Orbital selective metallicity in the valence bond liquid phase of Li₂RuO₃

Physics on a honeycomb lattice is intriguing because of the potential for novel phenomena, stemming from competitive interactions between the fundamental degrees of freedom (charge, orbital, lattice, spin). One such system is Li₂RuO₃ (LRO), which forms a valence bond crystal at room temperature. At high temperature ~ 500 K, it undergoes a phase transition that involves changes to its structural and transport properties, which leads to an exotic valence bond liquid state. The orbital degrees of freedom are expected to be critical in explaining the evolution of LRO properties across the phase transition.  We report temperature dependent broadband (100 cm^-1 - 26,000 cm^-1) reflectance measurements on single crystals of LRO to elucidate structural and transport properties through the high-temperature phase transition. We observe the expected structural transition via the change of the phonon spectrum. Additionally, the optical band gap closes at high temperature, only 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.