Vibronic excitations in the orbitally active A-site spinels FeSc$_2$S$_4$, FeCr$_2$O$_4$, and FeCr$_2$S$_4$

We investigated the low-lying excitations of the spinels FeSc$_2$S$_4$, FeCr$_2$O$_4$, and FeCr$_2$S$_4$ by THz spectroscopy. FeSc$_2$S$_4$ reportedly is in a spin-orbital singlet ground state [1,2], while the other two compounds exhibit complex magnetically ordered ground states and orbital ordering transitions [3]. In all compounds we observed excitations which we assign to transitions between vibronic levels of the Fe2+ ions in tetrahedral environment. We will discuss the evolution of these excitations in the case of orbital ordering transition and the competition of spin-orbit coupling and electron-phonon interaction as a source for (spin-)orbital frustration in these systems. \\[4pt] [1] A. Krimmel et al. Phys Rev Lett. 94, 237402 (2005).\\[0pt] [2] G. Chen et al. Phys Rev Lett. 102, 096406 (2009)\\[0pt] [3] V. Tsurkan, et al., Phys. Rev. B 81, 184426 (2010).