Emergent properties in films of transition metal oxides*

Transition metal oxides exhibit diverse emergent phenomena such as strongly correlated Mott insulating states, magnetic and structural phase transitions, and metal-insulator transitions. Many of these emergent states occur at nanometer length scales which cannot be accessed with traditional infrared focusing methods due to the Abbe diffraction limit. However, scattering-type scanning near-field infrared microscopy (S-SNIM) can circumvent the Abbe diffraction limit and probe optical properties with spatial resolution of tens of nanometers. We have coupled table-top plasma light sources developed in-house to a commercial S-SNIM apparatus from Neaspec GmbH to demonstrate broadband infrared nano-spectroscopy. This is used to study the novel nanoscale properties of ultrathin vanadium dioxide (VO₂) film on rutile titanium dioxide (TiO₂) substrate. Due to strain from the substrate, the metal-insulator transition in the VO₂ film occurs at a temperature of 305 K which is much lower than in bulk VO₂. We also study the surface metallic layer induced by vacuum-annealing of insulating strontium titanate (SrTiO₃) crystals and extract the dynamical properties of the free charge carriers.