Topological Superconductivity Could be a Twist Away

No one would dispute that graphene is an exciting material, or that its discoverers were worthy of a Nobel prize [1]. But if you’re a lover of materials in which electrons are strongly correlated, graphene’s single layer of carbon atoms hasn’t been much of a draw. That impression changed earlier this year, when two exceptional experiments were reported on “twisted graphene.” This atypical form of graphene consists of a pair of closely spaced graphene layers rotated so that their carbon atoms are slightly out of alignment (Fig. 1). The first experiment showed that a small twist angle caused the conduction electrons to form a Mott insulator [2], in which the charges are “frozen” onto distinct lattice sites by strong electron-electron interactions. The second experiment revealed that the insulator can be transformed into a superconductor by using various applied voltages to control the electron density in the graphene layers [3]. This observed transition from Mott insulator to superconductor suggests that graphene exhibits an unconventional form of superconductivity, as found in high-temperature (high-Tc) superconductors.