In 2018 Pablo Jarillo-Herrero of MIT and his colleagues demonstrated superconductivity in magic-angle bilayer graphene (see Physics Today, May 2018, page 15). A single layer of graphene is a two-dimensional sheet of carbon atoms and on its own is not superconducting. But two sheets (blue and black in the figure) vertically stacked at just the right angle θ —about 1.1°—with respect to each other have a superconducting transition around 1.7 K. The magic lies in the quasiperiodic structure, or moiré lattice, that forms at a larger length scale than the underlying graphene lattices, as seen in the figure. Provided the temperature is low enough, the resulting superconducting state can be summoned in and out of existence by changing the angle between the sheets of graphene or the charge carrier density with an applied voltage. Beyond that tunability, magic-angle graphene’s superconductivity is interesting because its relationship of temperature to carrier density resembles that of high-Tc cuprates.
Now Dmitri Efetov of the Institute of Photonic Sciences in Barcelona, Spain, and his colleagues have replicated Jarillo-Herrero’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.
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