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Abstract
Twisted two-dimensional structures open new possibilities in band structure engineering. At magic twist angles, flat bands emerge, which gave a new drive to the field of strongly correlated physics. In twisted double bilayer graphene dual gating allows changing of the Fermi level and hence the electron density and also allows tuning of the interlayer potential, giving further control over band gaps. Here, we demonstrate that by application of hydrostatic pressure, an additional control of the band structure becomes possible due to the change of tunnel couplings between the layers. We find that the flat bands and the gaps separating them can be drastically changed by pressures up to 2 GPa, in good agreement with our theoretical simulations. Furthermore, our measurements suggest that in finite magnetic field due to pressure a topologically nontrivial band gap opens at the charge neutrality point at zero displacement field.
Highlights
Twisted van der Waals heterostructures recently opened a new platform to explore correlated electronics phases
Electron interaction can become comparable to the kinetic energy and novel, correlated phases form.[3−13] The ability to control the size of the moiré unit cell size with the twist angle and implicitly the bandwidth of the low energy bands led to the discovery of various correlated phases[3,7−11] such as correlated insulator states,[3−6,8] ferromagnetic phase with a signature of the quantum anomalous Hall effect,[7−9,13] and unconventional superconducting phases resembling hightemperature superconductors[4−6,12] in twisted bilayer graphene (TBG)
As a result of pressure, the flat bands slightly narrow down and the dispersive bands shift down in energy for the electron side and close the gaps at ±ns. (Later, we show the D dependence of the gaps at p = 2 GPa.) in Figure 2a there is a sign of emerging correlated phases at half-filling, which disappears at p = 2 GPa, similar to what was observed in magic-angle bilayer graphene.[5,40,49]
Summary
Twisted van der Waals heterostructures recently opened a new platform to explore correlated electronics phases. These structures consist of two or more layers of 2D materials, e.g., graphene, placed on top of each other, with a well-defined rotation angle between their crystallographic axes. Electron interaction can become comparable to the kinetic energy and novel, correlated phases form.[3−13] The ability to control the size of the moiré unit cell size with the twist angle and implicitly the bandwidth of the low energy bands led to the discovery of various correlated phases[3,7−11] such as correlated insulator states,[3−6,8] ferromagnetic phase with a signature of the quantum anomalous Hall effect,[7−9,13] and unconventional superconducting phases resembling hightemperature superconductors[4−6,12] in twisted bilayer graphene (TBG). The correlation effects are often accompanied by nontrivial topology of the bands.[13−25]
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