Looking only at their subatomic particles, most materials can be placed 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 rubbe r—keep their electrons tightly bound and therefore cannot conduct electricity.
Insulators can become metals when hit by an intense electric field, offering surprising possibilities for microelectronics and supercomputing, but the physics behind this phenomenon called resistive switching is poorly understood.
Questions, such as how large an electric field is needed, are hotly debated by scientists, such as University at Buffalo condensed matter theorist Jong Han.
“I was obsessed with that,” he said.
Han, Ph.D., professor of physics in the College of Arts and Sciences, is the lead author of a study that calls for a new approach to solving a long-standing mystery about insulator-to-metal transitions. The study, “Correlated insulator collapse due to quantum avalanche through in-gap ladder states,” was published in May in Communication in Nature.
A quantum path allows electrons to move between bands
The difference between metals and insulators lies in quantum mechanical principles, which dictate that electrons are quantum particles and their energy levels fall into band gaps, Han said.
Since the 1930s, the Landau-Zener formula has served as a blueprint for determining the magnitude of the electric field required to push electrons in an insulator from its lower bands to its upper bands. But experiments in the decades since have shown that materials require a much smaller electric field—approximately 1,000 times smaller—than the Landau-Zener formula approximates.
“So, there’s a lot of difference, and we need to come up with a better theory,” Han said.
To solve this, Han decided to consider a different question: What happens when the electrons in the upper band of an insulator are pushed?
Han ran a computer simulation of resistive switching that accounted for the presence of electrons in the upper band. It shows that a relatively small electric field can cause the gap between the lower and upper bands to collapse, creating a quantum path for electrons to move up and down between the bands.
To make an analogy, Han said, “Imagine some electrons moving on a second floor. When the floor is tilted in an electric field, the electrons not only start to move but previously forbidden quantum transition opens and the very stability of the floor suddenly falls apart, making electrons on different floors flow up and down.
“Then, the question is no longer how the electrons in the lower floor jump, but the stability of the higher floors under an electric field.”
This idea helps solve some of the differences in the Landau-Zener formula, Han said. It also sheds some light on the debate over insulator-to-metal transitions being caused by the electrons themselves or those caused by extreme heat. Han’s simulation suggests that quantum avalanches cannot be triggered by heat. However, the full insulator-to-metal transition does not occur until the separate temperatures of the electrons and phonons—quantum vibrations of the crystal’s atoms—equilibrate. This shows that the mechanisms for electronic and thermal switching are not mutually exclusive, Han said, but can arise simultaneously.
“So, we found a way to understand some corner of this whole resistive switching phenomenon,” Han said. “But I think it’s a good starting point.”
Research advances microelectronics
The study was co-authored by Jonathan Bird, Ph.D., professor and chair of electrical engineering in UB’s School of Engineering and Applied Sciences, who provided experimental context. His team studied the electrical properties of emerging nanomaterials that exhibit new states at low temperatures, which can teach researchers many aspects of the complex physics that govern electrical behavior.
“While our studies are focused on solving fundamental questions about the physics of new materials, the electrical phenomena we reveal in these materials may provide the basis for new microelectronic technologies, such as compact memories for use in data-intensive applications such as artificial intelligence,” said Bird.
The research could also be important for areas such as neuromorphic computing, which tries to mimic the electrical stimulation of the human nervous system. “Our focus, however, is primarily on understanding the basic phenomenology,” Bird said.
Other authors include UB physics Ph.D. student Xi Chen; Ishiaka Mansaray, who received a Ph.D. in physics and is currently a postdoc at the National Institute of Standards and Technology, and Michael Randle, who received a Ph.D. in electrical engineering and is currently a postdoc at the Riken research institute in Japan. Other authors include international researchers representing the Swiss Federal Institute of Technology in Lausanne, Pohang University of Science and Technology, and the Center for Theoretical Physics of Complex Systems, Institute for Basic Science.
Since publishing the paper, Han has developed an analytic theory that fits well with computer calculations. However, there is still more for him to investigate, such as the exact conditions required for a quantum avalanche to occur.
“Someone, an experimenter, will ask me, ‘Why didn’t I see this before?'” Han said. “Some may have seen it, some may not. We have a lot of work ahead of us to solve it.”
More information:
Jong E. Han et al, Correlated insulator collapse due to quantum avalanche through in-gap ladder states, Communication in Nature (2023). DOI: 10.1038/s41467-023-38557-8
Provided by the University at Buffalo
Citation: ‘Quantum avalanche’ explains how non-conductors become conductors (2023, July 24) retrieved 24 July 2023 from https://phys.org/news/2023-07-quantum-avalanche-nonconductors-conductors.html
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