Quantum graphene magic: perfection overrated

The hexagonal lattice is a feature of graphene, and the wave symbolizes the movement of electrons. Credit: Tu Win

carbonaceous matter graphene It has excellent electronic properties. But is it also stable enough to be useful in practice? New accounts say: yes.

The new computer model demonstrates that graphene’s exceptional electronic properties remain stable, even with defects, confirming its potential in quantum technology and sensor applications.

Realism in materials research: the case of graphene

Nothing in the world is perfect. This is also true in materials research. In computer simulations, one often represents the system in a very idealistic way. For example, one could calculate what properties an absolutely perfect crystal would have. However, in practice we always have to deal with additional effects – with defects in the crystal lattice, with additional molecules that bind to matter, with complex interactions between molecules. So the crucial question is: Do these unavoidable additional effects change the properties of matter or not?

This is particularly interesting in the case of the two-dimensional material graphene, which consists of only one layer of carbon atoms. It has long been known that graphene has excellent electronic properties. But it was not yet clear how stable these properties were. Are the disturbances and additional influences that are unavoidable in practice destroy them, or do they remain intact? The Vienna University of Technology (TU Wien) has now succeeded in developing a comprehensive computer model of realistic graphene structures. It turns out that the desired effects are very stable. Even pieces of graphene that aren’t quite perfect can be put to good use in technological applications. This is good news for the global graphene community.

Electron movement in graphene

“We calculate on the atomic scale how electric current propagates in a small piece of graphene,” says Professor Florian Liebisch from the Institute for Theoretical Physics at TU Wien. “There are different ways an electron can move through matter. According to the rules of quantum physics, it is not necessary to choose one of these paths; an electron can take several paths at the same time.

These different pathways can then overlap in different ways. At very specific energy values, the pathways cancel each other out; At this energy, the probability of electrons passing through a piece of graphene is very low, and the electric current is minimal. This is called “destructive interference”.

“The fact that current flow decreases exponentially at very specific energy values ​​for quantum physical reasons is a very desirable effect from a technological point of view,” Florian Liebisch explains. “This could be used, for example, to process information on a micro-scale, similar to what electronic components do in computer chips.”

One could also use it to develop new quantum sensors: suppose a piece of graphene conducts no current at all. Then, all of a sudden, a molecule sticks from the outside to the surface of the graphene. “This molecule slightly changes the electronic properties of the piece of graphene, and that could be enough to suddenly increase the current flow very dramatically,” says Dr. Robert Stadler. “This could be used to make highly sensitive sensors.”

The intricacies and advances in graphene modeling

However, the physical effects that play a role in the details are very complex: “The size and shape of a piece of graphene is not always the same, and there are multiple-body interactions between several electrons that are difficult to calculate mathematically. “There might be unwanted extra atoms in some places, and the atoms always oscillate a bit – all of this has to be taken into account in order to be able to describe graphene in a really realistic way,” says Dr. Angelo Valli.

This is exactly what has now been achieved at TU Wien: Angelo Valli, Rupert Stadler, Thomas Fabian and Florian Liebisch have years of experience correctly describing different effects in materials in computer models. By combining their expertise, they have now succeeded in developing a comprehensive computer model that includes all relevant sources of error and perturbation effects present in the graphs.

By doing so, they were able to show that even in the presence of these sources of error, the desired effects are still visible. It is still possible to find a certain energy in which current flows only to a very small extent due to quantum effects. Experiments have already shown that this is plausible, but a systematic theoretical investigation is still missing.

This proves that graphene does not have to be perfect for use in quantum information technology or quantum sensing. For applied research in this area, this is an important message: global efforts to use quantum effects in graphene in a controlled way are indeed promising.

Reference: “Stability of destructive quantum interference resonances in electron transport through graphene nanostructures” By Angelo Valli, Thomas Fabian, Florian Liebisch, and Rupert Stadler, 10 August 2023, Available here. carbon.
doi: 10.1016/j.carbon.2023.118358

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