Signatures of non-Loudon-Fleury Raman scattering in the Kitaev magnet $\beta$-Li$_2$IrO$_3$

TL;DR

This study reveals non-Loudon-Fleury Raman scattering signatures in the Kitaev magnet $eta$-Li$_2$IrO$_3$, indicating complex magnetic excitations and challenging traditional theories of Raman processes in such materials.

Contribution

First experimental observation of non-Loudon-Fleury one-magnon scattering in a Kitaev magnet, highlighting the need to revise Raman scattering models for systems with strong spin-orbit coupling.

Findings

Detection of a broad continuum suggestive of fractional quasi-particles.

Observation of two sharp low-energy peaks with polarization dependence.

Experimental verification of non-Loudon-Fleury scattering processes.

Abstract

We investigate the magnetic excitations of the hyperhoneycomb Kitaev magnet $\beta$-$\text{Li}_2\text{IrO}_3$ 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 $\Gamma$ (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.