Near-Field Forces and Non-Contact Dissipation in VO₂ Films

Two theories can describe how VO₂ undergoes its insulator-to-metal transition (IMT) at 340 K: it may exhibit a Mott transition; or exhibit a Peierls or structural phase transition (SPT) due to electron-phonon interactions. To study the IMT of VO₂, we use the non-invasive technique of non-contact atomic force microscopy (nc-AFM). When the oscillating conducting tip of the nc-AFM approaches a VO₂ film grown by molecular beam epitaxy, we detect a resonant frequency shift of the tuning fork sensor, related to tip-sample forces. We also increase the driving bias to maintain a constant tip response amplitude, related to tip-sample power dissipation. Preliminary data shows a rise in the power dissipated in the high force regime, suggesting phononic dissipation which may signal proximity to a phase transition. We expect power dissipation to change across the structural phases of VO₂ due to differing lattice symmetries and phonon spectra. Thus, we study the role of phonon dispersion in the insulating and metallic states of VO₂ in order to understand the dissipative nature of the SPT.