BCS-BEC crossover: From high temperature superconductors to ultracold superfluids

摘要

We review the BCS to Bose-Einstein condensation (BEC) crossover scenario which is based on the well known crossover generalization of the BCS ground state wavefunction &UPsi;(0). While this ground state has been summarized extensively in the literature, this Review is devoted to less widely discussed issues: understanding the effects of finite temperature, primarily below T, in a manner consistent with &UPsi;(0). Our emphasis is on the intersection of two important problems: high T, superconductivity and superfluidity in ultracold fermionic atomic gases. We address the "pseudogap state" in the copper oxide superconductors from the vantage point of a BCS-BEC crossover scenario, although there is no consensus on the applicability of this scheme to high T, We argue that it also provides a useful basis for studying atomic gases near the unitary scattering regime; they are most likely in the counterpart psendogap phase. That is, superconductivity takes place out of a non-Fermi liquid state where preformed, metastable fermion pairs are present at the onset of their Bose condensation. As a microscopic basis for this work, we summarize a variety of T-matrix approaches, and assess their theoretical consistency. A close connection with conventional superconducting fluctuation theories is emphasized and exploited. © 2005 Elsevier B.V. All rights reserved.