Schematic view of the interplay of localized and itinerant states in twisted trilayer graphene giving rise to a heavy fermion state
Schematic view of the interplay of localized and itinerant states in twisted trilayer graphene giving rise to a heavy fermion state

Two researchers have combined their knowledge of strongly correlated physics, with a focus on heavy fermions, and 2D van der Waals materials to propose a novel way of advancing radiation-free quantum technology using subtly modified graphene. Aline Ramires from the Paul Scherrer Institute in Switzerland and Jose Lado from Aalto University in Finland have shown a new pathway to engineer the fundamental phenomena of rare-earth compounds, known as heavy fermion superconductors, using just graphene.

While these rare-earth compounds have been known about for decades, the ability to produce usable quantum entangled phenomena from them has not been possible as many of them contain radioactive compounds such as uranium and plutonium, making them of limited use in real-world quantum technologies. However, as reported in Physical Review Letters [Ramires A., Lado J. L., Phys. Rev. Lett. (2021) DOI: 10.1103/PhysRevLett.127.026401], this study demonstrated the potential to produce heavy fermions with inexpensive and non-radioactive materials through a combination of three twisted graphene layers.

A heavy fermion is a particle, here an electron, that behaves as if it has much more mass than it actually does due to the unique quantum many-body effects mostly only previously seen in some rare-earth compounds. This behavior is the key force of the phenomena needed for these materials to be applied in topological quantum computing.

The team showed that layered thin sheets of carbon in a specific pattern, with each sheet being rotated in relation to the other, would develop the quantum properties effect that result in the electrons in the graphene behaving like heavy fermions. Twisted van der Waals materials have been found to host a variety of tunable electronic structures, and here the twisted trilayer graphene was used as a platform to emulate heavy fermion physics, building on previous experimental breakthroughs that related van der Waals systems realize states usually only seen in compounds with strong correlations.

The proposal opens up a pathway towards the emulation of heavy fermions and their control in a completely new way – by just controlling the electric field by gates in the material, one can achieve what was before possible only by the application of high pressure or by doping the material. The heavy-fermion regime presented can also be efficiently controlled by external electric fields, making it experimentally realistic and demonstrating a superior tunability with respect to rare-earth based heavy fermion compounds.

Interestingly, recent breakthroughs by experimental groups showed the emergence of superconductivity in a similar structure. While the nature and origin of the superconducting state in this material remains unresolved, there could be a relation between the superconducting state observed and a hypothetical heavy fermion superconducting state. The researchers hope that this connection will allow some of the lessons learned in strongly correlated materials to be further used in van der Waals structures.