A new experimental platform based on two misaligned graphene layers could be used to investigate strongly correlated physics – that is, the physics of systems in which the interactions between electrons lead to novel phenomena. The platform, which can be tuned by simply applying an electric field, could help shed important light on the underlying mechanisms at play in superconductors, in particular high-temperature ones based on cuprates, for which a fundamental understanding is still lacking.
A team of researchers led by Pablo Jarillo-Herrero of the Massachusetts Institute of Technology (MIT) in the US made the platform by stacking two sheets of atomic-thick carbon (graphene) on top of each other. They then twisted the sheets so that the angle between them, known as the (theoretically predicted) “magic angle”, was 1.1°. They found that the material became a superconductor (that is, it conducted electricity without resistance) at 1.7 K.
“We were not looking for superconductivity when we began our experiments,” explains Jarillo-Herrero. “We chose to study these structures because there were some theoretical predictions that interesting electronic properties would occur in the graphene moiré superlattice if the two layers were stacked at this angle. Our intuition also told us that there would be some interesting physics, but what we discovered went far beyond what we had anticipated.”
The researchers studied the conductivity of the graphene sheets by applying a voltage to them and then measuring the current that circulated through them. They also measured the density of the particles that carry electronic charge inside the sheets.
Two breakthrough results
“We found two things: first that we can electrically tune the graphene system so that it becomes a correlated insulator, which can happen thanks to electrons localized in the moiré superlattice. This ‘Mott’ insulator is a material that should be a metal but which, because of strong repulsion between electrons, does not conduct. We reported this result in the first of our two Nature papers published this week.
“Secondly, we found that by adding a few extra charge carriers to this insulator state (by applying a small electric field), we could tune the graphene superlattice so that it became a superconductor. This result is detailed in our second Nature paper.”
The researchers say that graphene superlattices containing a record-low 2D charge carrier density of about just 1011 per cm2 can become superconducting. This means that they can superconduct electricity with just 10-4 of the electron density of conventional superconductors (that work at temperatures near absolute zero, and which can be described by the well-established Bardeen–Cooper–Schreiffer theory of superconductivity).
This behaviour (the presence of an insulating state so close to the superconducting one) is characteristic of so-called unconventional, high-temperature superconductors, known as cuprates. These complex copper oxides can conduct electricity without resistance at the relatively “high” temperature of 133 K. Although physicists have been studying these materials for decades now, in their quest to make superconductors that work at even higher temperatures, and ideally at room-temperature, they are still unable to explain the fundamental mechanisms at play in them.
Magic-angle graphene is magic
“The technique to make our new misaligned graphene sounds simple, but it took years to perfect,” says Jarillo-Herrero. “The good thing, however, is that there are several groups around the world that can carry it out. There are also many other groups that will now be able to replicate it too and so use the platform to study unconventional superconductivity in a simple system.”
Normally, when researchers study high-temperature superconductors, they need to subject the materials to extremely high magnetic fields, he explains. With graphene, they might be able to do this by simply applying a modest magnetic field.
“First and foremost, our discovery represents an advance in terms of fundamental science, and we hope that it will allow us to gain insight into the properties of strongly correlated systems, such as high-temperature superconductors and quantum spin liquids,” he tells nanotechweb.org. “What is more, our platform is a general one and could be applied to any 2D material, not just graphene.”
Quantum computers and photodetectors might benefit
Although there might be many potential applications most of these are likely to be a way off, realistically speaking, he adds. “For example, the most advanced technology to make prototype quantum computers today are based on superconducting devices. Magic-angle graphene superlattices could offer us a new type of electrically tunable superconductor, and who knows, they might one day be exploited in quantum computation and information technologies.
“Superconductors are also used in many other applications, such as ultrasensitive detectors of light, so our result may perhaps have an impact there too.”
There is no doubt that graphene is an exceptional material in so many ways. Its unique properties, such as extremely high mechanical strength (it is stronger than steel) and extremely high electrical conductivity, with electrons zipping through it at near-ballistic speeds, have been known for a while now. Although researchers had already shown that it could behave like a superconductor before too, the superconductivity was only observed when it was in contact with other superconducting materials. What is more, this could mostly be explained by the Bardeen–Cooper–Schreiffer theory, so it was considered to be conventional.
“The relatively high superconducting temperature of 1.7 K of twisted bilayer graphene that we observed, with its charge carrier density of just 1011 per cm2, now also makes this material among the strongest coupling superconductors known,” adds Jarillo-Herrero. He says that the material might be working in a regime close to the crossover between the Bardeen–Cooper–Schrieffer regime and a Bose–Einstein condensate (a state of matter in which all the particles in a system condense into a single state), but confirming or refuting this will be the subject of future research.