We report x-ray and optical absorption spectroscopy as well as hard x-ray photoemission spectroscopy on hexagonal Ba3CuSb2O9 in which a Cu 3d spin-orbital liquid state is suggested from absence of the Jahn-Teller distortion of Cu2+O6 octahedra and of the magnetic ordering. The experimental results and their cluster model analysis indicate that O 2p holes play a crucial role in realizing the spin-orbital liquid state of hexagonal Ba3CuSb2O9. These oxygen holes appear due to the "reaction"Sb5+→Sb3++ two O 2p holes, with these holes being able to attach to Cu ions. The present work opens avenues towards spin-charge-orbital entangled liquid state in transition-metal oxides with small or negative charge transfer energy.
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