Physics – Condensed Matter – Superconductivity
Scientific paper
2010-08-25
Physics
Condensed Matter
Superconductivity
17 pages, 4 figures
Scientific paper
The pseudogap state of cuprates has been often viewed as either a yet unknown competing order or a precursor state to superconductivity. While awaiting the resolution of this pseudogap problem, we show that local pairing fluctuations, vortex liquid dynamics and other precursor phenomena can emerge quite generally whenever fermionic excitations remain gapped across the superconducting transition, regardless of the gap origin. Our choice of a tractable model is a lattice-generated band insulator with short-range attractive interactions between fermions which can induce pairing in the s-wave channel. The analysis benefits from the accumulated knowledge about unitary scattering and the crossover between Bardeen-Cooper-Schrieffer (BCS) and Bose-Einstein condensate (BEC) regimes of fermionic superfluids. An effective BEC-BCS crossover can be identified in any band insulator above two dimensions, while in two dimensions only the BEC regime exists. The superconducting transition is ``unconventional'' (non-pair-breaking) in the BEC regime, identified by either the bosonic mean-field or XY universality class. The insulator adjacent to the superconductor in the BEC regime is a bosonic Mott insulator of Cooper pairs, which may be susceptible to charge density wave ordering. We construct a function of the many-body excitation spectrum whose non-analytic changes define a sharp distinction between band and Mott insulators. The corresponding ``second order transition'' can be observed out of equilibrium by driving a Cooper pair laser in the Mott insulator. We also prove that the Cooper pair insulator is a stable state by showing that the gap for charged bosonic excitations lies below the threshold for Cooper pair breakup in any BEC regime. Our discussion ends with a view of possible consequences for cuprates, where antinodal pair dynamics has certain features in common with our simple s-wave picture.
Nikolic Predrag
Tesanovic Zlatko
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