Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr3Ir2O7, a spin-orbit coupling driven magnetic insulator with a small charge gap, a magnon gap of ~92 meV for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of ~70 meV and implies a marked departure from the Heisenberg model, in stark contrast to the case of the single-layer iridate Sr2IrO4. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudo-dipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudo-dipolar interactions, may emerge in small charge-gap iridates.
@article{arxiv.1205.5337,
title = {Giant Magnon Gap in Bilayer Iridate Sr3Ir2O7: Enhanced Pseudo-dipolar Interactions Near the Mott Transition},
author = {Jungho Kim and A. H. Said and D. Casa and M. H. Upton and T. Gog and M. Daghofer and G. Jackeli and J. van den Brink and G. Khaliullin and B. J. Kim},
journal= {arXiv preprint arXiv:1205.5337},
year = {2012}
}