A Twist in the Tale: Almost Flat Bands and Strongly Interacting Electrons in Moire Structures

If two dimensional materials are stacked on top of each other, their electronic structure can be tuned by rotating the crystal axes of the layers with respect to each other. At specific values of twist angles, called magic angles, the low energy electrons have almost flat bands and strong electronic correlations determine the fate of the system. After a brief review of novel phenomena in this class of systems, I will focus on the twisted double bilayer graphene, which has small electron and hole pockets. This leads to characteristic signatures in magneto-transport in this material. The strong electronic interactions lead to formation of excitonic condensates at very low temperatures. At intermediate temperatures, Landau damped excitonic fluctuations in a critical metal lead to sublinear temperature dependence of resistivity. This non-Fermi liquid behaviour gives way to linear temperature dependence at higher temperatures.