Surface versus bulk characterization of the electronic inhomogeneity in a VO_{2} film

TL;DR

This study compares surface and bulk electronic inhomogeneity in VO₂ thin films during the metal-insulator transition, revealing different behaviors likely due to strain effects near the surface, with implications for correlated electron systems.

Contribution

It provides a combined surface and bulk analysis of the MIT in VO₂ films, highlighting the role of strain in surface inhomogeneity and its impact on transition dynamics.

Findings

Bulk measurements fit percolation theory well

Surface inhomogeneity evolves more slowly than bulk

Strain near the surface influences the MIT behavior

Abstract

We investigated the inhomogeneous electronic properties at the surface and interior of VO_{2} thin films that exhibit a strong first-order metal-insulator transition (MIT). Using the crystal structural change that accompanies a VO_{2} MIT, we used bulk-sensitive X-ray diffraction (XRD) measurements to estimate the fraction of metallic volume p^{XRD} in our VO_{2} film. The temperature dependence of the p$^{XRD}$ was very closely correlated with the dc conductivity near the MIT temperature, and fit the percolation theory predictions quite well: $\sigma$ $\sim$ (p - p_{c})^{t} with t = 2.0$\pm$0.1 and p_{c} = 0.16$\pm$0.01. This agreement demonstrates that in our VO$_{2}$ thin film, the MIT should occur during the percolation process. We also used surface-sensitive scanning tunneling spectroscopy (STS) to investigate the microscopic evolution of the MIT near the surface. Similar to the XRD results, STS maps revealed a systematic decrease in the metallic phase as temperature decreased. However, this rate of change was much slower than the rate observed with XRD, indicating that the electronic inhomogeneity near the surface differs greatly from that inside the film. We investigated several possible origins of this discrepancy, and postulated that the variety in the strain states near the surface plays an important role in the broad MIT observed using STS. We also explored the possible involvement of such strain effects in other correlated electron oxide systems with strong electron-lattice interactions.