Researchers turn graphene sheets into insulators or superconductors by applying voltage

Researchers at ETH Zurich have successfully turned specially prepared graphene sheets into insulators or superconductors by applying voltage
.
This technology can even work locally, which means that on the same graphene sheet, completely different physical properties can be achieved at the same time
.
The production of modern electronic components requires materials with very different properties
.
For example, there are isolators, which do not conduct electricity, and superconductors can carry current without loss
.
In order to obtain a specific function of a component, it is usually necessary to connect several such materials together
.
This is usually not easy, especially when dealing with nanostructures that are widely used today
.
A research team at the Solid State Physics Laboratory of ETH Zurich, led by Klaus Ensslin and Thomas Ihn, has now succeeded in making a material alternately behave as an insulator or a superconductor, and even behave as an insulator and a superconductor at the same time in different positions of the same material.
Need to apply voltage
.
Their results have been published in the scientific journal Nature Nanotechnology
.
This work was supported by QSIT (Quantum Science and Technology), the National Center for Scientific Research Competence
.
The material used by Enslin and his colleagues has a somewhat cumbersome name: "Magic Angle Twisted Double Graphene
.
" In fact, the name hides something quite simple and well-known, namely carbon, but it is twisted in a special form
.
The starting point for this material is a graphene sheet, which is a layer of carbon only one atom thick
.
The researchers placed two of these layers on top of each other so that their crystal axes were not parallel, but formed a "magic angle" of exactly 1.
06 degrees
.
"This is quite tricky, and it also requires accurate control of the temperature of the sheet during the production process
.
Therefore, it often makes mistakes.
However, in 20% of the attempts, it is effective.
The atomic lattice of the graphene sheet Will produce a so-called moiré pattern, in which the electronic behavior of the material is different from that of ordinary graphene
.
On top of the magic-angle graphene sheet, the researchers attached several additional electrodes that they can use to apply voltage to the material
.
When they cooled everything to a few percent above absolute zero, something extraordinary happened
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Depending on the applied voltage, graphene flakes behave in two completely opposite ways: either as a superconductor or as an insulator
.
This switchable superconductivity has been demonstrated at the Massachusetts Institute of Technology (MIT) in 2018
.
Even today, only a few groups in the world can produce such samples
.
Enslin and his colleagues have now taken another step forward
.
By applying different voltages to the electrodes, they turned magic-angle graphene into an insulator in one place, but it became a superconductor at a few hundred nanometers on one side
.
The researchers first tried to realize a Josephson junction, in which two superconductors are separated by an insulating layer as thin as a cicada's wings
.
In this way, the current cannot flow directly between the two superconductors, but has to tunnel through the insulator in quantum mechanics
.
This, in turn, causes the conductivity of the contact to change with the current in a peculiar way, depending on whether direct current or alternating current is used
.
ETH researchers succeeded in creating a Josephson junction twisted at a magical angle inside the graphene sheet by using different voltages applied to the three electrodes, and also measured its characteristics
.
An interesting aspect is that with the help of electrodes, graphene sheets can not only become insulators and superconductors, but also magnets or so-called topological insulators, where current can only flow in one direction along the edges of the material
.
This can be used to implement different kinds of qubits in one device
.