  Long-range entanglement in quantum spin liquids (QSLs) lead to novel low
energy excitations with fractionalised quantum numbers and (in 2D) statistics.
Experimental detection and manipulation of these excitations present a
challenge particularly in view of diverse candidate magnets. A promising probe
of fractionalisation is their coupling to phonons. Here we present Raman
scattering results for the S = 1/2 honeycomb iridate Cu2IrO3, a candidate
Kitaev QSL with fractionalised Majorana fermions and Ising flux excitations. We
observe anomalous low temperature frequency shift and linewidth broadening of
the Raman intensities in addition to a broad magnetic continuum both of which,
we derive, are naturally attributed to the phonon decaying into itinerant
Majoranas. The dynamic Raman susceptibility marks a crossover from the QSL to a
thermal paramagnet at ~120 K. The phonon anomalies below this temperature
demonstrate a strong phonon-Majorana coupling. These results provide for
evidence of spin fractionalisation in Cu2IrO3.
