DMFT-QE Symposium: October 21

Talk 1:

Orbital Ordering: Finding true Kugel-Khomskii materials

Eva Pavarini, Peter Grünberg Institut, Forschungszentrum Jülich

In a classic paper, Kugel and Khomskii demonstrated about 50 years ago that in strongly-correlated systems orbital ordering can arise from a purely electronic super-exchange mechanism, and not just from the co-operative Jahn-Teller effect.  Since then, finding materials in which the Kugel-Khomskii (KK) super-exchange mechanism dominates orbital physics has proven a challenge, however. For the two textbook cases for orbital ordering, KCuF3 and LaMnO3, it was ultimately shown that, although strong, KK super-exchange alone can not explain the presence of orbital ordering at very high temperature; other mechanisms, such as the (Coulomb-enhanced) Jahn-Teller effect and/or the Born-Mayer repulsion, are at work [1,2,3].  Similar conclusions were reached for other representative systems.

Recently, however, combining DMFT with irreducible tensors decompositions of the multi-orbital KK interactions, we have identified representative cases in which KK super-exchange is indeed key  [4,5,6].  In this talk I will discuss such paradigmatic examples and the unusual interplay between spin and orbital degrees of freedom that can occur in these materials.

[1] E. Pavarini, E. Koch, A.I. Lichtenstein, Phys. Rev. Lett. 101, 266405 (2008)
[2] E. Pavarini and E. Koch, Phys. Rev. Lett. 104, 086402 (2010)
[3] H. Sims, E. Pavarini, and E. Koch, Phys. Rev. B 96, 054107 (2017)
[4] X-J. Zhang, E. Koch and E. Pavarini, Phys. Rev. B 105, 115104 (2022)
[5] X-J. Zhang, E. Koch and E. Pavarini, Phys. Rev. B 106, 115110 (2022)
[6] X-J. Zhang, E. Koch and E. Pavarini, to be published (2024)

Talk 2:

Exceptional Topology of Quasiparticles

Jan Carl Budich, TU Dresden and MPIPKS Dresden

In a variety of physical scenarios ranging from classical meta-materials to correlated quantum many-body systems, non-Hermitian (NH) Hamiltonians have proven to be a powerful and conceptually simple tool for effectively describing dissipation. Recently, investigating the topological properties of such NH systems has become a broad frontier of research. In this talk, I will discuss how NH topological features such as stable exceptional degeneracies and a remarkable spectral sensitivity with respect to boundary conditions can emerge in the quasi-particle description of correlated electron systems that are globally described by a Hermitian Hamiltonian. In particular, I will demonstrate how magnetic fluctuations at the onset of ferromagnetism may promote the symmetry-protected nodal surface states of a topological insulator to a NH Weyl phase that is robust against generic perturbations.

For the full DMFT-QE Symposium go here.