| The cubic spinel compound FeCr2S4 exhibits an orbital ordering transition at about 10K which is preceded by a transition from a commensurate to an incommensurate magnetic structure at about 70K. Using infrared and THz spectroscopy we find that both transitions are accompanied by the occurrence of new modes of both electronic and phononic origin. Some optical phonons seem to become both Raman and infrared active indicating a loss of inversion symmetry in the groundstate. We will discuss these anomalies with respect to theoretical predictions based on vibronic coupling and a pseudo-spin flop transition. Moreover, we report on magnetic field-induced phase transitions in the orbitally-ordered ground state, indicating the presence of low-energy competing interactions in the systems. |
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