Magic-angle graphene reveals new phases

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.

The group measured the electrical resistance over a wide range of electron or hole densities—depending on whether the applied voltage was negative or positive. They saw the same superconducting state as MIT's Prof. Jarillo-Herrero, when the magic-angle graphene had a hole density of about 2 × 10¹² cm^−2, plus three new superconducting states at electron and hole densities as low as 0.5 × 10¹² cm^−2. For the original superconducting state, Efetov and his colleagues found a higher transition temperature, 3 K, than previously reported—perhaps due to their improved sample quality. The three new superconducting states had much lower transition temperatures in the hundreds of millikelvin.

At charge carrier densities between superconducting regimes, magic-angle graphene showed resistance peaks from correlated electron or hole states, which are described by collective rather than individual charge carrier behavior. Three of the correlated states were insulating, and three of them seemed semimetallic. Two of the noninsulating states were also topologically nontrivial with Chern numbers of 1 and 2. The correlated states occurred when there were an integer number of electrons or holes for each moiré unit cell.

The states are manifestations of electronelectron interactions, as are other quantum phases of matter including some types of superconductivity. But the mechanism behind graphene’s superconductivity is still unknown. Now, though, theorists have more data to work with.