Investigation of superconducting electrons in twisted graphene

Superconductors, substances capable of conducting electricity without resistance, have intrigued scientists for over a century. Initially identified in 1911 by Dutch physicist Heike Kamerlingh Onnes, who noted the phenomenon in solid mercury cooled to approximately minus 450 F (merely a few degrees above absolute zero) using liquid helium, superconductors have been pursued to transform lossless energy transmission, levitating trains, and even quantum computers.

Currently, employing specially designed microwave technology, a collective of investigators from Harvard, Massachusetts Institute of Technology, and Raytheon-BBN Technologies has unveiled unusual superconducting traits in twisted layers of graphene, which consists of a single atomic layer of carbon. Their study was published in Nature.

Graphene was identified in 2004 by Andre Geim and Konstantin Novoselov, earning them the Nobel Prize in physics a few years later. In 2018, an MIT team directed by Professor Pablo Jarillo-Herrero, a co-author of the recent paper, discovered superconductivity in a stack of twisted bilayer graphene.

“This groundbreaking research demonstrated that a slight twist between two graphene layers can result in radically different characteristics than a single layer alone, and since that time, researchers have also found that introducing additional layers of graphene with a minor twist can induce comparable superconducting properties,” explained Zeyu Hao, a doctoral student at the Kenneth C. Griffin Graduate School of Arts and Sciences who conducts research in the Kim Group lab at Harvard and serves as one of the paper’s co-leading authors. The group’s most notable observation is that the superconducting attributes of electrons in twisted layers of graphene vary from traditional superconductors such as aluminum. This distinction “necessitates thorough examinations of how these electrons synchronize — this ‘quantum dance’ — at extremely low temperatures,” Hao remarked.

Diving into the reasons electrons pair up instead of repelling each other, a natural reaction due to their negative charge, is essential for revealing how superconductivity forms. “Once electrons aggregate strongly enough, they coalesce into a superfluid that flows without energy loss,” stated Abhishek Banerjee, co-lead author of the study and a postdoctoral researcher in the Kim group. “In twisted graphene, electrons decelerate, and their interactions somehow combine with quantum mechanics in an unprecedented manner to forge a ‘glue’ force that binds them into pairs. We are still striving to fully comprehend how this pairing operates in this new array of superconductors, which is the reason we are designing new methods to investigate it.”

One method under exploration involves measuring the resonant vibrations of the superconducting electrons — a “superfluid” composed of paired electrons — by exposing them to microwaves, akin to “listening to the frequency” of the superfluid, stated Mary Kreidel, co-lead author of the paper who collaborated with Mallinckrodt Professor of Applied Physics and Physics Robert Westervelt at Harvard and Kin Chung Fong at Raytheon BBN Technology.

“It resembles playing a glass harp,” noted Hao. “Instead of blowing across containers filled with varying water levels to generate different notes, we utilize a microwave circuit as the ‘container,’ while the ‘water’ symbolizes the superfluid of paired electrons. When the volume of the superfluid fluctuates, the resonant frequency adjusts correspondingly. Essentially, we’ve created our glass containers using this microwave resonant circuit, with the water representing the electrons that pair up to form a superfluid, enabling them to flow without energy loss.”

“As the weight and volume of the superfluid — fundamentally the density of paired electrons — varies, so too does the musical tone,” added Kreidel.

Through these frequency variations, the team gathered unexpected insights regarding how these electrons might be pairing. “We discovered that the adhesive force among electrons can be substantial in certain directions and completely absent in others,” stated doctoral student Patrick Ledwith, who conducts research at Harvard with George Vasmer Leverett Professor of Physics Ashvin Vishwanath. This directional nature parallels observations seen in high-temperature superconductors made from oxide materials — a persistent enigma for scientists, even after four decades of inquiry. “Perhaps our revelations regarding twisted graphene can illuminate how electrons engage in this quantum dance across other two-dimensional superconducting materials,” suggested Professor of Physics and Applied Physics Philip Kim, the principal investigator of this study.

While graphene technologies have not yet achieved mass production, the researchers envision significant potential applications. Kreidel, now a postdoctoral researcher at NASA’s Jet Propulsion Laboratory, highlights that such materials could aid in the development of ultra-sensitive, energy-efficient detectors for space exploration. “In the near vacuum of space, there’s minimal light,” she noted. “We need compact, lightweight detectors that consume little power while providing extremely high resolution. Twisted graphene may represent a promising option.”

This initiative was partially funded by the U.S. Department of Energy and the National Science Foundation.