Signatures of non-Loudon-Fleury Raman scattering in the Kitaev magnet ${\hat{I}}^2$-Li${}_2$IrO${}_3$

Abstract

We investigate the magnetic excitations of the hyperhoneycomb Kitaev magnet ${\hat{I}}^2$-${\text{Li}}_2{\text{IrO}}_3$ by means of inelastic Raman scattering. The spectra exhibits a coexistence of a broad scattering continuum and two sharp low-energy peaks at 2.5 meV and 3 meV, with a distinctive polarization dependence. While the continuum is suggestive of fractional quasi-particles emerging from a proximate quantum spin liquid phase, the sharp peaks provide the first experimental signature of the `non-Loudon-Fleury' one-magnon scattering processes proposed recently [Phys. Rev. B 104, 144412 (2021)]. The corresponding microscopic mechanism is similar to the one leading to the symmetric off-diagonal exchange interaction $\hat{I}“$ (as it involves a combination of both direct and ligand-mediated exchange paths), but is otherwise completely unexpected within the traditional Loudon-Fleury theory of Raman scattering. The present experimental verification therefore calls for a drastic reevaluation of Raman scattering in similar systems with strong spin orbit coupling and multiple exchange paths.

5 pages, 4 figures, Supplementary material