Spin dynamics and possible topological magnons in non-stoichiometric pyrochlore iridate Tb${}_2$Ir${}_2$O${}_7$ studied by RIXS

Abstract

We report a resonant inelastic X-ray scattering study on a single crystal of a non-stoichiometric pyrochlore iridate Tb${}_{2+x}$Ir${}_{2-x}$O${}_{7-y}$ ($x\simeq 0.25$) that magnetically orders at $T_{\mathrm{N}}\simeq 50$ K. We find that the strength of the spin-orbit coupling and the trigonal distortion of the IrO${}_6$ 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.

manuscript: 7 pages, 4 figures; Supplemental Material: 6 pages, 7 figures, 1 table. Accepted at PRB Letters (2024)