&Bullet; physics 14, p83
Numerical simulations show that discrepancies between experiments on graphene bilayers are due to minute strains exerted on the samples.
Stacking two layers of graphene and rotating them relative to each other by a certain “magic” angle induces a rich palette of phases in the system, including superconductivity (see Viewpoint: Graphene Reveals its Strange Side). Experimentalists have been mapping these phases since 2018, when the idea of the “twisted bilayer graphene” (TBG) was first presented. But where some groups have found an insulating phase, others have observed a semimetal. Now Daniel Parker from Harvard University and colleagues, using numerical simulations, have identified elongation as the decisive factor that could explain these different results  .
In TBG with magic angle, the grids of two graphene layers are offset by about 1.1 °. This misalignment creates a moiré pattern that repeats over a length scale that is much longer than that of the crystal structure of graphene. (The unit cell of graphene consists of only a few carbon atoms, that of TBG consists of thousands.) This crystalline complexity poses a challenge for researchers who study the material using numerical methods.
Parker and colleagues got around this problem by simulating a small number of low-energy electronic bands around the Fermi level of the material and “freezing” electrons with energies outside this range. The simulation of only this subset enabled them to model a sufficiently large TBG system to reveal the special properties of the material.
The results of their simulations show that the phase transition from an insulator to a semi-metal can be controlled by applying a heterovoltage of 0.1–0.2% to the graphene layers. This small stretch falls within the range expected for experiments. The researchers say it will be difficult to eliminate the stress from experiments, but if possible, the stress could eventually become an important “tuning regulator” for the material.
Marric Stephens is the corresponding editor for physics based in Bristol, UK.
- DE Parker et al., “Stress-induced quantum phase transitions in graphs with a magic angle”, Phys. Rev. Lett.127, 027601 (2021).