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Squeezed graphene becomes a superconductor

28 Jan 2019
Bilayer graphene
Come together: squeezing bilayer graphene can induce superconductivity. (Courtesy: Shutterstock/Mopic)

Twisted bilayer graphene can be made into a superconductor by simply squeezing the two layers closer together – according to an international team of physicists. Observation of the effect confirms a key prediction about the causes of correlated electron phenomena in bilayer graphene and could potentially help to unravel the puzzle of unconventional superconductivity.

Graphene is a sheet of carbon just one atom thick and its remarkable electronic properties have captivated physicists since the free-standing material was first isolated in 2004. While much work has been done on the properties of electrons within graphene sheets, researchers have also become interested in the weak coupling that occurs between electrons in bilayers of graphene. Indeed, the Physics World 2018 Breakthrough of the Year went to Pablo Jarillo-Herrero of the Massachusetts Institute of Technology and colleagues who showed that the electronic properties of a bilayer are strongly influenced by the relative orientation of its two graphene sheets.

This effect was predicted several years ago by independent teams in Chile and the US, who calculated that when two layers are twisted by a “magic angle” of about 1.1° relative to each other, “flat bands” occur, in which the kinetic energy of the electrons is almost independent of their momentum.

“Exotic things”

This can have some strange effects, explains condensed-matter physicist Cory Dean of Columbia University. “In normal materials, the behaviour of the most energetic electrons is typically determined by the kinetic energy term, and they don’t care about the other electrons,” he explains. “But if the band is very flat, even the most energetic electrons have very low kinetic energy. In that case, the system becomes more dominated by the electron-electron interaction energy. Systems can often do exotic things to minimize that interaction energy.”

Graphene is usually an extremely good electrical conductor, but in 2018, Jarillo-Herrero’s group showed that when the flat bands were exactly half filled with electrons, the bilayer behaves like an insulator. The researchers attributed this to the localization of electrons by electron-electron interactions between the graphene layers. Furthermore, by injecting or withdrawing electrons from this correlated insulator, they could produce electron-doped or hole-doped superconductors respectively.

Many features of this superconductivity – such as its proximity to an insulating state – bear striking similarities to that of type-II superconductors such as cuprates and pnictides. Discovered in 1986, these materials are of significant technological interest because they remain superconductors at relatively high temperatures and magnetic field strengths

Systematic change

The mechanism for type-II superconductivity has remained elusive, in part because the materials are difficult to study in a systematic way. “If you want to change anything in the system, such as doping or lattice constant, then you have to make a completely new material,” explains Dean. “Then you get caught up in arguments about what else has changed.” This has complicated efforts to optimize type-II materials and perhaps produce room-temperature superconductors.

Now, Dean and colleagues in the US and Japan have produced multiple samples of bilayer graphene – some with twist angle 1.1°; some with greater twist angles. As expected, samples with twist angle 1.1° showed superconductivity before being compressed whereas those with greater twist angles did not. However, when pressure was applied to samples with twist angles greater than 1.1°, the bilayers became superconductors.

Dean explains that value of the magic angle is related to the nature of the interlayer coupling, which is changed by compressing the bilayer at pressures greater than 10,000 atm. Tantalizingly, the critical temperature of the superconductor also increased slightly from 1 K to 3 K. This is in line with a theoretical prediction that the critical temperature of the superconductor increases systematically with magic angle and pressure – but Dean says this “needs to be properly tested”.

Indeed, Dean says that compressed bilayers are a useful potential platform for investigating type-II superconductors: “[We have] demonstrated how incredibly tunable these systems are. We can go from superconducting to metal to insulating states over large ranges of temperature, pressure and magnetic field without changing the composition. That could make this type of superconductivity a much more tractable problem.”

“I think it’s a very significant result,” says Jarillo-Herrero. “First of all, by being the first [experiment] that confirms our results, it gives credibility to the whole field. Second, though the tuning by pressure is difficult to apply, and I’m not sure how many other groups will follow it up, it gives many opportunities to think of complementary experiments and apply them to other 2D systems.”

The research is described in Science.

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