Correlated electron metal properties of the honeycomb ruthenate Na2RuO3

We report the synthesis and characterization of polycrystalline ${\mathrm{Na}}_{2}{\mathrm{RuO}}_{3}$, a layered material in which the ${\mathrm{Ru}}^{4+}$ ($4{d}^{4}$ configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered ${\mathrm{Na}}_{2}{\mathrm{RuO}}_{3}$ ($C2/c$ phase), as assessed from the refinement of the time-of-flight neutron powder diffraction. Magnetic susceptibility measurements reveal a large temperature-independent Pauli paramagnetism $[{\ensuremath{\chi}}_{0}\ensuremath{\sim}1.42(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ 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 (${\ensuremath{\chi}}_{0}$) together with the Sommerfeld coefficient of $\ensuremath{\gamma}=11.7(2)$ mJ/Ru mol ${\mathrm{K}}^{2}$ estimated from heat capacity measurements gives an enhanced Wilson ratio of ${R}_{W}\ensuremath{\approx}8.9(1)$, 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 ${\mathrm{Na}}_{2}{\mathrm{RuO}}_{3}$ is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that ${\mathrm{Na}}_{2}{\mathrm{RuO}}_{3}$ is an antiferromagnetic insulator at low temperatures.