Unconventional superconductivity in graphene bilayers

Much of the appeal of the two-dimensional materials zoo—which includes graphene, hexagonal boron nitride, molybdenum disulfide, and many others—lies in the multitude of ways the atomically thin sheets can be stacked and combined to create new structures with novel properties. (See the article by Pulickel Ajayan, Philip Kim, and Kaustav Banerjee, Physics Today, September 2016, page 38 .) Compounding the number of potential arrangements is the possibility of tuning the twist angle between successive layers. Now MIT’s Pablo Jarillo-Herrero and colleagues have shown that the twist angle between two sheets of graphene can be exploited to dramatic effect: At a so-called magic angle of approximately 1.1°, the two-layer stack becomes a superconductor.

A graphene monolayer’s electronic properties are dominated by the electrons’ kinetic energy. Free to roam the honeycomb lattice, the electrons behave quasi-relativistically. In the magic-angle bilayer, on the other hand, electrons are largely concentrated on the regions (shown in yellow in the figure) where the hexagons in the two layers line up. Confined to such close quarters, the electrons’ Coulomb interactions dominate over their kinetic energy. The physics of strong correlations comes to the fore—just as it does in the cuprates and other unconventional superconductors.

The graphene bilayer has a critical temperature T_c of just 1.7 K, which seems low in absolute terms. But in light of the material’s minuscule charge-carrier density, which limits how many carriers can pair and condense into a superfluid, it’s actually unexpectedly high. And as a platform for studying the notoriously mysterious high-T_c superconductivity, graphene offers a big experimental advantage over more standard high-T_c superconductors such as the cuprates and the pnictides. In a material family such as the cuprates, tuning the carrier-doping level requires fabricating a new material of slightly different composition. In contrast, doping a graphene bilayer with charge carriers is as simple as applying a small gate voltage, so researchers can explore the whole phase diagram with a single sample. (Y. Cao et al., Nature, in press, doi:10.1038/nature26160 .)