Graphene is a 1-atom-thick layer of tightly bonded carbon atoms arranged in a hexagonal lattice. Graphene the world’s first 2D nanomaterial, is widely regarded as the “wonder material” of the 21st century due to the combination of its extraordinary properties. As a single layer of graphite, it is the thinnest material (monoatom thick), transparent, 200 times stronger than steel, yet as flexible as rubber, more conductive than copper, excellent thermal conductor and impermeable to moisture and gases. Graphene is also extraordinarily light at 0.77 mg/m2, which is roughly 1,000 times lighter than 1 m2 of paper. It is fire resistant yet retains heat.
Due to its exceptional strength, graphene is used today to reinforce products such as tennis rackets, car tires or aircraft wings. But it is also an interesting subject for fundamental research, as physicists keep discovering new, astonishing phenomena that have not been observed in other materials.
In 2018, MIT researchers discovered that they could induce superconductivity in graphene by stacking two layers of the atom-thick material at a precise 1.1 ° angle. They called the material that they produced “magic-angle twisted graphene,” so named for the seemingly magical effects of this exact way for stacking graphene thin films.
Not only this, but the researchers found that magic angle twisted bilayer graphene is highly similar to other materials that are superconductive at temperatures near or over 100 K. Scientists agreed that this material provided an exciting platform for a new branch of research into unconventional superconductivity, which may lead to more insight into the still little understood phenomenon of superconductivity writ large.
Twisted graphene has around 10,000 times less charge-carrier density than aluminum. A low charge-carrier density means that the pairing mechanism that makes electrons join up and condense into a superfluid is more constrained: a limited number of available electrons to pair up.
This means that twisted graphene, which becomes superconductive at a low critical temperature despite a low charge-carrier density, is one of the most strongly correlated superconductors we know of.
Benefits of a Tunable Superconductivity
Twisted graphene has a much lower critical temperature (Tc) than other superconductive materials, only around 1 K. An extremely low critical temperature usually makes a material unsuitable for industrial superconductivity needs, as excessive amounts of energy and water are needed to cool it down. But for research purposes, a low critical temperature is not necessarily a limiting factor.
Its electronic properties are more similar to superconductors with high critical temperatures, such as cuprates. And because graphene’s carrier doping can be tuned easily by adjusting the voltage on the gate electrode, superconductive twisted graphene can be explored across all parts of the phase diagram with one device
Two groups—including the pioneering MIT group—are now delivering on that promise by turning twisted graphene into working devices, including superconducting switches like those used in many quantum computers.
That control could simplify quantum computers. Those being developed by Google and IBM rely on Josephson junctions with properties that are fixed during fabrication. To operate the finicky qubits, the junctions must be manipulated jointly in cumbersome ways. With twisted graphene, however, qubits could come from single junctions that are smaller and easier to control.
Kin Chung Fong, a Harvard University physicist and member of Raytheon BBN Technologies’s quantum computing team, is enthusiastic about another potential use for the material. In April 2020, he and his colleagues proposed a twisted graphene device that could detect a single photon of far infrared light. That could be useful for astronomers probing the faint light of the early universe; their current sensors can spot lone photons only in the visible or nearly visible parts of the spectrum.
Twisted Graphene for Quantum technology
In 201, a team of researchers led by Klaus Ensslin and Thomas Ihn at ETH Zurich’s Laboratory for Solid State Physics was able to demonstrate that twisted graphene could be used to create Josephson junctions, the fundamental building blocks of superconducting devices.
Based on this work, researchers were now able to produce the first superconducting quantum interference device, or SQUID, from twisted graphene for the purpose of demonstrating the interference of superconducting quasiparticles. Conventional SQUIDs are already being used, for instance in medicine, geology and archaeology. Their sensitive sensors are capable of measuring even the smallest changes in magnetic fields. However, SQUIDs work only in conjunction with superconducting materials, so they require cooling with liquid helium or nitrogen when in operation.
In quantum technology, SQUIDs can host quantum bits (qubits); that is, as elements for carrying out quantum operations. “SQUIDs are to superconductivity what transistors are to semiconductor technology—the fundamental building blocks for more complex circuits,” Ensslin explains.
The spectrum is widening
The graphene SQUIDs created by doctoral student Elías Portolés are not more sensitive than their conventional counterparts made from aluminum and also have to be cooled down to temperatures lower than 2 degrees above absolute zero. “So it’s not a breakthrough for SQUID technology as such,” Ensslin says. However, it does broaden graphene’s application spectrum significantly. “Five years ago, we were already able to show that graphene could be used to build single-electron transistors. Now we’ve added superconductivity,” Ensslin says.
What is remarkable is that the graphene’s behavior can be controlled in a targeted manner by biasing an electrode. Depending on the voltage applied, the material can be insulating, conducting or superconducting. “The rich spectrum of opportunities offered by solid-state physics is at our disposal,” Ensslin says.
Also interesting is that the two fundamental building blocks of a semiconductor (transistor) and a superconductor (SQUID) can now be combined in a single material. This makes it possible to build novel control operations. “Normally, the transistor is made from silicon and the SQUID from aluminum,” Ensslin says. “These are different materials requiring different processing technologies.”
An extremely challenging production process
The challenge is that scientists have to carry out several delicate work steps one after the other: First, they have to align the graphene sheets at the exact right angle relative to each other. The next steps then include connecting electrodes and etching holes. If the graphene were to be heated up, as happens often during cleanroom processing, the two layers re-align the twist angle vanishes. “The entire standard semiconductor technology has to be readjusted, making this an extremely challenging job,” Portolés says.
The study is published in Nature Nanotechnology.
Stacked carbon sheets used to make switches that some quantum computers rely on
The MIT group went further, electrically transforming their Josephson junctions into other submicroscopic gadgets, “just as proof of concept, to show how versatile this is,” says lab leader Pablo Jarillo-Herrero, whose group posted its results to arXiv on 4 November. By tuning the carbon into a conductor-insulator-superconductor configuration, they were able to measure how tightly the electron pairs were yoked together—an early clue to the nature of its superconductivity and how it compares with other materials. The team also built a transistor that can control the movement of single electrons; researchers have studied such single-electron switches as a way to shrink circuits and diminish their thirst for energy.
Magic angle graphene devices are unlikely to challenge consumer silicon electronics anytime soon. Graphene itself is easy to make: Sheets of it can be stripped off blocks of graphite with nothing more than Scotch tape. But the devices must be chilled nearly to absolute zero before they can superconduct. And maintaining the precise twist is awkward, as the sheets tend to wrinkle, disrupting the magic angle. Reliably creating smoothly twisted sheets even just 1 micron or two across is still a challenge, and researchers don’t yet see a clear path toward mass production. “If you wanted to do a real complex device,” Jarillo-Herrero says, “you’d need to create hundreds of thousands of [graphene substrates] and that technology doesn’t exist.”
The field of twistronics remains in its infancy, and the fussy process of twisting microscopic specks of graphene to the magic position still requires sleight of hand, or at least deft lab work. But regardless of whether twisted graphene finds its way into industrial electronics, it’s already profoundly changing the world of materials science, says Eva Andrei, a condensed matter physicist at Rutgers University, New Brunswick, whose lab was one of the earliest to notice twisted graphene’s peculiar properties.
“It’s a really new era,” she says. “It’s a totally new way of making materials without chemistry.”
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