Seminars

Moiré floppiness: phasons, strain, and electrons in twisted bilayer graphene

Sliding and twisting the layers of van der Waals materials give rise to superstructures with new physics. In these systems, structural inhomogeneities and vibrational modes can strongly impact the electronics due totheir geometrical amplification in the moiré beating pattern. In this talk, I will argue that the long-wavelength lattice dynamics of these generically incommensurate structures are dominated by new collective modes, phasons. These modes correspond to coherentsuperpositions of optical phonons describing the sliding motion of stacking domain walls separating regions of partial interlayer commensuration. The low stiffness of this soliton network makes the system very sensitive to structural disorder in the form ofstrains accumulated between the layers, or heterostrain. I will explain how the presence of heterostrain favors different forms of symmetry breaking in twisted bilayer graphene, which could be the origin of some of the disparities observed in transport measurementsin different devices. In particular, I will show that heterostrain favors an excitonic instability of the Fermi surface close to the neutrality point, where the spontaneous condensation of electron-hole pairs breaks time reversal and valley symmetries. Thismechanism may explain the occurrence of insulating states in the most homogenous samples, where uniform strain fields contribute both to stabilizing the relative orientation between layers and to the formation of an excitonic gap.