First-principles calculation of gate-tunable magnetism in twisted bilayer graphene*

Magic angle twisted bilayer graphene (MAtBLG) is believed to be a highly tunable platform for investigating strongly correlated phenomena such as high-T_c superconductors and quantum spin liquids, due to easy control of doping level through gating and sensitive dependence of the magic angle on hydrostatic pressure. Experimental observations of correlated insulating states, unconventional superconductivity and ferromagnetism in MAtBLG indicate that rich exotic phases exist in this system, which is also suggested by various theoretical works. In this work, with a combination of density functional theory and effective screening medium method, we systematically study the ground state of twisted bilayer graphene at magic twist angle 2.88° under pressure and simulate how the ground state evolves when doping level is gate-tuned. We find that at zero doping, a ferromagnetic solution showing half-metallic behavior with spin density localized at AA stacking sites is lower in energy than the trivial spin non-polarized solution. Interestingly, the magnetic moment per moiré unit cell of this ferromagnetic state decreases upon both electron and hole doping and vanishes at four electron/holes doped per moiré unit cell.