Current-Driven Insulator-To-Metal Transition in Strongly Correlated ${\mathrm{VO}}_2$

Despite extensive studies on the insulator-to-metal transition (IMT) in strongly correlated ${\mathrm{VO}}_2$, the fundamental mechanism underlying the current-driven IMT in ${\mathrm{VO}}_2$ is still not well understood. Although it is generally believed that the mechanism is Joule heating leading to a rise in temperature to above the normal transition temperature, there is ample experimental evidence demonstrating that the transition could be driven by nonthermal electronic processes. Here we formulate a phase-field model to demonstrate that the electric current may drive the IMT isothermally via the current-induced electron-correlation weakening. We discover that a current with a large density (on the order of $10{\mathrm{nA}/\mathrm{nm}}^2$) induces ultrafast resistive switching on the order of a few nanoseconds, consistent with experimental measurements. We also construct the temperature-current phase diagram and investigate the influence of the current on domain walls. This work is expected to provide guidance for understanding the current-driven IMT in ${\mathrm{VO}}_2$ and for designing ${\mathrm{VO}}_2$-based electric switching devices.

Figure 1

The geometry used in the simulations. The blue and the gold parts are the substrate and electrodes, respectively. The length of the ${\mathrm{VO}}_2$ sample is $L$ and is set to $100$ nm in the simulations.

Figure 2

Simulated temporal evolution of various variables during the current-driven ultrafast switching in ${\mathrm{VO}}_2$ at $T=320$ K, $J_{\mathrm{b2}}=57.8{\mathrm{nA}/\mathrm{nm}}^2$, and $n_{\mathrm{b1}}\approx 0.6$ per unit cell. During the process, ${\eta }_3$ (${\mu }_3$) is the same as ${\eta }_1$ (${\mu }_1$), and ${\eta }_2={\eta }_4=0$ and ${\mu }_2={\mu }_4=0$. The dashed lines indicate the positions of the insulator-metal interface at different times.

Figure 3

Switching time as a function of the current density in ${\mathrm{VO}}_2$ at $T=320$ K. The line is the hyperbola fit to the simulation data, $\mathrm{\Delta }t=c/J_{\mathrm{b2}}$, with the fitted constant $c=412\mathrm{ns}\mathrm{nA}/\mathrm{n}{\mathrm{m}}^2$.

Figure 4

Calculated temperature-versus-current-density phase diagram of ${\mathrm{VO}}_2$. The dots with error bars are the calculated points on the phase boundaries (the presence of the error bars results from the discretely sampled calculation points on the phase diagram), and the lines are a guide for the eye. The dashed line represents the discontinuous point. The dotted red line is the boundary line for the Joule-heating-induced IMT: for $J_{\mathrm{b2}}$ exceeding this line, the R phase will eventually be induced by the Joule-heating effect (see the text).

Figure 5

Simulated temporal evolution of various variables during the current-driven domain-wall motion in ${\mathrm{VO}}_2$ at $T=320$ K, $J_{\mathrm{b2}}=0.0811{\mathrm{nA}/\mathrm{nm}}^2$, and $n_{\mathrm{b1}}\approx 0.6$ per unit cell. During the process, ${\eta }_1$ (${\mu }_1$) has a nearly uniform value of $0.76$ ($-0.84$) along the sample except at the boundaries, and ${\eta }_2={\eta }_4=0$ and ${\mu }_2={\mu }_4=0$. The dashed lines indicate the positions of the twin wall within the M1 phase at different times. The range of the finite net-charge region at the $x=L$ boundary is within $\lambda \triangleq 5$  nm, which justifies this setting of $\lambda$.

Figure 6

Average speed of the twin-wall motion from $x=50$ nm to $x=0$ nm as a function of the current density in ${\mathrm{VO}}_2$ at $T=320$ K. The line is a guide for the eye.