Spin-orbiton and quantum criticality in FeSc2S4
  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. 
L. Mittelst\"adt, M. Schmidt, Zhe Wang, F. Mayr, V. Tsurkan, P.   Lunkenheimer, D. Ish, L. Balents, J. Deisenhofer, and A. Loidl
cond-mat.str-el