Vibronic and Magnetic Excitations in the Spin-Orbital Liquid State ofFeSc2S4

Inelastic neutron scattering results on the spin-orbital liquid in ${\mathrm{FeSc}}_2{\mathrm{S}}_4$ are presented. This sulfospinel reveals strong geometric frustration in the spin and in the orbital sector. In the present experiments the orbital liquid is evidenced by a clear spectroscopic signature of a dynamic Jahn-Teller effect with a vibronic splitting $3\Gamma \approx 2\mathrm{meV}$ in agreement with theoretical estimates. The excitations of the spin liquid reveal strong dispersion and can be characterized as cooperative spin excitations in a supercooled paramagnet with a spin gap of $\Delta \approx 0.2\mathrm{meV}$.