Two-Component Superconductivity

Abstract

There are two essential attributes of electrons for superconductivity: mobility and pairing. While this is not directly obvious, these two attributes compete against each other. High-temperature superconductivity may be obtained by combining two-components, one which provides pairing and the other mobility. In the local-strong-pairing (negative-U lattice) limit of superconductivity Tc is controlled by the hopping of electron pairs rather than by the pair binding energy. Coupling a negative-U lattice to delocalized electron states increases the hopping and thus the critical temperature. In parallel, Cooper-pair superconductivity is induced in the delocalized electrons. In the normal state both Bosonic and Fermionic states exist, and below Tc Bosonic states exist in the Fermionic gap. It is suggested that superconductivity in the new class of oxide superconductors is due to locally paired electrons on the lattice of oxygen vacancies combined with conducting metal-oxide layers. The discussion includes the superconducting properties Tc, Δ, Hc and ξ, long wave collective excitations, normal state properties including resistance and tunneling, and the isotope shift. Unusual properties are predicted including neutral Fermion excitations due to hybridization of electrons and holes, a spreading of the Femionic gap onset, a separation between the resistive transition Tc’ and the evaporation of the condensate Tc, anomalies in sound and bulk modulii at Tc, linear temperature dependence of normal state resistivity, linear voltage dependence in normal state tunneling conductance, and finite zero bias conductance in superconducting state tunneling. A new signature of structural coherence which can be seen in channeling experiments and other structural probes is indicated.