Field-Driven Mott Gap Collapse and Resistive Switch in Correlated Insulators

Mott insulators are “unsuccessful metals” in which Coulomb repulsion prevents charge conduction despite a metal-like concentration of conduction electrons. The possibility to unlock the frozen carriers with an electric field offers tantalizing prospects of realizing new Mott-based microelectronic devices. Here we unveil how such unlocking happens in a simple model that shows the coexistence of a stable Mott insulator and a metastable metal. Considering a slab subject to a linear potential drop, we find, by means of the dynamical mean-field theory, that the electric breakdown of the Mott insulator occurs via a first-order insulator-to-metal transition characterized by an abrupt gap collapse in sharp contrast to the standard Zener breakdown. The switch on of conduction is due to the field-driven stabilization of the metastable metallic phase. Outside the region of insulator-metal coexistence, the electric breakdown occurs through a more conventional quantum tunneling across the Hubbard bands tilted by the field. Our findings rationalize recent experimental observations and may offer a guideline for future technological research.

Figure 1

(a) Zero-field phase diagram. Schematic representation of the equilibrium phase diagram. The enlargement at $\mathrm{\Delta }=0.4$ highlights the range of the interaction relevant for this study. $U_c$ marks the Mott transition critical value (red diamond). $U_s$ marks the spinodal point (green square). The arrows indicate the interaction strengths used in (c) and (d) (purple crosses). (b) Sample geometry. The sample is a layered slab subject to a linear voltage drop $\mathrm{\Delta }V$, corresponding to a uniform and static electric field along the slab direction. (c),(d) Electric-field-induced insulator-to-metal transition. Layer-resolved local spectral densities before (left) and after (right) the field-driven insulator-to-metal transition for two values of the interaction strength: $U=8.5$ (c) and $U=8.1$ (d), respectively, outside and inside the coexistence region. The intensities of the applied fields are $E=0.2$ (left) and $E=0.6$ (right) for (c) and $E=0.01875$ (left) and $E=0.025$ (right) for (d). (e) Electric-field versus interaction phase diagram. Threshold field $E_{\mathrm{th}}$ in units of the zero-field insulating gap ${\mathrm{\Delta }}_{\text{gap}}$ versus $U/U_c$. We also show ${\mathrm{\Delta }}_{\text{gap}}$ for each value of $U/U_c$ (upper $x$ axis).

Figure 2

Metal-insulator coexistence. (a) Internal energy $⟨H⟩$ of the metallic (blue circles) and the insulating (red diamonds) solutions, as a function of the electric field $E=\mathrm{\Delta }V/N$. The solid (open) symbols mark the stable (metastable) character of the solution for each value of $E$. The dashed blue line is a quadratic fit to the energy of the metallic solution (see the text). The red full line is a guide to the eye. (b) Hysteresis loop for the orbital polarization $m=n_1-n_2$ across the zero-bias Mott transition. The red and blue arrows define the directions of continuous evolution for the insulating and metallic phases, respectively. The big green arrow indicates the electric-field-induced switch between the stable insulator and the metastable metal. Lines are guides to the eye. Inset: Effective crystal field ${\mathrm{\Delta }}_{\mathrm{eff}}$ in units of the bandwidth $W$ as a function of the correlation strength $U$. The metallic state is destabilized for ${\mathrm{\Delta }}_{\mathrm{eff}}>W$. (c) Polarization profile for the metastable metal for increasing electric field strength (gray arrow). Red diamonds (blue circles) refer to a system with an insulating (metallic) ground state.