Imaging the nanoscale phase separation in vanadium dioxide thin films at terahertz frequencies

TL;DR

This study employs broadband THz-SNOM to visualize nanoscale phase separation in VO2 thin films during the insulator-to-metal transition, revealing frequency-dependent inhomogeneities and domain responses that challenge classical transition models.

Contribution

It introduces broadband THz-SNOM imaging of VO2, demonstrating frequency-dependent nanoscale phase behavior and domain dynamics during the IMT.

Findings

Spatial inhomogeneities depend on probing frequency.

Individual domains show temperature-dependent optical responses.

Results align with dynamical mean-field theory predictions.

Abstract

We use apertureless scattering near-field optical microscopy (SNOM) to investigate the nanoscale optical response of vanadium dioxide (VO2) thin films through a temperature-induced insulator-to-metal transition (IMT). We compare images of the transition at both mid-infrared (MIR) and terahertz (THz) frequencies, using a custom-built broadband THz-SNOM compatible with both cryogenic and elevated temperatures. We observe that the character of spatial inhomogeneities in the VO2 film strongly depends on the probing frequency. In addition, we find that individual insulating (or metallic) domains have a temperature-dependent optical response, in contrast to the assumptions of a classical first-order phase transition. We discuss these results in light of dynamical mean-field theory calculations of the dimer Hubbard model recently applied to VO2.