Article

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

07/2007; DOI:doi:10.1103/PhysRevB.76.075118
Source: arXiv

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. Comment: 27 pages and 7 figures

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### Keywords

bulk-sensitive X-ray diffraction

correlated electron oxide systems

crystal structural change

electronic inhomogeneity

exhibit

inhomogeneous electronic properties

microscopic evolution

percolation theory predictions

postulated

slower

strain effects

strain states

strong electron-lattice interactions

strong first-order metal-insulator transition

STS

STS maps

surface-sensitive scanning tunneling spectroscopy

systematic decrease

temperature dependence