ETH Zurich researchers have transformed specially prepared graphene flakes into superconductors or insulators using an electric voltage. This technique can be used locally to create graphene flake regions with different physical properties.
Materials with a variety of properties are required for the production of modern electronic parts. For example, superconductors transport the current without losses, and isolators don’t conduct it. It is common to combine several materials to achieve specific functionality. This is especially true when you consider nanostructures, which are common today.
Klaus Ensslin at ETH Zurich and Thomas Ihn at Laboratory for Solid State Physics have succeeded in changing the behavior of a material to act as either an insulator or a superconductor – simply by applying an electric voltage. The results were published in the scientific journal Nature Nanotechnology. The National Centre of Competence in Research QSIT (Quantum Science and Technology) supported the work.
Graphene with a magic Angle
Ensslin and his coworkers use “Magic Angle Twisted Bilayer graphene.” This name is too complicated. It hides something simple and well-known: carbon, although in a specific form and with a unique twist. Graphene flakes are carbon layers of only one atom thickness. They are the starting point for this material. Researchers placed two layers of graphene flakes on top of one another so that their crystal angles are not parallel but instead create a “magic angle,” which is precisely 1.06 degrees. This is a difficult task, as we need to control the temperature of the flakes in production. It often goes wrong,” says Peter Rickhaus, a postdoc involved with the experiments.
It works in twenty percent of cases. The atomic crystal lattices in the graphene flakes create a moire pattern in which electrons behave differently to ordinary graphene. Moire patterns can be seen on television. The interplay between a patterned garment and the scanning lines in the television image can create interesting optical effects. Researchers attach additional electrodes to the magic angle graphene flakes to apply an electrical voltage. Amazing things happen when they cool everything to just a few hundredths above zero. The graphene flakes can behave in opposite ways depending on the voltage applied. They can either act as superconductors or insulators, depending on your choice. The switchable superconductivity of graphene was demonstrated at the Massachusetts Institute of Technology (MIT), USA, in 2018. Only a few groups are capable of producing such samples today.
Insulators and superconductors are made from the same material.
Ensslin, along with his colleagues, is going even further. They apply different voltages to individual electrodes to transform the magic angle graphene from an insulator to a superconductor by placing it a few hundred nanometers away.
Fokko deVries, a postdoc at Ensslins, says, “When we saw this, we first attempted to realize a Josephson Junction.” A thin, wafer-thin insulation layer separates two superconductors in such junctions. This prevents current from flowing directly between the superconductors and allows it to tunnel through the insulator. This causes the conductivity to change characteristically as a function of the present, depending on whether it is direct or alternating.
Potential applications of quantum technology
Researchers at ETH created a Josephson junction in graphene flakes twisted with the magic angle using different voltages to measure its properties. De Vries says, “Now that we have that working, we can experiment with more complicated devices like SQUIDs.” SQUIDs (superconducting quantum interference devices) are two Josephson junctions connected to form a circle. These devices can measure small magnetic fields and modern technologies like quantum computers. One exciting feature of graphene flakes that could be used in quantum computers is the ability to transform them with the help of electrodes into superconductors, insulators, magnets, or so-called topological insulations. In these cases, current can only flow in one direction around the edge of the material. This could create different quantum bits (qubits) in one device.
A keyboard to hold materials
Ensslin states that “so far, however,” is speculation. He is still excited about the potential of electrical control. “With the electrodes, we can practically play on the graphene the piano.” The physicists also hope this will allow them to gain new insight into the mechanisms that create superconductivity in magic-angle graphene.