Orbital, charge, and spin couplings in Ru 25+O9 dimers of Ba3CoRu2O9

e magnetic, transport, and structural properties of cold-pressed Ba3CoRu2O9 show (i) an antiferromagnetic transition, (ii) a semiconductor-semiconductor electronic transition where the resistivity is dominated by the electron hopping between the Ru25+O9 dimers with itinerant Ru electrons at high temperatures, and (iii) a hexagonal to orthorhombic structural phase transition. All three transitions occur at 93 K and are related to the Ru5+ ions in the Ru2O9 dimers. The Ru-O bond distortion below 93 K further indicates a possible orbital ordering for Ru5+ ions in isolated Ru2O9 dimers, which accounts for the strong orbital, charge, and spin couplingsThis work is supported by NSF-DMR-0654118 and the State of Florida. J.S.B. and A.K. acknowledge support from NSF-DMR-1005293S