnetic state. Using this scheme, we succesfully described the experimental magnetic/orbital phase diagram of both LaMnO${}_3$ and KCuF${}_3$ in crystal structures when the Jahn-Teller distortions are neglected. Hence, we conclude that the orbital ordering in both compounds is purely electronic in origin.

10 pages, 5 figures

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