Strongly correlated superconductivity

Band theory and BCS theory are arguably the most successful theories of condensed matter physics. Yet, in a number of materials, in particular the high-temperature superconductors and the layered organic superconductors, they fail. In these lecture notes for an international school, I emphasize that even though the low energy properties of a phase of matter are generally emergent and entirely determined by the broken symmetry, there are many differences between a strongly correlated and a weakly correlated state of matter. For example, spin waves are an emergent property for antiferromagnets, but for weak correlations the normal phase of a (Slater) antiferromagnet is metallic, whereas it is insulating (Heisenberg) for strong correlations. As a function of interaction strength, above the antiferromagnetic phase, the crossover between a metal and a local moment paramagnetic insulator is described by the Mott transition, whose mean-field theory down to T=0 is best formulated with dynamical mean-field theory. A similar situation occurs for superconductors: despite similar emergent properties, there are many differences between both kinds of superconductors. Experimental evidence suggests that hole-doped cuprates are strongly correlated whereas the correlations are weaker in the electron-doped case, especially near optimal doping. Evidence for a pseudogap arising from antiferromagnetic fluctuations is strong in the latter case, whereas in the hole-doped case the pseudogap temperature appears as a consequence of Mott physics. In cluster dynamical mean-field theory, the pseudogap line T* is a Widom line arising from a T=0 first-order transition that terminates at a finite T critical point. Strongly correlated superconductors are much more resilient to near-neighbor repulsion than their weakly correlated counterpart. Many different methods to attack these problems theoretically are described.