Resolving the ${\mathrm{VO}}_2$ controversy: Mott mechanism dominates the insulator-to-metal transition

We consider a minimal model to investigate the metal-insulator transition in ${\mathrm{VO}}_2$. We adopt a Hubbard model with two orbitals per unit cell, which captures the competition between Mott and singlet-dimer localization. We solve the model within dynamical mean-field theory, characterizing in detail the metal-insulator transition and finding new features in the electronic states. We compare our results with available experimental data, obtaining good agreement in the relevant model parameter range. Crucially, we can account for puzzling optical conductivity data obtained within the hysteresis region, which we associate with a metallic state characterized by a split heavy quasiparticle band. Our results show that the thermal-driven insulator-to-metal transition in ${\mathrm{VO}}_2$ is compatible with a Mott electronic mechanism, providing fresh insight to a long-standing “chicken-and-egg” debate and calling for further research of “Mottronics” applications of this system.

Figure 1

Phase diagram showing the coexistence (grayed) of metal and insulator states (black lines from IPT and red from QMC), where the approximate position of the first-order lines is indicated. MI denotes Mott insulator and BI denotes bond insulator; the crossover regions have bad metal behavior (see the text and Ref. [40]). The top panels show the $t_{\perp }-U$ plane. The left one shows lower temperature $T=0.001$ (IPT) and 1/200 (CT-QMC), and the right one shows higher temperature $T=0.03$ (IPT) and 1/64 (CT-QMC). Lower panels show the $U-T$ plane. The left one is for fixed $t_{\perp }=0$ (i.e., the single-band Hubbard model), and right one is for $t_{\perp }=0.3$.

Figure 2

Top: The scattering rate $\text{Im}\left[{\mathrm{\Sigma }}_{11}(\omega =0)\right]$ for the metal (solid) at fixed $t_{\perp }=0.3$ values of $U$ from 0 to 3 in steps of 0.5 (upward). The experimentally relevant values $U=2.5$ and 3 are highlighted with thick lines. The inset shows the $U$ dependence at fixed $T=0.04$. Bottom: The effective intradimer hopping $t_{\perp }^{\text{eff}}=t_{\perp }+\mathrm{Re}\left[{\mathrm{\Sigma }}_{12}\right]\left(0\right)$ as a function of $T$ for the same parameters as the top panel. Metal states are shown by solid (blue) lines, and the insulator is shown by dashed (red) lines for $U=2.5$ and 3. The calculations are done with IPT.

Figure 3

Electronic dispersion for the metal (left top) and insulator (left bottom) in the coexistence region for parameter values $t_{\perp }=0.3, U=2.5$, and $T=0.01$. Right panels show the respective $\mathrm{DOS}\left(\omega \right)$. The calculations are done with IPT (cf. the Supplemental Material [43]).

Figure 4

The optical conductivity $\sigma \left(\omega \right)$ of the metal and the insulator within the coexistence region for parameters $t_{\perp }=0.3, U=2.5$, and $T=0.01$. The calculations are done with IPT. Inset: the experimental optical conductivity adapted from Ref. [12].