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NOVIDADES
Researchers at ETH Zurich have succeeded in turning specially prepared graphene flakes either into insulators or into 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 material keyboard realized by the ETH Zurich researchers. By applying electric voltages (“keys”) at different points, the magic-angle graphene can become locally superconducting (electron pairs) or isolating (barrier on the right). Image: ETH Zurich / F. de Vries
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 ("Gate-Defined Josephson Junctions in Magic-Angle Twisted Bilayer Graphene"). “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,” explains Peter Rickhaus, who was involved in the experiments as a postdoc. In twenty percent of the attempts, however, it works, and the atomic crystal lattices of the graphene flakes then create a so-called moiré pattern in which 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 which they can use to apply an electric voltage to the material. When they then 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 USA. Even today only a few groups worldwide are able to produce such samples. Electron microscope image of the Josephson junction (false colours). Using the electrodes (bright and dark gold) as piano keys, an insulating layer only 100 nanometres thick can be created between the two superconducting regions. Image: ETH Zurich / F. de Vries
“Now that that’s worked as well, we can try our hands at more complex devices such as SQUIDs”, says de Vries. In SQUIDs (“superconducting quantum interference device”) two Josephson junctions are connected to form a ring. Practical applications of such devices include measurements of tiny magnetic fields, but also 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. ETH Zurich. Posted: May 05, 2021.
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