Regulating graphene structures for potential quantum materials

Researchers have developed a technique to control ion channels in 2D graphene, making them promising candidates for quantum devices

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The possible uses for quantum devices are ever-increasing, thanks to the potential of quantum mechanics to solve complex problems significantly faster than classical computers.

However, for this potential to be realised, researchers are working to overcome the phenomenon of decoherence – when a material loses its quantum properties. This is a key step to reduce error rates and scaling limitations, achieving information flow without loss.

One of the promising materials in quantum devices is bilayer graphene – a one-atom-thick layer of carbon atoms arranged in a hexagonal lattice. As its name suggests, the material is formed by stacking two layers of graphene together, similar to Lego blocks, into a 2D structure.

When the upper and lower layers are twisted, a superlattice is formed, creating protected areas called topological channels between them.

Preserving these protected states is crucial for developing quantum devices because they exhibit various novel properties necessary for information flow including electron transport.

The basis of quantum data – the qubit (including trapped ions and superconductors) – is the quantum analog of the classical bit.

However, many current forms of qubits are susceptible to noise and decoherence because they encode information in the particles themselves. In topological quantum computing, information is not encoded in the quasiparticles themselves, but in the way they interact, offering potential for a more robust system.

New research led by the University of Melbourne, Chongqing University, and Tokyo University has shown for the first time that these topological structures can be controlled while preserving their topological nature.

The team visualised the 2D graphene structure by using low-energy dark field electron microscopy (LEEM) which maps the atomic three-dimensional arrangement into image contrast.

The graphene structure was exposed to Lithium (Li) atoms and using LEEM, researchers observed the intercalation (addition without a change in structure) of Li into the graphene layers.

They found that Li did not uniformly intercalate (observed as transitioning from dark to bright in LEEM images) at the same rate in all regions. As the intercalation process progressed, the bright areas expanded, but some regions remained dark producing zebraic and triangular patterns that have been previously observed in graphite.

By interpreting these patterns it was possible to observe the relationship between intercalation and structural evolution, providing insights into how to control topological channels.

Researchers also observed that the channels could be controllably moved and the sizes changed by varying the amount of intercalating lithium atoms, while the domains remained robust.

Figure 1: Simulation results (left): moiré patterns of overlapping grids in graphene structure and Experimental observation (right) of Li intercalation within these patterns

Next steps

The ability to maintain topological structures has not been observed previously and opens exciting possibilities for realising future topological quantum devices.

This knowledge could also have implications for improving Li-ion batteries, given that similar patterns have also been observed in graphite systems.

Publication

Endo, Y., Yan, X., Li, M. et al. Dynamic topological domain walls driven by lithium intercalation in graphene. Nat. Nanotechnol. 18, 1154–1161 (2023). https://doi.org/10.1038/s41565-023-01463-7

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