Unveiling intermediate metallicity in epitaxial VO2 using nano-spectroscopy aided by machine learning

A thin film of VO2 was grown on a TiO2 substrate to bring the Mott insulator-to-metal transition to room temperature. The metallic state in VO2 forms in puddles that percolate as the temperature is increased [1]. Using Nanoscale Imaging and Spectroscopy with Machine learning Assistance (NISMA), we probe coexisting insulating and metallic domains at variable temperature and identify a third, intermediate phase at boundaries between insulating and metallic domains. We will present a detailed analysis of the electronic properties of these three phases at room temperature and provide a comparison to the high-temperature metallic state. We also investigate the accompanying monoclinic-to-rutile transition by analysis of the optical phonon response of the metallic puddles. This lays the groundwork for disentangling electronic transitions from the slower structural transitions in Mott insulators [2]. This has broader implications for the realization of Mott insulators in high-speed electronic applications such as resistive switching and neuromorphic computing.

1. Qazilbash, M. M. et al. Science 318, 1750–1753 (2007).

2. Morrison, V. R. et al. Science 346, 445–448 (2014).