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

TL;DR

This study reveals how heterointerfaces in VO2 bilayers induce collective phase transitions between insulating and metallic states, with temperature-dependent phase separation observed via layer-specific spectroscopy.

Contribution

It demonstrates that interface effects can drive collective phase transitions in VO2 bilayers, providing detailed spectroscopic insights into the electronic and structural changes.

Findings

Layer-specific spectroscopy shows phase transition from monoclinic to rutile in VO2 layers.

Temperature induces in-plane phase separation of metallic and insulating phases.

Interface-driven transition is governed by static energy balance between interfacial and bulk energies.

Abstract

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/V0.99W0.01O2 (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.