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

Abstract

We analyze the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature TBKT as a function of attractive pairing interaction of strength g, in strongly correlated 2D superconductors. Our approach combines the BKT theory for bosons with BCS-BEC crossover theory for fermionic systems. Here we adopt a formulation of the BKT instability which is based on a widely applied Quantum Monte Carlo analysis. This determines TBKT, in terms of bosonic variables corresponding to their density and mass; these, in turn, are evaluated using a strong pairing fluctuation approach for BCS-BEC crossover. Our calculations lead to the surprising observation that TBKT vs g decreases with g in the more accessible, intermediate coupling regime. This leads to a robust superconducting dome generally followed by a long tail from the BEC asymptote. Importantly, these features are also consistent with observations in atomic Fermi superfluids. We show how, given the measured size of TBKT, our calculations establish the magnitude of related quantities such as the excitation (pseudo)gap and coherence length; this should be relevant to the broad class of strongly correlated 2D superconductors.