Four-spin ring exchange in triangular lattice compounds*

In the quest for exotic magnetic states, triangular lattice compounds offer a rich playground.  An important question in this context is, which exchange terms are necessary to construct an appropriate minimal magnetic model. For materials where the hopping amplitudes t are of comparable magnitude to the Coulomb repulsion U, higher order terms such as the four-spin ring exchange become significant. In this talk, we discuss ab-initio four-spin ring exchange for S=1/2 triangular lattice compounds, which fall in this regime.

A significant ring exchange may result from effectively reduced U, which is realized in κ-phase organic Mott insulators. Here, ring exchange is argued to suppress conventional magnetic order in a QSL/VBS candidate material. In contrast, in a recently synthesized organic compound we confirm that ring exchange induces a 'spin vortex crystal' order [1].

Another option to enter this regime are large hopping amplitudes due to heavy ligand elements. This situation is necessarily accompanied with strong spin-orbit coupling, resulting in anisotropic ring exchange that forms a potential new avenue for exotic magnetic states.

[1] Riedl et al., Phys. Rev. Lett. 127, 147204 (2021).