Physics – Condensed Matter – Superconductivity
Scientific paper
2008-05-27
Physics
Condensed Matter
Superconductivity
12 pages, 5 figures
Scientific paper
It is demonstrated that SC mechanism of doped Fe-based compounds is characteristic for itinerant electron systems with coexistence of both (e-e)- and (e-h)-pairing arising due to electron-phonon and Coulomb interactions, respectively. The higher Tc of the SC transition here (as compared with conventional (LTSC) BCS-systems without (e-h)-pairing) is a natural consequence of (e-e)-pairing at the background of high density of states which arises in the narrow energy range near dielectric-(SDW)-gap (pseudogap) edges due to removing of electronic states from the energy region of dielectric (SDW) gap (pseudogap) (already formed at the part of the Fermi surface in the normal state with onset temperature T* (near the same for corresponding structural transition) due to (e-h)-pairing). Below Tc the system enters the coexistence (SC+SDW) state. The SDW formed is incommensurate with lattice and dynamic in character. The phase diagram for such system is determined: the doping dependence of the SC gap (and Tc) has a maximum (optimal doping) while the dielectric (SDW) gap (pseudogap) (and T*) is a decreasing function of doping. These conclusions follow from detailed analysis of available resistivity and another data for Fe-based superconductors on the basis of model with partial dielectrization of electron energy spectra. The picture obtained and manifestation of two order parameters (SC and SDW) in experiments, first of all, in threshold phenomena are discussed. The comparison with the case of cuprates is performed.
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