Raman spectroscopy study of spin-orbital liquid candidate FeSc$_2$S$_4$*

The A-site cubic spinel FeSc$_2$S$_4$, containing Fe$^{2+}$ ions in a tetrahedral S$_4$ environment, represents a rare candidate of a spin-orbital liquid, where spin and orbital order remain suppressed down to the lowest measurable temperature [1]. We studied phonon spectrum and orbital excitations in FeSc$_2$S$_4$ by Raman spectroscopy on single crystals. At temperatures below 100K we observe widening of sulfur 330 cm$^{-1}$ T$_{2g}$ and 365 cm$^{-1}$ A$_{1g}$ phonon modes with an additional weak mode emerging at 400 cm$^{-1}$. These changes can indicate weak lattice distortions associated with the sulfur sites. Below 100K we also observe orbital excitations at frequencies of approximately 2000 cm$^{-1}$. We discuss the result in terms of a competition of spin-orbital liquid and a magnetically ordered state. [1] L. Mittelstadt et al., Phys. Rev. B 91 125112 (2015).