Infrared spectroscopy and nano-imaging of the insulator-to-metal transition in vanadium dioxide

TL;DR

This study uses infrared spectroscopy and nano-imaging to investigate the insulator-to-metal transition in VO2 thin films, revealing phase coexistence, static metallic regions, and the role of electronic correlations.

Contribution

It introduces a combined far-field and near-field infrared analysis using Bruggeman theory and the extended dipole model to study VO2's IMT at nanoscale.

Findings

Percolative IMT observed with temperature changes.

Coexisting insulating and metallic regions are static.

Divergent effective carrier mass indicates strong electronic correlations.

Abstract

We present a detailed infrared study of the insulator-to-metal transition (IMT) in vanadium dioxide (VO2) thin films. Conventional infrared spectroscopy was employed to investigate the IMT in the far-field. Scanning near-field infrared microscopy directly revealed the percolative IMT with increasing temperature. We confirmed that the phase transition is also percolative with cooling across the IMT. We present extensive near-field infrared images of phase coexistence in the IMT regime in VO2. We find that the coexisting insulating and metallic regions at a fixed temperature are static on the time scale of our measurements. A novel approach for analyzing the far-field and near-field infrared data within the Bruggeman effective medium theory was employed to extract the optical constants of the incipient metallic puddles at the onset of the IMT. We found divergent effective carrier mass in the metallic puddles that demonstrates the importance of electronic correlations to the IMT in VO2. We employ the extended dipole model for a quantitative analysis of the observed near-field infrared amplitude contrast and compare the results with those obtained with the basic dipole model.