6-GHz lattice response in a quantum spin-orbital liquid probed by time-resolved resonant x-ray scattering

Long-sought quantum spin liquids usually emerge in geometrically frustrated magnets. Less commonly, unfrustrated magnets can harbor a variety of quantum spin liquids if charge and/or orbital degrees of freedom are involved. Jahn-Teller distortion of Cu2+O6 octahedra is absent in hexagonal Ba3CuSb2O9 suggesting a Cu 3d spin-orbital liquid state. Here, by means of time-resolved resonant x-ray scattering, we show that hexagonal Ba3CuSb2O9 exhibits a charge-orbital dynamics which is absent in the orthorhombic phase of Ba3CuSb2O9 with Jahn-Teller distortion and Cu 3d orbital order. The time scale of charge-orbital dynamics manifests in the coherent phonons at 6 GHz. The present work reveals a role of electron-lattice entanglement in the quantum spin-orbital liquid state.