Defects in the structure of insulating materials can trigger the transformation into a conductor

Researchers at Università Cattolica, Brescia campus, have discovered that the transition from insulating to conductive behavior in certain materials is driven by topological defects in the structure. Specifically, a study published in Nature Communications  (https://doi.org/10.1038/s41467-024-53726-z) and coordinated by Professor Claudio Giannetti, director of the Interdisciplinary Laboratories of Advanced Material Physics in the Department of Mathematics and Physics at the university, shows that so-called "Mott materials"—a type of insulator fundamentally different from conventional insulators, capable of switching from an insulating to a conductive state (a process known as "resistive switching")—can change state precisely due to topological defects in their crystal structure. This study, conducted specifically on a particular vanadium oxide (V₂O₃), reveals that these defects trigger this transition.

The study was carried out in collaboration with IMDEA Nanociencia, KU Leuven, SISSA, and Diamond Light Source.

BACKGROUND

Vanadium oxide V₂O₃ is a material that is studied for its ability to rapidly change its electrical and optical properties under the influence of temperature or an electric field, a phenomenon known as the “Mott phase transition.” It is mainly used in smart windows to control transparency and regulate sunlight; advanced electronics and resistive memories for high-speed memory devices and switches; optical devices such as lenses and filters that respond to light and heat. Vanadium oxides show promise for applications in power electronics, energy-saving technology, and neuromorphic computing devices.

THE STUDY

Using advanced X-ray microscopy techniques, the researchers observed the formation of metallic channels at the nanoscale under electrical tension.

“Resistive switching is the fundamental process behind the sudden change in electrical properties of solid-state devices under the action of intense electric fields,” first author of the work Alessandra Milloch, Università Cattolica in Brescia explains. “Despite its technological relevance, this process was previously thought to be stochastic in nature, driven by local and uncontrollable fluctuations. We decided to delve deeper and investigate the true nature of this phenomenon in planar devices consisting of two metallic contacts deposited on a thin V₂O₃ film, which is one of the most celebrated examples of a Mott insulator.”

“Mott insulators are fundamentally different from conventional insulators, since their poor conductivity is determined by the strong interactions among the electrons moving throughout the lattice.”, adds Ignacio Figueruelo, IMDEA Nanociencia, Madrid. “In this class of materials, comprising many transition metal oxides, a small change of the external parameters, such as the applied voltage, can induce a dramatic change of the electric properties and the release of a number of conducting electrons of about 1 per unit cell, which is many orders of magnitude larger than what can be achieved in conventional semiconductors by means of doping. This intrinsic non-linearity triggered a huge interest to address the possible use of these materials as building blocks for a novel family of devices, thus leading to the development of a new field dubbed Mottronics.”

Experts found that these transitions are not random but depend on defects in the structure’s topology.

Mariela Menghini, one of the coordinators from IMDEA Nanociencia, explains: “in previous experiments we discovered that the insulating ground state of this material is inherently inhomogeneous and forms a spatial nanotexture of stripy nanometric domains, whose boundaries follow well defined mathematical rules. When these constraints cannot be respected at some peculiar intersection points, nanometric topological defects are formed. Indeed, it is at these specific points that the insulator-to-metal switching takes place when a well defined threshold voltage is overcome. The observation of this peculiar nanotextured Mott insulating state was possible thanks to the high-quality V₂O₃ samples grown by the group of Prof. Jean-Pierre Locquet at KU Leuven.”

“This discovery required a state-of-the-art experiment combining nanometric microscopy, sensitive to the nanotexture of the material, with the ability to apply an in-situ voltage during measurements,” Professor Giannetti, full professor of Physics at Università Cattolica explains. “We conducted this experiment at the Diamond Light Source laboratories in the United Kingdom, where it was possible to capture images of the formation of a metallic channel while applying an above-threshold voltage across the device. This discovery is important because it sheds light on the mechanism behind the transformation from insulator to metal in these materials. Our experimental results are well-supported by a theoretical model developed by Professor Michele Fabrizio from SISSA. With the new knowledge that the transformation is triggered by topological defects, we can design new experiments to pin down these defects and control the resistive switching process, with the goal of achieving complete control over the process and engineering devices that operate at unprecedented speeds and with extremely low power dissipation.”

Thanks to this discovery, for example, it may be possible to develop different types of innovative devices, including resistive memories (ReRAM)—devices that store data by changing their resistance, using topological defects to switch between insulating and conductive states. These would be faster and more energy-efficient than traditional memories. Neuromorphic devices could also be developed, mimicking human synapses for use in artificial intelligence, as well as low-energy electrical switches to reduce energy consumption in various types of electronics.

These devices could improve the efficiency and performance of computing and memory systems, with applications ranging from computers to advanced artificial intelligence systems, the study concludes.