  Significant efforts have focused on the magnetic excitations of relativistic
Mott insulators, predicted to realize the Kitaev quantum spin liquid (QSL).
This exactly solvable model involves a highly entangled state resulting from
bond-dependent Ising interactions that produce excitations which are non-local
in terms of spin flips. A key challenge in real materials is identifying the
relative size of the non-Kitaev terms and their role in the emergence or
suppression of fractional excitations. Here, we identify the energy and
temperature boundaries of non-Kitaev interactions by direct comparison of the
Raman susceptibility of RuCl3 with quantum Monte Carlo (QMC) results for the
Kitaev QSLs. Moreover, we further confirm the fractional nature of the magnetic
excitations, which is given by creating a pair of fermionic quasiparticles.
Interestingly, this fermionic response remains valid in the non-Kitaev range.
Our results and focus on the use of the Raman susceptibility provide a
stringent new test for future theoretical and experimental studies of QSLs.
