Fig. 3: Two-site energy levels and bilinear magnetic couplings.

a Evolution of lowest energy eigenvalues for two-site Z1-bond cluster. States are colored according to their composition in terms of Jeff states. For Ut2g ≤ 5 eV, Jeff = 2 states intrude into the low-energy space, invalidating a pure Jeff = 1 model. b, c Evolution of bilinear couplings in Jeff = 1 model for large Ut2g for first and third neighbors, respectively. Second neighbor couplings are an order of magnitude smaller and are not shown.