Correlated electron metal properties of the honeycomb ruthenate Na$_2$RuO$_3$

TL;DR

This study synthesizes and characterizes Na$_2$RuO$_3$, revealing it to be a moderately correlated electron metal with no magnetic order, contrasting previous reports of it being an insulator.

Contribution

First comprehensive synthesis and characterization of Na$_2$RuO$_3$ showing its metallic and correlated electron properties with detailed magnetic and transport analysis.

Findings

Na$_2$RuO$_3$ is a nearly ordered layered honeycomb material.

It exhibits large temperature-independent Pauli paramagnetism without magnetic ordering.

Transport measurements suggest nonmetallic behavior, but spectroscopic data indicates metallicity.

Abstract

We report the synthesis and characterisation of polycrystalline Na$_2$RuO$_3$, a layered material in which the Ru$^{4+}$ ($4d^4$ configuration) form a honeycomb lattice. The optimal synthesis condition was found to produce a nearly ordered Na$_2$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 ($\chi_0 \sim 1.42(2)\times10^{-3}$ 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 coeficient of $\gamma=11.7(2)$ mJ/Ru mol K$^2$ estimated from heat capacity measurements, gives an enhanced Wilson ratio of $R_W\approx8.9(1)$, suggesting that magnetic correlations may be present in this material. While transport measurements on pressed pellets show nonmetallic behaviour, photoemission spectrocopy indicate 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 Na$_2$RuO$_3$ is a moderately correlated electron metal. Our results thus stand in contrast to earlier reports that Na$_2$RuO$_3$ is an antiferromagnetic insulator at low temperatures.