  The combination of electronic correlation and spin-orbit coupling is thought
to precipitate a variety of highly unusual electronic phases in solids,
including topological and quantum spin liquid states. We report a Raman
scattering study that provides evidence for unconventional excitations in
$\alpha$-RuCl$_3$, a spin-orbit coupled Mott insulator on the honeycomb
lattice. In particular, our measurements reveal unusual magnetic scattering,
typified by a broad continuum. The temperature dependence of this continuum is
evident over a large scale compared to the magnetic ordering temperature,
suggestive of frustrated magnetic interactions. This is confirmed through an
analysis of the phonon linewidths, which show a related anomaly due to
spin-phonon coupling. These observations are in line with theoretical
expectations for the Heisenberg-Kitaev model and suggest that $\alpha$-RuCl$_3$
may be close to a quantum spin liquid ground state.
