Raman spectroscopic signature of fractionalized excitations in the harmonic-honeycomb iridates $\beta$- and $\gamma$-Li$_2$IrO$_3$

TL;DR

This study uses Raman spectroscopy to detect signatures of fractionalized Majorana fermions in three-dimensional iridates, providing evidence for spin fractionalization in a 3D Kitaev system.

Contribution

First experimental identification of Majorana fermion signatures in 3D harmonic-honeycomb iridates via polarization-resolved Raman spectroscopy.

Findings

Observation of a broad Raman scattering continuum with polarization dependence

Temperature-dependent spectral weight indicating fermionic excitations

Evidence supporting Majorana fermions from spin fractionalization

Abstract

The fractionalization of elementary excitations in quantum spin systems is a central theme in current condensed matter physics. The Kitaev honeycomb spin model provides a prominent example of exotic fractionalized quasiparticles, composed of itinerant Majorana fermions and gapped gauge fluxes. However, identification of the Majorana fermions in a three-dimensional honeycomb lattice remains elusive. Here we report spectroscopic signatures of fractional excitations in the harmonic-honeycomb iridates $\beta$- and $\gamma$-Li$_2$IrO$_3$. Using polarization resolved Raman spectroscopy, we find that the dynamical Raman response of $\beta$- and $\gamma$-Li$_2$IrO$_3$ features a broad scattering continuum with distinct polarization and composition dependence. The temperature dependence of the Raman spectral weight is dominated by the thermal damping of fermionic excitations. These results suggest the emergence of Majorana fermions from spin fractionalization in a three-dimensional Kitaev-Heisenberg system.