In 2018, researchers at MIT demonstrated superconductivity in magic-angle bilayer graphene. Now, Dmitri Efetov of the Institute of Photonic Sciences in Barcelona, Spain, and his colleagues have replicated MIT's results and discovered even more states in magic-angle graphene. By preparing a high-quality device, Efetov’s team could measure the electronic phases more accurately and resolve previously hidden electronic states.
To realize the magic angle, the researchers use an established technique: They take one sheet of graphene and tear it in two. They then rotate one of the pieces just past the magic angle, by about 1.2°, and stack it on top of the other. In most electrical devices, the final step is annealing to clean the sample and get rid of any air bubbles between the layers. But in magic-angle graphene, with the layers misaligned by such a small angle, heating the sample snaps the graphene layers back into alignment. Instead of annealing, Efetov and his colleagues rolled the top layer down gradually, starting from one edge, rather than dropping the second layer directly down onto the first. That method squeezes out any air bubbles as they form. The result is a relative angle that varies by only 0.02° over a 10 µm device, a record for magic-angle graphene. The fabrication overall is tricky; it was reported that in three months of trying, just 2 of the 30 devices worked.