Coupled spin-lattice fluctuations in a compound with orbital degrees of freedom: The Cr-based dimer system Sr3Cr2O8

We report on an extended fluctuation regime in the spin dimer system Sr3Cr2O8 based on anomalies in Raman active phonons and magnetic scattering. The compound has two characteristic temperatures. The Jahn-Teller transition at T-S = 275 K is related to structural distortions and orbital ordering and the second instability at T* approximate to 150 K is due to further changes in the orbital sector. Below T-S quasielastic scattering marks strong fluctuations and in addition phonon anomalies are observed. For temperatures below T* we observe an exponential decrease of one phonon linewidth and determine a gap of the orbital excitations. At low temperatures the observation of two- and three-magnon scattering allows the determination of the spin excitation gap.