  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.
