Spin dynamics and possible topological magnons in non-stoichiometric pyrochlore iridate Tb₂Ir₂O₇ studied by RIXS

We report a resonant inelastic X-ray scattering study on a single crystal of a non-stoichiometric pyrochlore iridate Tb₂₊ₓIr₂₋ₓO7-y (x 0.25) that magnetically orders at TN 50 K. We find that the strength of the spin-orbit coupling and the trigonal distortion of the IrO₆ octahedra are comparable with the ones obtained in other pyrochlore iridates. We observe a propagating gapped magnon mode at low energy, and model it using a Hamiltonian consisting of a Heisenberg exchange [$J = 16.2(9)$ meV] and Dzyaloshinskii-Moriya interactions [$D = 5.2(3)$ meV], which shows the robustness of interactions despite Tb-stuffing at the Ir-site. Strikingly, the ratio $D/J = 0.32(3)$ supports possible non-trivial topological magnon band crossing. This material may thus host coexisting fermionic and bosonic topology, with potential for manipulating electronic and magnonic topological bands thanks to the $d-f$ interaction.