Orbital-selective metallicity in the valence-bond liquid phase of Li₂RuO₃

Li2RuO3 (LRO) forms a valence bond crystal at room temperature. It undergoes a high temperature phase transition that involves structural, magnetic, and electronic changes leading to an exotic valence bond liquid state. The orbital degrees of freedom are thought to be fundamental to the evolution of LRO properties across the phase transition. We report temperature dependent broadband (100-26 000 cm(-1)) reflectance measurements on single crystals of LRO to elucidate structural and transport properties. Specifically, the phonon and electronic properties of LRO were investigated through the phase transition. We report that above the transition temperature (T-c approximate to 500 K), the optical band gap closes for electrons in the d(xz)/d(yz) orbitals, but the d(xy) electrons remain gapped. This behavior at high temperature can be associated with an orbital selective metallic state which to our knowledge has not been previously reported in LRO.