Interface-induced collective phase transition in VO₂-based bilayers studied by layer selective spectroscopy

We investigated the origin of collective electronic phase transitions induced at the heterointerface between monoclinic insulating VO₂ and rutile metallic electron-doped VO₂ layers using in situ soft X-ray photoemission spectroscopy (PES) and X-ray absorption spectroscopy (XAS) on nanoscale VO₂/V_0.99W_0.01O₂ (001)_R bilayers. Thanks to the surface sensitivity of PES and XAS, we determined the changes in the electronic structure and V-V dimerization in each constituent layer separately. The layer selective observation of the electronic and crystal structures in the upper VO₂ layer of the bilayer indicates that the monoclinic insulating phase VO₂ layer undergoes a transition to the rutile metallic phase by forming the heterointerface. Detailed temperature-dependent measurements reveal that the rutile metallic phase VO₂ undergoes a transition to the monoclinic insulating phase with a decrease in temperature, as in the case of a VO₂ single-layer film. Furthermore, during the temperature-induced phase transition in the VO₂ layer, the spectra are well described by an in-plane phase separation of the rutile metallic and monoclinic insulating phases. These results suggest that the interface-induced transition from the monoclinic insulating to the rutile metallic phase in the VO₂ layer of bilayers occurs as a collective phase transition derived from the static energy balance between the interfacial energy and the bulk free energies of the constituent layers.