Events

Abstract: 
In a seminal paper 1, Kugel and Khomskii demonstrated about 50 years ago that, in strongly-correlated systems, orbital-ordering can arise from a purely electronic mechanism, orbital super-exchange. Identifying Kugel-Khomskii (KK) materials is a classic chicken-and egg problem, however. In fact, the hallmark of orbital ordering is a secondary effect, a Jahn Teller-like co-operative lattice distortion. The same distortion can arise from the conventional Jahn-Teller effect, i.e., via electron-lattice coupling; this in turn can result in orbital ordering. We have shown how to overcome the chicken-and-egg problem in Ref. 2, by explicitly disentangling the two mechanisms. Finding true KK materials has proven a challenge, however. For the textbook cases of orbital ordering, KCuF3 and LaMnO3, we found that KK superexchange alone can not explain the persistence of orbital ordering at very high temperature 2,3. Other mechanisms, such as the Coulomb-enhanced Jahn-Teller effect 2,3 and/or the Born-Mayer repulsion 4, are at work. Similar conclusions were drawn for other families of proposed KK candidates. 
Recently, we have however identified representative materials in which KK super-exchange is indeed key 5. In this talk, after a general introduction to the problem, I will discuss such paradigmatic examples and the surprising consequences of this finding.


1 K. I. Kugel’ and D. I. Khomskii, Zh. Eksp. Teor. Fiz. 64, 1429 (1973). 2 E. Pavarini, E. Koch, A.I. Lichtenstein, Phys. Rev. Lett. 101, 266405 (2008) 3 E. Pavarini and E. Koch, Phys. Rev. Lett. 104, 086402 (2010) 4 H. Sims, E. Pavarini, and E. Koch, Phys. Rev. B 96, 054107 (2017) 5 X-J. Zhang, E. Koch and E. Pavarini, Phys. Rev. B 106, 115110 (2022); Phys. Rev. Lett. 135, 026508 (2025)