Correlated Electron Metal Properties Of The Honeycomb Ruthenate Na2ruo3

We report the synthesis and characterization of polycrystalline Na2RuO3, a layered material in which the Ru4+ (4d(4) configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered Na2RuO3 (C2/c phase), as assessed from the refinement of the time-of-flight neutron powder diffraction. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetism [chi(0) similar to 1.42(2) x 10(-3) emu/mol Oe] with no evidence of magnetic ordering down to 1.5 K, and with an absence of dynamic magnetic correlations, as evidenced by neutron scattering spectroscopy. The intrinsic susceptibility (chi(0)) together with the Sommerfeld coefficient of gamma = 11.7(2) mJ/Ru mol K-2 estimated from heat capacity measurements gives an enhanced Wilson ratio of R-w approximate to 8.9(1), suggesting that magnetic correlations may be present in this material. While transport measurements on pressed pellets show nonmetallic behavior, photoemission spectroscopy indicates a small but finite density of states at the Fermi energy, suggesting that the bulk material is metallic. Except for resistivity measurements, which may have been compromised by near-surface and interface effects, all other probes indicate that Na2RuO3 is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that Na2RuO3 is an antiferromagnetic insulator at low temperatures.