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The arrangement of atoms in solids and molecules, together with the resulting distribution of electrons within them, dictates many of their physical properties. However, emergent phenomena may arise when solids or molecules are combined to form superstructures. A prime example is the ability to stack atomically thin two-dimensional (2D) crystals into heterostructures (1). Such van der Waals (vdW) heterostructures enable tailoring of electronic properties through control over the twist angle between layers. On page 1059 of this issue, Yankowitz et al. (2) show that applying pressure to modify the interlayer separation of a twisted bilayer graphene device provides a second control parameter to tune between regimes with strong and weak electronic interactions in a single sample.

Collective effects that result from interacting electrons underlie many exotic physical phenomena across multiple materials systems, ranging from strongly correlated materials such as high-temperature superconductors to the fractional quantum Hall effect in 2D electron systems subjected to a strong magnetic field. The common thread that connects these phenomena is that Coulomb repulsion dominates over the kinetic energy of individual electrons because the electronic bands are flat, which allows many electronic states to pile up over a narrow range of energies.

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Category: Science