Abstract

Twistronics is a new and exciting field in physics in which the components of layered systems are twisted or strained by small amounts with respect to one another, radically changing the properties of the system. In 2018 it was discovered that by twisting the layers in bilayer graphene by the ‘magic angle’ of 1.05 degrees, the system could exhibit superconducting or insulating behavior, neither of which are possible when untwisted. Very recently it was discovered twisted systems comprised of nonpolar layers could exhibit ferroelectricity [Nature 588, 71–76 (2020)]. However, the mechanism remains unclear.

Twisted systems form complex domain structures due to the interplay between the in-plane strains and the out-of-plane stacking energy. These domain structures are highly sensitive to the twist angle and lattice mismatch. I will discuss theoretical work and first-principles calculations which show that the domain structure can also be tuned using an applied electric field, and in a single sample. I will discuss how the domain structures may be responsible for ferroelectricity due to the polar response to their strain gradients via flexoelectricity.