BEC-BCS crossover, condensed matter experiments

The BCS-BEC crossover indicates the qualitative change in physical properties of Fermions with attractive interactions from the weakly to strongly interacting regime, offering an important guideline for maximizing Tc with given interactions. Though Uemura et al. (1991) pointed out that high-Tc cuprates and other exotic superconductors can be close to the crossover regime, the superconducting gap, Δ, is typically more than two orders of magnitude smaller than the Fermi energy, EF. This suggests that these materials are still far from the crossover regime. Recently, several classes of new superconductors with comparable Δ and EF have been discovered. Furthermore, the density-controlled BCS-BEC crossover has been achieved in gate-controlled two-dimensional superconductors. We review the recent progress of superconductor research from the viewpoint of the BCS-BEC crossover.