Observation of an anomalous quasi-one-dimensional behavior in Na2Ru4O9- delta single crystals.

Abstract

The magnetic susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathrm{\ensuremath{\parallel}}}$(H\ensuremath{\parallel}b), ${\mathrm{\ensuremath{\chi}}}_{\mathrm{\ensuremath{\perp}}}$(H\ensuremath{\perp}b) (2\ensuremath{\le}T\ensuremath{\le}700 K) and electrical resistivity \ensuremath{\rho}(T) (2\ensuremath{\le}T\ensuremath{\le}300 K) of single crystals ${\mathrm{Na}}_{2}$${\mathrm{Ru}}_{4}$${\mathrm{O}}_{9\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ (\ensuremath{\delta}\ensuremath{\ge}0) with various oxygen content were measured. The crystal structure is monoclinic with single, double, and triple chains along the b axis. The most striking feature is the drastically large anisotropy reflected in the resistivity which exhibits metallic behavior along chains and semiconducting behavior perpendicular to the chains. The resistivity ratio for these two directions ${\mathrm{\ensuremath{\rho}}}_{\mathrm{\ensuremath{\perp}}}$/${\mathrm{\ensuremath{\rho}}}_{\mathrm{\ensuremath{\parallel}}}$ is larger than ${10}^{5}$. This ratio is exceptionally large, indicating that the anisotropy of the bandwidth is substantial. While the magnetic susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathrm{\ensuremath{\parallel}}}$(H\ensuremath{\parallel}b) undergoes a drastic change due to oxygen removal, the electrical resistivity along the conducting chains ${\mathrm{\ensuremath{\rho}}}_{\mathrm{\ensuremath{\parallel}}}$(T) alters only slightly, however, the resistivity perpendicular to the chain ${\mathrm{\ensuremath{\rho}}}_{\mathrm{\ensuremath{\perp}}}$(T) changes more significantly at low temperatures. It is argued that the different sensitivity to oxygen content reflected in magnetic and transport properties may indicate decoupling of spin and charge, and can be attributed the complexity of the crystal structure. \textcopyright{} 1996 The American Physical Society.