Physics (Internal)

Mechanism for Superconductivity and Competing Excitonic Order in Honeycomb Superlattices

This event is part of the Preliminary Oral Exam.

Honeycomb superlattices — periodic potential wells with the same symmetries as graphene but in which the lattice spacing and potential profile can be tuned experimentally — have attracted immense interest in recent years, in particular in the context of Moire and semiconductor heterostructures. I will explain how these materials can realize a new mechanism for superconductivity, which arises due to the presence of a winding number in the electron wavefunction. The winding number of the Dirac fermion causes destructive interference between states with opposite momenta, leading to the novel effect whereby electrons can partially avoid the Coulomb repulsion by forming Cooper pairs. This gives rise to an effective pairing attraction, which is enhanced by doping due to an antiscreening effect, eventually leading to superconductivity. In systems where the superlattice is tuned to flatten the bands, the effective attraction gives rise to an excitonic insulating order which competes with superconductivity. The resulting phase diagram contains several phenomenologically novel states, and the effects of Rashba and intrinsic spin orbit on the phase diagram are discussed.