Graphene rotated to a precise degree could transform electricity and solve a mystery that has puzzled scientists for decades.

Part of the beauty of science is serendipitous discoveries. Many accidental findings, like nuclear fission, X-rays, penicillin and plastic, have led to innovations that have changed our world radically.

In the tradition of scientific tinkering, a team of MIT and Harvard scientists recently discovered that graphene – isolated from pencil graphite into a single layer of carbon and arranged in a honeycomb – can behave both as a superconductor and as an insulator. The ability to tune graphene to behave as one or the other is in stark contrast to other materials and methods used today to achieve the same or similar effect.

This fortuitous discovery could transform how electricity is transmitted and help solve a mystery that has puzzled scientists for decades: the mechanism of superconductivity, a phenomena in which there is zero electrical resistance when certain materials are cooled to very low temperatures.

The scientific findings, published as two papers in Nature, could provide answers to a decades-long pursuit for room-temperature superconductors. The papers are titled Unconventional superconductivity in magic-angle graphene superlattices and Correlated insulator behaviour at half-filling in magic-angle graphene superlattices.

An accidental discovery

Theorists have predicted that counteracting atoms between layers of two-dimensional materials at a specific angle might encourage the electrons to interact in unique ways, but they didn’t know how.

When Pablo Jarillo-Herrero, an investigator in the Moore Foundation’s Emergent Phenomena in Quantum Systems Initiative, and his colleagues set up their experiment, they were curious to find out. What resulted was a surprise discovery.

By taking two sheets of graphene, layering them on top of each other and then twisting the material at a precise angle of 1.1. degrees, they discovered a model system that behaves similarly to a special class of insulating, or electrically non-conducting, materials known as Mott insulators. This is important because Mott insulators are known to exhibit unconventional superconductivity when provided with a few extra charge carriers (electrons) and when cooled sufficiently.

Also important is the material itself. Graphene is considered a simple material because it is made up only of carbon atoms in a single layer. In scientific experiments, having a simple material is more favorable as there is less to contend with when trying to understand how something works.

With this discovery, scientists now have a simpler system to better understand the underlying mechanisms of unconventional superconductivity.

“We can now use graphene as a new platform for investigating unconventional superconductivity”

-Pablo Jarillo-Herrero, Moore Foundation EPiQS Investigator

“One can also imagine making a superconducting transistor out of graphene, which you can switch on and off, from superconducting to insulating. That opens many possibilities for quantum devices," Jarillo-Herrero says.

In general, superconductors are classified into two types: conventional or unconventional. A conventional superconductor is exhibited by many metals, such as copper or silver, when cooled to a certain temperature, and is well described by theory. Conversely, unconventional superconductors display superconductivity, but do not conform to conventional theory. An example of this is the 1986 discovery of cuprates, a closely related group of materials that all contain copper and oxygen atoms. The materials are distinguished by having different dopants (impurity atoms) and by having different atomic arrangements.

High-temperature and room-temperature superconductors

Most superconductors work only at temperatures close to absolute zero – the lowest temperature that is theoretically possible. Zero kelvin (−273.15 °C) is defined as absolute zero. Jarillo-Herrero and team saw superconductivity when graphene was cooled to 1.7°K degrees above absolute zero, suggesting it may conduct electricity much closer to room temperature.

Superconductors that can work at room temperature eliminate the need for expensive cooling and could transform how electricity is transmitted, with the potential for greater efficiency in power stations to medical scanners. In a Nature podcast, Jarillo-Herrero talks about what this discovery could mean, providing an example of efficiencies to be gained in our energy grid. He says, “About 20 to 30 percent of the energy carried by the electrical grid is dissipated just in transportation – from where it is originated to its ultimate destination. If we could have superconducting transmission lines carrying that electricity, we could gain back some of the lost electricity.”

Cause for celebration

In the past, researchers, including Jarillo-Herrero, have put graphene in contact with other superconducting metals, such as copper, allowing it to inherit superconducting behaviors. However, this time around, he and his colleagues found a way to make graphene conduct on its own, demonstrating that superconductivity can be an intrinsic quality of the carbon-based material. The behavior observed by graphene when rotated to this precise angle is what has scientists both excited and curious. It could help explain the superconducting mechanism of the elusive cuprates.

While physicists cannot yet say that the superconducting mechanism is the same in graphene and cuprates, Robert Laughlin, a physicist and Nobel laureate at Stanford University said, “but enough of the behaviors are present in these new experiments to give cause for cautious celebration.” Physicists, he adds, have been “stumbling around in the dark for 30 years” trying to understand cuprates and “many of us think that a light just switched on.”

Providing scientists with the means and freedom to explore stems from our founders, Gordon and Betty Moore. In the Statement of Founders’ Intent, they write that “expanding knowledge is both intellectually satisfying and often of practical value,” and that the “rate of expansion of knowledge can be increased by funding potentially high-impact areas that do not fit conventional funding sources.”

Through the foundation’s Emergent Phenomena in Quantum Systems Initiative, we aim to stimulate breakthroughs that fundamentally change our understanding of the organizing principles of complex matter. Other scientists supported through our initiative have used graphene to develop new techniques that could be used to build quantum computers and discovered a new type of collective electronic behavior when pairing the material with another.

“One of our approaches is to support experimental researchers and to help them maximize their creativity,” said Ernie Glover, Ph.D., program officer. “Supporting scientists, like Pablo Jarillo-Herrero and our other EPiQS Investigators, to experiment freely is transforming our understanding of how complex quantum materials work, further accelerating scientific discoveries and expanding knowledge.”

Courtesy of the researchers
Image: Physicists at MIT and Harvard University have found that graphene, a lacy, honeycomb-like sheet of carbon atoms, can behave at two electrical extremes: as an insulator, in which electrons are completely blocked from flowing; and as a superconductor, in which electrical current can stream through without resistance. Credit: Courtesy of the researchers. 
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