Mott resistive switching initiated by topological defects

Abstract

Avalanche resistive switching is the fundamental process that triggers the sudden change of the electrical properties in solid-state devices under the action of intense electric fields. Despite its relevance for information processing, ultrafast electronics, neuromorphic devices, resistive memories and brain-inspired computation, the nature of the local stochastic fluctuations that drive the formation of metallic regions within the insulating state has remained hidden. Here, using operando X-ray nano-imaging, we have captured the origin of resistive switching in a V₂O₃-based device under working conditions. V₂O₃ is a paradigmatic Mott material, which undergoes a first-order metal-to-insulator phase transition together with a lattice transformation that breaks the threefold rotational symmetry of the rhombohedral metallic phase. We reveal a new class of volatile electronic switching triggered by nanoscale topological defects appearing in the shear-strain based order parameter that describes the insulating phase. Our results pave the way to the use of strain engineering approaches to manipulate such topological defects and achieve the full dynamical control of the electronic Mott switching. Topology-driven, reversible electronic transitions are relevant across a broad range of quantum materials, comprising transition metal oxides, chalcogenides and kagome metals.

Fig. 1. Nanotexture of monoclinic V₂O₃.

a Non-primitive hexagonal unit cell of the V₂O₃ high-temperature rhombohedral metallic phase. b Schematic of the rhombohedral-to-monoclinic distortion along each of the three equivalent hexagonal axes. c PEEM experimental setup. X-ray radiation, with tunable energy resonant with the vanadium L_2,3 edge, impinges on the sample surface and the emitted electrons are collected and imaged through electrostatic and magnetic lenses. The V₂O₃ film is coated with gold metal electrodes, allowing to drive a current through the device (see sketch of a typical resistive switching current-voltage curve in the bottom panel) while simultaneously acquiring XLD-PEEM images.

Fig. 2. PEEM imaging of resistive switching.

XLD-PEEM images before (a) and during (b–j) the application of an electric current at T = 120 K. The homogeneous regions at the top and bottom of each image are the gold electrodes. The area in between is the exposed V₂O₃ antiferromagnetic monoclinic phase, exhibiting a striped domain nanotexture. For currents larger than 1.5 mA, the striped domains disappear in the region delimited by the white dashed lines, demonstrating the appearance of a rhombohedral metallic filament, which widens as the current is increased.

Fig. 3. Topological defect.

a Detail of the XLD-PEEM image shown in Fig. 2a in the region where the metallic filament is formed upon the application of a current above the threshold I_th. b Schematic of the monoclinic domains crossing at 60^∘ and forming a topological defect. Blue, red and yellow areas identify the three possible monoclinic domains corresponding to the three equivalent order parameter directions ${\widehat{\mathit{ϵ}}}_n$. The order parameter at the boundaries between different domains is oriented along ${\widehat{\mathit{ϵ}}}_1+{\widehat{\mathit{ϵ}}}_2$ (2π/3) for the red-blue interface and along ${\widehat{\mathit{ϵ}}}_2+{\widehat{\mathit{ϵ}}}_3$ (4π/3) for the blue-yellow interface. The mixed red-yellow triangular region indicates the local suppression of the strain at the topological defect. The energy functionals shown on the left and right, illustrate how a topological defect (green plot, solid line) decreases the insulator-metal energy difference, Δ.

Fig. 4. Metallic channel formation.

a I−V curve measured during the XLD-PEEM imaging. The sudden drop in the voltage measured at I_th=1.05 mA indicates the first resistive switch. b Line profiles of the XLD-PEEM images in Fig. 2. The grey shaded area indicates the progressive widening of the metallic rhombohedral filament. The direction of the line profiles is shown by the white dashed line in the XLD-PEEM image on top, where we report a detail of Fig. 2j. c Width, d, of the metallic filament as a function of current. The blue/red markers represent the values of d obtained from the XLD-PEEM images below/above I_th. The errorbars indicate the uncertainty in the filament width, estimated from the line profiles in panel b; when no filament is observed (first four data points, d = 0), the errorbar is fixed to the experimental spatial resolution. The green solid line shows an estimate of d, derived from a parallel resistors model predicting a sudden jump of d to 200 μm at I_th (see Supplementary Note 5).