First-order transition from superfluid to bose-metal state in systems with resonant pairing.

Systems showing resonant superfluidity, driven by an exchange coupling of strength g between uncorrelated pairs of itinerant fermions and tightly bound ones, undergo a first-order phase transition as g increases beyond some critical value gc. The superfluid phase for ggc this state gives way to a phase-uncorrelated bosonic liquid with a q2 spectrum.

Bogoliubov shadow bands in the normal state of superconducting systems with strong pair fluctuations.

On the basis of a scenario where electron pairing is induced by resonant two-particle scattering (the boson-fermion model), we show how precursors of the superconducting state-in the form of overdamped Bogoliubov modes-emerge in the normal state upon approaching the transition temperature from above. This result is obtained by a renormalization technique based on continuous unitary transformations (the flow equations), projecting out the coherent contributions in the electron spectral function from an incoherent background.

Pseudogap phase in high- T(c) superconductors

We describe the approach of the superconducting state as a sequence of crossover phenomena. As the temperature is decreased, uncorrelated pairing of the electrons leads to the opening of a pseudogap at T(*)(F). Upon further lowering the temperature those electron pairs acquire well behaved itinerant features at T(*)(B), leading to partial Meissner screening and Drude-type behavior of the optical conductivity. Further decrease of the temperature leads to their condensation and superconductivity at T(c). The analysis is done on the basis of the boson-fermion model in the crossover regime between 2D and 3D.