Direct Observation of Decoupled Structural and Electronic Transitions and an Ambient Pressure Monocliniclike Metallic Phase of ${\mathrm{VO}}_2$

We report the simultaneous measurement of the structural and electronic components of the metal-insulator transition (MIT) of ${\mathrm{VO}}_2$ using electron and photoelectron spectroscopies and microscopies. We show that these evolve over different temperature scales, and are separated by an unusual monocliniclike metallic phase. Our results provide conclusive evidence that the new monocliniclike metallic phase, recently identified in high-pressure and nonequilibrium measurements, is accessible in the thermodynamic transition at ambient pressure, and we discuss the implications of these observations on the nature of the MIT in ${\mathrm{VO}}_2$.

Figure 1

Sequence of LEEM images of ${\mathrm{VO}}_2(110)$ recorded at 10 eV. The symbols correspond to the simultaneous acquisition of a PES spectrum [see Fig. 3]. LEED patterns at both end points are also shown; the (0,0) spot is circled, and the arrow indicates the extra reflections in the low-symmetry monoclinic phase.

Figure 2

Correspondence between LEEM and XPEEM images, recorded at $56°\mathrm{C}$. (a) LEEM image at 10 eV, and (b) XPEEM image of the spatially resolved work function at a photon energy of 50 eV. In (c), the XPEEM intensity in (b) is mapped as false color onto the surface of the LEEM intensity in (a). (d) Autocorrelation of the LEEM and XPEEM images perpendicular to the stripe direction, shown alongside their cross-correlation. ${\lambda }_1$ indicates the location of the first peak in the autocorrelation of the LEEM intensity, and the temperature dependence of its magnitude is shown in the inset.

Figure 3

(a) PES measurements of ${\mathrm{VO}}_2(110)$ across the MIT recorded on the same part of the sample as the LEEM images of Fig. 1. The upper inset illustrates the evolution of the leading edge of the spectra with temperature. (b) The fraction of the metallic phase extracted from LEEM and PES (the lines are guides for the eye). The inset shows the regions of interest used for the separate analysis of O $2p$ and V $3d$ states. (c) Histogram of the LEEM intensity at $59°\mathrm{C}$. (d) Magnified view of the V $3d$ states in the insulating and metallic phases. (e) Results of fitting the insulating and metallic end points to the $58°\mathrm{C}$ PES data in the V $3d$ region (89% metallic fraction). The spectrum corresponding to the LEEM results (50% metallic fraction) is also shown for comparison.