Tuning flat bands in twisted bilayer graphene with pressure

Twisted bilayer graphene (tBLG) with rotational mismatch of ~1.1°, referred to as the “magic angle,” has emerged as an exciting new platform to host strongly correlated electronic states due to its very flat low-energy bands. The electronic bandwidth – and consequentially the strength of the correlated states – is determined by an interplay between the separation of the Dirac cones of the two graphene layers in momentum space and the strength of the interlayer electronic coupling which hybridizes the bands. Here we demonstrate the capability to vary the bandwidth at fixed rotation angle by using hydrostatic pressure to modify the interlayer coupling [1]. For angles larger than the native magic angle, we can tune to the flat band condition with pressure, inducing correlated insulating states and superconductivity for both hole- and electron-type carriers. For a 1.27° twist angle, T_c increases above 3 K at ~1.3 GPa, and then diminishes with further pressure. This behavior is consistent with theoretical efforts to model the relationship between bandwidth and interlayer coupling strength, establishing layer compression as a viable route to engineering the energy scale of the superconductivity in this system. Improvements in device quality additionally allow us to resolve new sequences of quantum oscillations emerging from quarter-, half-, and three-quarters-filling of the moiré unit cell associated with the presence of new Fermi surfaces.

[1] M. Yankowitz et al., arXiv:1808.07865 (2018)