  In FeSc2S4 spin-orbital exchange competes with strong spin-orbit coupling,
suppressing long-range spin and orbital order and, hence, this material
represents one of the rare examples of a spin-orbital liquid ground state.
Moreover, it is close to a quantum-critical point separating the ordered and
disordered regimes. Using THz and FIR spectroscopy we study low-lying
excitations in FeSc2S4 and provide clear evidence for a spin-orbiton, an
excitation of strongly entangled spins and orbitals. It becomes particularly
well pronounced upon cooling, when advancing deep into the quantum-critical
regime. Moreover, indications of an underlying structureless excitation
continuum are found, a possible signature of quantum criticality.
