Upping the pressure in bilayer graphene: The discovery of superconductivity and exotic insulating phases in twisted bilayer graphene has established this material as a model system of strongly correlated electrons. To achieve superconductivity, the two layers of graphene need to be at a very precise angle with respect to each other. Yankowitz et al. now show that another experimental knob, hydrostatic pressure, can be used to tune the phase diagram of twisted bilayer graphene (see the Perspective by Feldman). Applying pressure increased the coupling between the layers, which shifted the superconducting transition to higher angles and somewhat higher temperatures. Science , this issue p. 1059 ; see also p. 1035
Twisted bilayer graphene goes magnetic: When two layers of graphene in a bilayer are twisted with respect to each other by just the right, “magic,” angle, the electrons in the system become strongly correlated. As the electronic density is tuned by gating, the system goes through several exotic phases, including superconductivity. Now, Sharpe et al. show that, at a particular electronic density, magic-angle graphene becomes magnetic (see the Perspective by Pixley and Andrei). The finding is supported by the observation of a large anomalous Hall effect. Science , this issue p. 605 ; see also p. 543
Quantum anomalous Hall goes intrinsic: Quantum anomalous Hall effect—the appearance of quantized Hall conductance at zero magnetic field—has been observed in thin films of the topological insulator Bi 2 Se 3 doped with magnetic atoms. The doping, however, introduces inhomogeneity, reducing the temperature at which the effect occurs. Two groups have now observed quantum anomalous Hall effect in intrinsically magnetic materials (see the Perspective by Wakefield and Checkelsky). Serlin et al. did so in twisted bilayer graphene aligned to hexagonal boron nitride, where the effect enabled the switching of magnetization with tiny currents. In a complementary work, Deng et al. observed quantum anomalous Hall effect in the antiferromagnetic layered topological insulator MnBi 2 Te 4 . Science , this issue p. 900 , p. 895 ; see also p. 848