Strong pairing in two dimensions: Pseudogaps, domes, and other implications

The recent interest in superconductivity of strongly correlated 2D materials is driven by exciting discoveries of novel superconductors such as magic-angle twisted bilayer graphene (MATBG), FeSe monolayers and transition metal dichalcogenides. These are generally thought to be distinct from BCS-Eliashberg superconductors, and can be argued, from the Uemura plot, to be intermediate between BCS and Bose-Einstein condensation (BCS-BEC). In this talk we compute the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature T_BKT and pairing onset T* as a function of pairing interaction strength g and of density using this crossover theory. Rather than solving directly for the phase stiffness parameter we follow the literature on atomic Bose systems (which is based on a Quantum Monte Carlo analysis) and which constrains the total phase space density at the transition. Our results for T_BKT compare favorably with Fermi gas experiments and yield the expected BCS and BEC asymptotes for a film with lattice dispersion. Given the measured T_BKT, we provide estimates for the pairing gap and for T* in concrete systems such as MATBG.