We investigated the inhomogeneous electronic properties at the surface and interior of V O2 thin films that exhibit a strong first-order metal-insulator transition (MIT). Using the crystal structural change that accompanies a V O2 MIT, we used bulk-sensitive x-ray diffraction (XRD) measurements to estimate the fraction of metallic volume pXRD in our V O2 film. The temperature dependence of the pXRD was very closely correlated with the dc conductivity near the MIT temperature and fitted the percolation theory predictions quite well: σ∼ (p- pc) t with t=2.0±0.1 and pc =0.16±0.01. This agreement demonstrates that in our V O2 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.
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - 16 Aug 2007|