Anomalous spectral-weight transfers unraveling oxygen screening and electronic correlations in the insulator-metal transition of ${\mathrm{VO}}_2$

Vanadium dioxide $\left({\mathrm{VO}}_2\right)$ undergoes an unusual insulator-metal transition (IMT), and after decades of study, the origin of the IMT remains hotly debated. Here, by analyzing spectral-weight transfers (SWTs) of x-ray absorption spectroscopy at the $\mathrm{V}{L}_{3,2}$ and O K edges on specially designed ${\mathrm{VO}}_2$ films, we observe $d_{\left|\right|}\left(d_{x^2-y^2}\right)$ band splitting at the $\mathrm{V}{L}_{3,2}$ edges across the IMT, accompanied by anomalous SWTs as high as $\sim 12\mathrm{eV}$ at the $\mathrm{O}K$ edge, indicating strong electronic correlations. Surprisingly, a few oxygen vacancies induce dramatic SWTs at the $\mathrm{O}K$ edge, but the sample remains conducting. Supported by theoretical calculations, we find that in the metallic state, direct $\mathrm{V}\left(3d_{\parallel }\right)\text{-V}\left(3d_{\parallel }\right)$ and $\text{O}\left(2p\right)\text{-V}\left(3d_{\parallel }\right)$ hybridized orbital correlations are screened by $\text{O}\left(2p\right)\text{-V}\left(3d_{\pi }\right)$ hybridized orbitals, while in the insulating state they are strongly correlated due to changes in the oxygen orbital occupancy. Our result shows the importance of screenings and electronic correlations for IMTs in ${\mathrm{VO}}_2$.

Figure 1

(a) Temperature-dependent resistivity of ${\mathrm{VO}}_{2-\epsilon }$ (B) and ${\mathrm{VO}}_{2+\delta }$ (M) films. The transport of the ${\mathrm{VO}}_{2+\delta }$ film shows temperature-dependent hysteresis and an insulator-metal transition (IMT) of three orders of magnitude. In contrast, ${\mathrm{VO}}_{2-\epsilon }$ shows only semimetallic behavior without IMT. Inset: ${\mathrm{VO}}_{2-\epsilon }$ resistivity over a wider temperature range. (b) Crystal structure diagram of ${\mathrm{VO}}_2$ in the (110) plane showing two $\text{V}\left(3d_{\parallel }\right)$ (also known as $d_{x^2-y^2}$) orbitals and nearby O atoms. O($2p$) orbitals are directed along the $x$, $y$, and $z$ axes. An example of a possible oxygen vacancy is illustrated.

Figure 2

X-ray absorption spectroscopy intensities of ${\mathrm{VO}}_{2+\delta }$ and ${\mathrm{VO}}_{2-\epsilon }$ taken using the fluorescent yield mode at the (a) V $L_{2,3}$ and (b) O K edges. Intensities of XAS are plotted as functions of photon energy for both samples at temperatures above and below $T_c$.

Figure 3

Temperature-dependent XAS intensities. Integrated spectral weight of selected regions in Fig. 2 as a function of temperature. (a) 512.6–513.9 eV; (b) 527.3–529 eV; (c) 529.1–530.4 eV; (d) 531.2–533.4 eV. For comparison, the transport data of ${\mathrm{VO}}_{2+\delta }$ are replotted along in (c). Insets show a wider temperature range.