An international team of researchers from Spain, the U.S., China and Japan has found that superconductivity in bilayer graphene can be turned on or off with a small voltage change, increasing its usefulness for electronic devices. This follows previous findings regarding twisted bilayer graphene and its ability to exhibit alternating superconducting and insulating regions.
"It's kind of a holy grail of physics to create a material that has superconductivity at room temperature," University of Texas at Austin physicist Allan MacDonald said. "So that's part of the motivation of this work: to understand high-temperature superconductivity better."
The discovery is a significant advance in an emerging field called Twistronics, whose pioneers include MacDonald and engineer Emanuel Tutuc, also from The University of Texas at Austin. It took several years of hard work by researchers around the world to turn MacDonald's original insight into materials with these strange properties, but it was worth the wait.
In 2011, MacDonald, a theoretical physicist who uses quantum mathematics and computer modeling to study two-dimensional materials, made an unexpected discovery. Along with Rafi Bistritzer, a postdoctoral researcher, he was working on building simple but accurate models of how electrons behave in stacked 2D materials, when one layer is slightly twisted relative to the others. The seemingly un-computable problem, MacDonald believed, could be greatly simplified by focusing on one key parameter of the system.
The strategy MacDonald and Bistritzer employed proved successful. The surprise came later. When they applied their method to twisted bilayer graphene, they found that at a very specific angle of about 1.1 degrees - which they dubbed the "magic angle" - the electrons behaved in a strange and extraordinary way, suddenly moving more than 100 times more slowly.
In the short term, the finding was largely ignored or dismissed. The result seemed too unusual to believe. Moreover, it was not obvious that creating a physical example of such a system, with such a precise placement of the two-dimensional sheets, was physically achievable.
But not everyone was incredulous or intimidated by the results. A few experimentalists around the world took note of the prediction and chose to pursue the "magic angle." When in 2018, for the first time, physicists at the Massachusetts Institute of Technology created a system of layered graphene twisted by 1.1 degrees, they found, as MacDonald had predicted, that it exhibited remarkable properties - in particular, superconductivity at a surprisingly high temperature.
"There's no simple explanation for why electrons suddenly slow down," MacDonald said. "Thanks to recent work by theorists at Harvard, there's now a partial explanation related to models often studied in elementary particle physics. But there's now a whole world of related effects in different layered 2D materials. Twisted bilayer graphene is just a peek into one part of it."
Superconducting materials have no electrical resistance, allowing electrons to travel endlessly without dissipating energy. They are used in quantum computing and could be game changers for electrical transmission if they did not require expensive refrigeration.
The discovery of superconductivity in twisted bilayer graphene has since provided fuel for a flourishing subfield with a catchy name - Twistronics - and a rush to develop the technology further.
"My work is all about predicting unusual phenomena that haven't been seen before, or trying to understand phenomena that are not well understood," MacDonald said. "I'm drawn to theory that connects directly to things that actually happen, and I'm interested in the power of math and theory to describe the real world."
The strange properties of layered 2D materials seems to relate to interactions, which become much more crucial when electrons slow down, inducing strong correlations between individual electrons. Typically, electrons circle nearly separately around the nucleus in atomic orbitals, settling into quantum states with the lowest available energies. This does not seem to be the case in magic angle graphene.
"Basically, nothing much interesting can happen when the electrons organize themselves the way they do in an atom by occupying the lowest energy orbitals," MacDonald said. "But once their fate is determined by interactions between the electrons, then interesting things can happen."
In recent years, MacDonald and his team have explored stacks of three, four or five layers of graphene, as well as other promising materials, particularly transition metal chalcogenides, searching for unusual - and potentially useful - phenomena.
Now, MacDonald and the international team have published a study in Nature of magic angle graphene that showed the material can exhibit alternating superconducting and insulating phases that can be turned on or off with a small voltage change, similar to the voltages used in integrated circuits, increasing its usefulness for electronic devices. To achieve this result, team members from the Catalan Institute for Optical Physics produced graphene superlattices with more uniform twists than previously possible. In so doing, they discovered that the pattern of interleaved insulating and superconducting states is even more intricate than predicted.
TACC supercomputers are a critical tool in MacDonald's research and were used for the theoretical modeling of the data in the recent Nature paper.
"Many of the things we do, we could not do without a high-performance computer," he asserted. "We start out running on a desktop and then we quickly get bogged down. So very often, using a supercomputer is the difference between being able to get a satisfactory answer and not being able to get a satisfactory answer."
Though the results of computational experiments may seem less immediate or "real" than those in a lab, as MacDonald has shown, the results can expose new avenues of exploration and help illuminate the mysteries of the universe.
"The thing that's energized my work is that nature is always posing new problems. And when you ask a new type of question, you don't know in advance what the answer is," MacDonald said. "Research is an adventure, a community adventure, a collective random walk, by which knowledge moves forward."