The superexchange interactions in Mott insulators are responsible not only for magnetic exchange constants, but also for the distribution of spectral weight in optical excitations. While in many cases the mean field approach gives already satisfactory explanation of the trends observed in experiments [1], we show that large composite spin-orbital fluctuations due to quantum entanglement of spin and orbital variables in the superexchange may destroy this “classical” picture, leading to an apparent violation of the classical Goodenough-Kanamori rules [2]. In such cases mean field decoupling of spin and orbital operators cannot be used, and spin exchange interactions fluctuate over positive and negative values. [1] A.M. Oles, P. Horsch, G. Khaliullin, L.F. Feiner, Phys. Rev. B 72, 214431 (2005). [2] A.M. Oles, P. Horsch, L.F. Feiner, G. Khaliullin, Phys. Rev. Lett. 96, 147205 (2006).
M. Smoluchowski Institute of Physics, Jagellonian University, Cracow, Poland