Strain-Induced Ferromagnetism in Twisted-Bilayer Graphene

Twisted bilayer graphene has recently proved to be an interesting two-dimensional system. Because the twisting between two graphene layers creates unit moiré superlattice cells on the order of the magnetic length for a perpendicular magnetic field of several tesla, the Hofstadter butterfly spectrum can be observed in the laboratory. When the twist angle is at 1.1 degrees, also known as the 'magic angle', the high density of states causes it to exhibit superconductivity, correlated insulating states, ferromagnetism and topological behavior, among others. This wide range of phenomena has raised the question: why do device properties vary so much from one device to the next? One idea is that the fabrication process introduces strain into the lattice, and the different strain experienced by different devices causes disparity in the results. To test this, we isotropically strained magic angle and near-magic angle TBLG devices to study how the observed phenomena changed with strain. We observed a device that behaved like a conductor in the absence of strain become ferromagnetic upon the application of strain. While this result suggests that strain does play an important role, more experiments are needed to fully understand the effects of strain.