Rapidly fluctuating orbital occupancy above the orbital ordering transition in spin-gap compounds

Several spin systems with low dimensionality develop a spin-dimer phase within a molecular orbital below T(S), competing with long-range antiferromagnetic order. Very often, preferential orbital occupancy and ordering are the actual driving force for dimerization, as in the so-called orbitally driven spin-Peierls compounds (MgTi(2)O(4), CuIr(2)S(4), La(4)Ru(2)O(10), NaTiSi(2)O(6), etc.). Through a microscopic analysis of the thermal conductivity kappa(T) in La(4)Ru(2)O(10), we show that the orbital occupancy fluctuates rapidly above T(S), resulting in an orbital-liquid state. The strong orbital-lattice coupling introduces dynamic bond-length fluctuations that scatter the phonons to produce a kappa (T) similar to T (i.e., glasslike) above T(S). This phonon-glass to phonon-crystal transition is shown to occur in other spin-dimer systems, like NaTiSi(2)O(6), pointing to a general phenomenon.