  A key characteristic of quantum spin liquids(QSL) is the presence of
fractional excitations related to their entanglement properties, yet
experimental verification of their statistics is missing. For example, in the
potential Kitaev spin liquid, RuCl3 experiments uncovered signs of fractional
particles, though not their Fermi statistics. Here we employ Raman scattering
to reveal the true nature of the magnetic excitations, using improved
experimental methods and analysis to remove the influence of laser heating and
thermal fluctuations. Via the energy loss and gain spectra, we extract the
energy and temperature dependence of the Raman susceptibility to prove RuCl3's
magnetic response is given by pair creation of fermionic excitations.
Furthermore, by comparing with quantum Monte Carlo (QMC) results for the exact
Kitaev limit, we are able to discern the energy and temperature range where
additional interaction terms are important. Our results open new directions in
QSL research by providing a new way to investigate fractional excitations and
the importance of terms causing spinon confinement.
