Unraveling the Mystery of Insulator-to-Metal Transitions: A Quantum Avalanche Study

Most materials can be classified into two categories based on their subatomic particles: metals and insulators. Like copper and iron, metals possess free-flowing electrons that enable them to conduct electricity, while insulators, such as glass and rubber, tightly bind their electrons, making them non-conductive.

The intriguing aspect of insulators is that they can be transformed into metals when exposed to a powerful electric field, presenting exciting possibilities for microelectronics and supercomputing. However, the physics underlying this phenomenon, resistive switching, remains poorly understood and is the subject of vigorous debate among scientists like Jong Han, a condensed matter theorist at the University at Buffalo.

Dr. Han, a Ph.D. professor of physics in the College of Arts and Sciences, spearheaded a study titled “Correlated insulator collapse due to quantum avalanche via in-gap ladder states,” published in Nature Communications in May.

The Difference Lies in Quantum Mechanical Principles

The fundamental difference between metals and insulators lies in quantum mechanical principles. Electrons, being quantum particles, exhibit energy levels organized into bands that contain forbidden gaps, as explained by Han.

Historically, the Landau-Zener formula, formulated in the 1930s, served as a blueprint for determining the electric field strength needed to push an insulator’s electrons from lower bands to upper bands. However, experiments over the decades have revealed a significant discrepancy—the actual electric field required is approximately 1,000 times smaller than predicted by the formula.

A New Approach: Analyzing the Upper Band Electrons

To address this enigma, Dr. Han explored a different question: What happens when electrons already exist in the upper band of an insulator and are subjected to an electric field?

Running computer simulations of resistive switching that accounted for the presence of electrons in the upper band, Han made a remarkable discovery. A relatively modest electric field could trigger a collapse of the gap between the lower and upper bands, establishing a quantum path for electrons to move up and down between the bars.

An Analogy to Grasp the Concept

Dr. Han offered an analogy to illustrate this phenomenon: Imagine electrons moving on a second floor. When an electric field tilts the floor, previously forbidden quantum transitions occur, causing the floor’s stability to break down. As a result, electrons can now freely flow between different floors.

This new insight helps reconcile the discrepancies in the Landau-Zener formula and clarifies the debate over insulator-to-metal transitions—whether caused by electrons or extreme heat. Dr. Han’s simulations suggest that heat does not trigger the quantum avalanche. 

However, the complete insulator-to-metal transition only occurs when the separate temperatures of the electrons and phonons equilibrate, indicating that electronic and thermal switching mechanisms can coexist.

Potential for Advancements in Microelectronics

The study, co-authored by Dr. Jonathan Bird, a professor, and chair of electrical engineering in UB’s School of Engineering and Applied Sciences, holds promise for advancing microelectronics. 

Bird’s team has been studying emergent nanomaterials with novel electrical states at low temperatures, providing insights into complex physics governing electrical behavior. 

These findings could lay the groundwork for new microelectronic technologies, such as compact memories for data-intensive applications like artificial intelligence.

Toward the Future: Investigating the Quantum Avalanche

Since publishing the paper, Dr. Han has developed an analytic theory that aligns well with computer simulations. However, there is more to explore, such as determining the conditions necessary for a quantum avalanche. 

Dr. Han eagerly looks forward to further investigation, anticipating that experimentalists will contribute to sorting out the intricacies of this intriguing phenomenon.

Read the original article on Phys.

Read more: Using Gravitational Waves to Hunt for Dark Matter.