THE CUPRATE PSEUDOGAP: PRECURSOR SUPERCONDUCTIVITY WITHOUT PREFORMED PAIRS

Abstract

We investigate a scenario for the cuprate pseudogap which is based on strong pairing correlations above T c. This picture is motivated by the small size of the coherence length, which suggests that these superconductors are in a crossover regime between the BCS (large pair size) and Bose–Einstein (small pair size) limits. Moreover, the observed smooth evolution of the pseudogap to the superconducting gap reinforces the notion that pairing correlations may be responsible for the pseudogap. Our new theoretical approach to this well-known crossover is based on a diagrammatic theory proposed by Kadanof and Martin (and extended by Patton), which self consistently treats the coupling between the single particle and pair propagators. The pseudogap, which is present only for an intermediate pair size (or equivalently, intermediate values of the coupling constant), correlates well with the onset of a resonance in the pair propagator, as expected for a state intermediate between free and bound fermions. Our results are consolidated into a phase diagram for T c and T*. We therefore demonstrate how the onset of superconductivity occurs in a pseudogapped normal state, and contrast our approach with alternate theories in the literature.