Macroscopic structural coherence in two-component superconductivity

Abstract

In two-component theory 1,2 pairing arises from localized negative-U states and mobility arises from extended single particle states. A small hybridization of localized and extended states enables mobility and pairing to provide a high T c. RPA analysis of the “normal” state implies uncondensed charged pairs carry current, while long lived single particle excitations are neutral electron-hole hybrids. At T c pairs condense and single particle states undergo Cooper pairing. In the superconducting state pair-pair excitations exist in the BCS-like fermionic gap. Signatures of this theory range from distinctive T c, Δ, H c, ξ, conductance anomalies in sound and bulk modulii at T c, linear temperature dependence of normal state resistivity, 2e charge carriers in the normal state, linear voltage dependence in normal-state-tunneling conductance, and finite zero-bias conductance in superconducting state tunneling. Quantitative comparisons with superconducting properties of YBa 2Cu 3O 7 were presented. 1 A distinctive signature is the prediction of dynamical structural correlations which are local above T c and macroscopic below T c. Experiments provide direct evidence for such dynamical correlations: neutron diffraction “thermal ovals”, 3 channeling experiment cross section changes as a function of temperature near T c, 4,5 pair-distribution-function neutron diffraction including enelastic and elastic scattering showing direct evidence for dynamic correlations which change at T c, 6 and EXAFS showing a large dynamical displacement of oxygen atoms tunneling between sites separated by 0.13Å, 7 In two-component theory strong lattice coupling is consistent with low isotope shifts since tunneling occurs by a virtual Frank-Condon transition. 1,2 Predictions for the dynamical structure factor are presented.