Seminarium ISQI - dr hab. Ipsita Mandal

Date, Time: 23.03, 15:00 - 16:00

Experiments on graphene bilayers, where the top layer is rotated with respect to the one below, have displayed insulating behavior when the moiré bands are partially filled. In the first part of the talk, I will elaborate on our calculations to find the static charge configurations in these phases, and to estimate the excitation gaps. In the second part of the talk, I will focus on the properties of a metal in a triangular lattice that undergoes a quantum phase transition to a valley-polarized nematic state. Besides breaking the threefold rotational symmetry of the triangular moiré superlattice, this type of order also breaks twofold rotational and time-reversal symmetries. At zero temperature, the ordered state displays a pseudo-Goldstone mode due to the existence of a dangerously irrelevant coupling λ in the 6-state clock model that describes the valley-polarized nematic quantum critical point. Using a two-patch model, we compute the fermionic self-energy to show that down to very low energies, the Yukawa-like coupling between the pseudo-Goldstone mode and the electronic degrees of freedom promotes the emergence of non-Fermi liquid behavior. Below a crossover energy scale, Fermi liquid behavior is recovered.