Theory of NMR and specific heat for extended s-wave and d-wave pairing states in high Tc superconductors
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The recent discoveries of the various high-temperature oxide superconductors have given rise to many questions regarding the actual mechanism for the BCS pairing in these materials, beyond the usual phonon-exchange mechanism. In the presence of strong Coulomb correlations and spin fluctuations, there are new theoretical attempts to abandon even the concept of the BCS pairing in these materials. A recent formulation of the BCS pairing approach, suitable for the new materials with layered structures, will be used to discuss various possible phonon and electronic exchange mechanisms. It will be argued that for a quantitative comparison of experimental results with the theoretical predictions of the BCS pairing theory, one has to go beyond the so called “BCS predictions” which are valid only in the framework of an extremely simplified one is still far away from establishing the exact pairing mechanism in these new materials, there is no convincing reason at present to show that the BCS pairing theory will not be applicable to them.
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The BCS theory models electron correlations with pure zero-momentum pairs. I consider a family of pairing Hamiltonians, where the electron correlations are modelled with pure arbitrary-momentum pairs. I present exact solutions to this family. The BCS pairing, the \eta pairing and the d -wave pairing in T_c superconductivity are the family members. These models, expect for the BCS model, are legitimate only on an xy plane defined by the electron spin S_z, compatible with the two dimensionality of high temperature superconductivity. Surprisingly, all parings are on equal footing in the xy plane, suggesting a unification of the s-wave and d-wave theoretical mechanisms in high T_c superconductivity. I also give the extension that includes all members of this family.
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Pairing symmetry which characterizes the superconducting pairing mechanism is normally determined by measuring the superconducting gap structure ($|{\mathrm{\ensuremath{\Delta}}}_{k}|$). Here, we report the measurement of a strain-induced gap modulation ($\ensuremath{\partial}|{\mathrm{\ensuremath{\Delta}}}_{k}|$) in uniaxially strained ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ utilizing angle-resolved photoemission spectroscopy and in situ strain tuning. We found that the uniaxial strain drives ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ into a nematic superconducting state which breaks the fourfold rotational symmetry of the superconducting pairing. The superconducting gap increases on the ${d}_{yz}$ electron and hole pockets while it decreases on the ${d}_{xz}$ counterparts. Such orbital selectivity indicates that orbital-selective pairing exists intrinsically in non-nematic iron-based superconductors. The ${d}_{xz}$ and ${d}_{yz}$ pairing channels are balanced originally in the pristine superconducting state, but become imbalanced under uniaxial strain. Our results highlight the important role of intraorbital scattering in mediating the superconducting pairing in iron-based superconductors. It also highlights the measurement of $\ensuremath{\partial}|{\mathrm{\ensuremath{\Delta}}}_{k}|$ as an effective way to characterize the superconducting pairing from a perturbation perspective.
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A theoretical study of BCS pairing in an extended Hubbard model with on-site repulsion (U) and a BCS pairing field (V) is presented. Treating the effect of U in the Gutzwiller approximation, we study the effect of increasing U in the Fermi-liquid phase, on the BCS pairing due to V. It is found that the superconducting energy gap (\ensuremath{\Delta}) is strongly enhanced in the correlated metallic phase near half-filling of the band due to the localization effects of U. Further, the ground state is found to be superconducting even when \ensuremath{\Vert}V\ensuremath{\Vert}U, contrary to the prediction in a Hartree-Fock treatment.
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