Isotropic strain effects on twisted bilayer graphene near the magic angle

In recent years, the magic angle of graphene has caused many researchers to focus on twisted bilayer graphene at the magic angle and the various ground states it manifests including superconductivity, ferromagnetism, topological states, etc. However, graphene is also attractive for its extremely high mechanical strength and endurance and it may be asked how these states respond to high levels of mechanical strain. Indeed, it is thought that built-in strain during sample fabrication may explain some sample-to-sample variation that exists in the literature [1]. By fabricating samples on flexible substrates and applying displacement to them out of plane isotropically, we are able to apply isotropic strains up to several percent, which we calibrate by directly measuring the shift of alignment marks on the substate and by recording the response of single layer graphene Landau levels to such strain. Applying this technique to twisted bilayer graphene samples away from the magic angle, we are able to tune the Hofstadter butterfly of the system with strain. Finally, studying samples closer to the magic-angle, in preliminary data, we find that strain tunes a correlated non-magnetic state at moire miniband filling factor equal to one (quarter filling) to a ferromagnetic state, detected via a gate-tunable anomalous Hall effect. Thus, we expect our strain technique to play an important role in tuning the superlattice and resultant ground states of twisted bilayer graphene.

[1] C. N. Lau, M. W. Bockrath, K. F. Mak, and F. Zhang, Nature 602, 7895 (2022)