Finding new quantum phenomena in graphene
Hong Kong researchers are the first to observe a certain quantum phenomenon in the wonder material graphene, which could lead to new applications and even bigger discoveries.
Graphene was discovered in 2004 and was the thinnest ‘two-dimensional’ material, made of a layer of carbon just one atom thick arranged in a hexagonal lattice. It has excited scientists for its strength, lightness, and electrical properties, with applications in development including touch screens, energy storage, and filtration.
A new configuration of graphene has potentially even more exciting properties: twisted bilayer graphene. Now, a team from the University of Hong Kong and the Hong Kong University of Science and Technology have discovered a quantum phenomenon operating in this graphene called the nonlinear Hall effect.
The observation and modelling of this effect adds to the understanding of how twisted bilayer graphene operates and opens up the possibility of new applications in sensing, materials, and computing. The results, produced by a team led by Dr Zi Yang Meng from the Department of Physics at HKU and Professor Ning Wang from the Department of Physics at HKUST, are published in the journal Physical Review Letters.
Creating a new effect
Twisted bilayer graphene is created by laying one graphene sheet over another, and twisting slightly, to make an angle between the lattice structures of the sheets. This creates a ‘moiré’ pattern, where the carbon atoms of each sheet form new clusters or ‘super cells’. This concentration of carbon atoms enhances the interactions of electrons in the material, creating whole new phenomena.
In this study, the HKUST team experimentally created a twisted bilayer graphene with a particular twist angle and applied a current to it. The voltage pattern resulting from variations in the current suggested to them something interesting was happening, so they asked the HKU theoretical team for help interpreting the results.
The structure of twisted bilayer graphene corrals electrons into ‘flat’ bands, meaning the relationship between their momentum and energy is such that the electrons are almost ‘stuck’ in certain places. This means their interactions with each other become the dominant physics, creating unusual phenomena.
Meng and his team had been thinking about one such phenomenon that might occur, and seeing the HKUST team’s results they had an intuitive sense of what was going on. They theorised that there were features called Berry curvature hotpots in the energy bands, which dominate the transport behaviour of the electrons.
These Berry curvature hotspots then create the nonlinear Hall effect. Essentially, when a horizontal current is applied to the twisted bilayer graphene, a vertical voltage is generated. Importantly, this voltage is twice that expected from the input current. It’s a nonlinear response, where 1 in does not equal 1 out.
High-performance modelling
However, though they had a sense that this was what was creating the voltage observations, to really understand what was happening in the material they needed to model all the possibilities. While twisted bilayer graphene can create interesting phenomena without extreme experimental conditions like high magnetic fields or cleanliness, the material still has complexities that need to be taken into account. Graphene sheets are still prepared by hand (incredibly, largely by using Scotch tape to peel atom-thick layers from graphite), and the twist angle can be tricky to standardise.
Each possible combination of variables like this needed to be modelled, or, as Meng puts it: “There was only one experiment performed, but we had to imagine a thousand setups.” The team used the High-Performance Computing Platform HPC2021 at the Information Technology Services, HKU, and the ‘Blackbody’ supercomputer at the Department of Physics at HKU to make these computations. From the model outputs of these scenarios, they found that there were a few that closely matched the pattern of voltage observed in the experiment.
The result marks the first systematic observations of the nonlinear Hall effect in graphene. It’s an achievement that required a lot of computational power to model the complex interactions of the electrons, but, says Meng, there are still a lot of unknowns.
Searching for superconductivity
At a ‘magic angle’, and at supercool temperatures, the interactions of electrons in twisted bilayer graphene are known to be even stronger and create a phenomenon much sought after: superconductivity. Superconductivity is the transport of current through a material with no resistance. When electricity is sent from a power station, for example, some of the current is lost as the cable material has some resistance, causing it to heat up and lose energy.
Finding a superconductive material that can be manufactured at scale and without extreme conditions needed would make energy transport extremely efficient. Twisted bilayer graphene could one day be this material, but first, the task is to understand how it generates superconductivity.
In comparison, Meng says, the task of modelling the nonlinear Hall effect was ‘simple’. At even stronger interactivity, several electron processes are thought to act at the same time, so while the Berry curvature may still play a role, other phenomena like Coulomb interactions will also be exerting influence, making the modelling even harder.
But it’s something Meng wants to work towards. He says: “For the system we studied in this paper, we now know how the moiré pattern creates the Berry curvature hotspots, and how in turn they create the nonlinear Hall effect. But with higher electron interactions, there could be new phenomena beyond the Berry curvature; there could be a new law to discover.
“We need to understand how the moiré pattern works at the magic angle, to know why superconductivity happens in these systems. Knowing the mechanisms means we could amplify them, making materials with remarkable properties easier to synthesise.”