The Wolf Prize is one of the most prestigious scientific prize, and is often considered to identify future laureates of the Nobel Prize. Yesterday, the prize ahs been anounced and in Physics, it goes to Prof. Pablo Jarillo-Herrero (MIT), Prof. Allan H. Macdonald (UT Austin) and Dr Rafi Bisritzer (Applied Materials Israel), “for pioneering theoretical and experimental work on twisted bilayer graphene”.
Graphene has been discovered in 2004 and led to the Nobel Prize in Physics in 2010, awarded to Prof. Andre Geim and Prof. Kostya Novoselov at the University of Manchester (UK). It is considered as the best platform to study 2 dimensional electron gases (2DEG) and discover novel physical phenomenon.
Now, the work awarded by the Wolf foundation rewards both the theoretical prediction and experimental work that made the discovery of superconductivity in twisted bilayer graphene one of the most important event in 2018. Back in 2011, Allan Macdonald and Rafi Bisritzer imagined a system comprised of two graphene layers, stacked together, with a twist angle between them. They calculated the tunnelling velocity of electrons between layers, and eventually found that it depends on the misalignment angle. Particularly, at 1.1° difference, they predicted the band to be flat. What is interesting is that under the flat band condition, electron-electron interactions, interlayer coupling, weak kinetic energy of the bands, magnetic interactions, etc. are likely to compete. This is an eventual condition for superconductivity.
This paper had been forgotten for several years, until 2018, when the group led by Pablo Jarillo-Herrero at MIT achieved sufficient control to be able to stacking two graphene layers with a misfit angle of 1.1° (called magic angle!). In two Nature papers in 2018, he reported an interesting phase diagram for this system, with a Mott-insulating phase, as well as a superconducting phase. The paper sparked a lot of interest, and triggered many other works in the past 2 years. This is interesting because this system is the ground of many-body physics, the same kind observed in cuprates (high temperature superconductors) and cold atom lattices, with a lattice size in between those two systems. At the time, there is no theoretical framework to explain the superconductivity in cuprates. Thus, the magic-angle twisted bilayer graphene system may provide a much better framework for the understanding of this kind of behaviour, thus leading to the realisation of cuprate-based superconductors at even higher temperatures.
Finally, this prize shows the usual way condensed matter physics works: At first, graphene has been discovered, and it has been shown that van der Waals heterostructures combining 2D materials could be realised. Second, theoretical physicists imagined a system “with a twist” and calculated its expected properties, that have been realised experimentally several years later.