New research solves the mystery of the transition of an insulator to metal. The study explored insulator-to-metal transitions, revealing inconsistencies in the traditional Landau-Zener formula and presenting a new understanding of resistive switching. Using computer simulations, the work illuminates the quantum mechanics involved and suggests that electronic and thermal switching could occur simultaneously with potential applications in microelectronics and neuromorphic computing.

When looking at only subatomic particles, most materials fall into one of two categories. Metals such as copper and iron have free-flowing electrons that allow them to conduct electricity, while insulators such as glass and rubber keep their electrons tightly bound and therefore do not conduct electricity.

Insulators can transform into metals when hit by an intense electric field, creating exciting possibilities for microelectronics and supercomputers, but the physics of this phenomenon, called resistive switching, is poorly understood.

The secret of the transition of an insulator to metal

Questions such as how large an electric field is needed are hotly debated by scientists such as University at Buffalo condensed matter theorist John Hahn.

“I was obsessed with it,” he says.

Professor of physics in the Faculty of Arts and Sciences, Ph.D. Hahn is lead author of a study that uses a new approach to answer a long-standing mystery about insulator-to-metal transitions. The study titled “Corulated Insulator Collapse by Quantum Avalanche via Ladder States in the Gap” was published in May. Nature Communication.

Electrons move in quantum paths

Hahn says the difference between metals and insulators lies in the principles of quantum mechanics, which states that electrons are quantum particles and their energy levels are located in bandgap regions.

Since the 1930s, the Landau-Zener formula has served as a model for determining the magnitude of the electric field required to push the electrons of an insulator from its lower regions to its upper regions. But experiments over the last few decades have shown that materials require a much smaller (about 1000 times less) electric field than predicted by the Landau-Zener formula.

“So there’s a huge inconsistency and we need a better theory,” says Hahn.

Resolution of disputes

To solve this problem, Khan decided to consider another question: What happens when electrons in the upper band of the insulator repel each other?

Hahn performed a computer simulation of resistive switching that takes into account the presence of electrons in the upper band. This showed that a relatively small electric field can cause the gap between the lower and upper regions to collapse, creating a quantum path for electrons to move up and down between regions.

To make an analogy, Hahn says, “Imagine a few electrons moving on the second floor. When the ground is tilted by the electric field, not only do the electrons start to move, but the previously forbidden quantum transitions are opened and the stability of the ground is significantly deteriorated, causing electrons in different grounds to flow up and down.

“Then the question is no longer how the electrons in the lower layer bounce up, but the stability of the upper layers under the electric field.”

Hahn says this idea helps resolve some inconsistencies in the Landau-Zener formula. It also clarifies some of the debate over whether the insulator-to-metal transitions are caused by the electrons themselves or by extreme temperatures. Hahn’s simulations show that the quantum avalanche is not caused by heat. However, the complete transition of the insulator to a metal will not occur until the individual temperatures of the electrons and phonons – the quantum vibrations of the crystal atoms – are balanced. This shows that the electronic and thermal switching mechanisms are not mutually exclusive, but can occur simultaneously, Khan said.

“So we found a way to understand some of this resistive switching phenomenon,” says Hahn. “But I think that’s a good starting point.”

Research could improve microelectronics

D., a professor and head of electrical engineering in the University’s School of Engineering and Applied Sciences, who provided the experimental context. Co-written by Jonathan Bird. His team is studying the electrical properties of new nanomaterials that exhibit new states at low temperatures, which could teach researchers a lot about the complex physics that govern electrical behavior.

“While our research is focused on solving fundamental questions in the physics of new materials, the electrical phenomena we detect in these materials may eventually form the basis for new microelectronic technologies such as compact memory for use in data-intensive applications such as artificial intelligence,” says Byrd.

Potential applications

The research could also be crucial for areas such as neuromorphic computing, which seek to mimic electrical stimulation of the human nervous system. “However, our focus is primarily on understanding basic phenomenology,” Byrd says.

After the paper was published, Khan developed an analytical theory that fits very well with computer computations. With all that, he still has something to investigate, like the exact conditions for a quantum avalanche to occur.

“Someone who is an experimenter asked me, ‘Why haven’t I seen this before?’ he will ask. “Some could see, some couldn’t. We have a lot of work ahead of us to figure out.” Source