  Kitaev quantum spin liquid is a topological magnetic quantum state
characterized by Majorana fermions of fractionalized spin excitations, which
are identical to their own antiparticles. Here, we demonstrate emergence of
Majorana fermions thermally fractionalized in the Kitaev honeycomb spin lattice
{\alpha}-RuCl3. The specific heat data unveil the characteristic two-stage
release of magnetic entropy involving localized and itinerant Majorana
fermions. The inelastic neutron scattering results further corroborate these
two distinct fermions by exhibiting quasielastic excitations at low energies
around the Brillouin zone center and Y-shaped magnetic continuum at high
energies, which are evident for the ferromagnetic Kitaev model. Our results
provide an opportunity to build a unified conceptual framework of
fractionalized excitations, applicable also for the quantum Hall states,
superconductors, and frustrated magnets.
