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Graphene made into insulators or superconductors by applying electric voltages

By applying electric voltages at different points, magic-angle graphene can become locally superconducting (electron pairs) or isolating (barrier on the right). [Image credit: ETH Zurich/F. de Vries]





Researchers at ETH Zurich (a public research university in Switzerland) have succeeded in turning specially prepared graphene flakes into either insulators or superconductors by applying an electric voltage. This technique even works locally, meaning that in the same graphene flake regions with completely different physical properties can be realized side by side.

The production of modern electronic components requires materials with very diverse properties. There are isolators, for instance, which do not conduct electric current, and superconductors that transport it without any losses.

To obtain a particular functionality of a component, one usually has to join several such materials together. Often, that is not an easy task, in particular when dealing with nanostructures that are in widespread use today.

A team of researchers at ETH Zurich led by Klaus Ensslin and Thomas Ihn at the Laboratory for Solid State Physics have now succeeded in making a material behave alternately as an insulator or as a superconductor -- or even as both at different locations in the same material -- by simply applying an electric voltage. Their results have been published in the scientific journal Nature Nanotechnology.

The material Ensslin and his co-workers use bears the somewhat cumbersome name of "Magic Angle Twisted Bilayer Graphene." In fact, this name hides something rather simple and well-known, namely carbon -- albeit in a particular form and with a special twist.

The starting point for the material is graphene in flake form, which is carbon layers that are only one atom thick. The researchers put two of those layers on top of each other in such a way that their crystal axes are not parallel, but rather make a "magic angle" of exactly 1.06 degrees. "That's pretty tricky, and we also need to accurately control the temperature of the flakes during production. As a result, it often goes wrong," said Peter Rickhaus, who was involved in the experiments as a postdoc.

In 20% of the attempts to make the material, however, it works, and the atomic crystal lattices of the graphene flakes then create a so-called moiré pattern, where the electrons of the material behave differently than in ordinary graphene. Moiré patterns are familiar from television, for instance, where the interplay between a patterned garment and the scanning lines of the television image can lead to interesting optical effects.

On top of the magic angle graphene flakes, the researchers attach several additional electrodes that they can use to apply an electric voltage to the material. After they cool everything down to a few hundredths of a degree above absolute zero, something remarkable happens. Depending on the applied voltage, the graphene flakes behave in two completely opposite ways: either as a superconductor or as an insulator. This switchable superconductivity was already demonstrated in 2018 at the Massachusetts Institute of Technology (MIT) in the United States. Even today, only a few groups worldwide are able to produce such samples.

Ensslin and his colleagues are now going one step further. By applying different voltages to the individual electrodes, they turn the magic angle graphene into an insulator in one spot, but a few hundred nanometers to one side it becomes a superconductor.

"When we saw that, we obviously first tried to realize a Josephson junction," said Fokko de Vries, who is also a postdoc in Ensslins laboratory. In such junctions, two superconductors are separated by a wafer-thin insulating layer. In this way, current cannot flow directly between the two superconductors but rather has to tunnel quantum mechanically through the insulator. That, in turn, causes the conductivity of the contact to vary as a function of the current in a characteristic fashion, depending on whether direct or alternating current is used.

Electron microscope image of the Josephson junction (false colors). Using the electrodes (bright and dark gold) as piano keys, an insulating layer only 100 nanometers thick can be created between the two superconducting regions. [Image credit: ETH Zurich/F. de Vries]





The ETH researchers managed to produce a Josephson junction inside the graphene flakes twisted by the magic angle by using different voltages applied to the three electrodes, and also to measure its properties.

"Now that that's worked as well, we can try our hands at more complex devices such as SQUIDs," said de Vries. In SQUIDs (superconducting quantum interference device), two Josephson junctions are connected to form a ring. Practical applications for such devices include measurements of tiny magnetic fields, but they could also be used in modern technologies such as quantum computers. For possible uses in quantum computers, an interesting aspect is that, with the help of the electrodes, the graphene flakes can be turned not just into insulators and superconductors, but also into magnets or so-called topological insulators in which current can only flow in one direction along the edge of the material. This could be exploited to realize different kinds of quantum bits (qubits) in a single device.

"So far, however, that's just speculation," Ensslin said. Still, he is enthusiastic about the possibilities that arise from the electrical control even now. "With the electrodes, we can practically play the piano on the graphene," he added. Amongst other things, the physicists hope that this will help them to gain new insights into the detailed mechanisms that bring about superconductivity in magic angle graphene.

Source: ETH Zurich

Published May 2021

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