Giant Magnon Gap in Bilayer Iridate Sr3Ir2O7: Enhanced Pseudo-dipolar Interactions Near the Mott Transition

TL;DR

This study reveals an exceptionally large magnon gap in bilayer iridate Sr3Ir2O7, driven by enhanced pseudo-dipolar interactions near the Mott transition, indicating potential for novel magnetic states beyond the Heisenberg model.

Contribution

It demonstrates the significant role of pseudo-dipolar interactions in the magnetic properties of bilayer iridates near the Mott transition, contrasting with single-layer counterparts.

Findings

Magnon gap of ~92 meV exceeds magnon bandwidth of ~70 meV.

Giant magnon gap caused by enhanced pseudo-dipolar interactions.

Potential for Kitaev-like magnetism in small charge-gap iridates.

Abstract

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.