Superconductivity in iron pnictides is studied by using a two-orbital Hubbard model in the large U limit. The Coulomb repulsion induces an orbital-dependent pairing between charge carriers. The pairing is found mainly from the scattering within the same Fermi pocket. The interpocket pair scatterings determine the symmetry of the superconductivity, which is extended s wave at small Hund's coupling, and d wave at large Hund's coupling and large U. The former is consistent with recent experiments of angle-resolved photoemission spectroscopy and Andreev reflection spectroscopy.
Possible superconductivity in the recently discovered (Tl,K)FexSe2 compounds is studied from the viewpoint of doped Mott insulator. The Mott insulating phase is examined to be preferred in the parent compound at x = 1.5 due to the presence of Fe-vacancies. Partial filling of vacancies at the Fe-sites introduces electron carriers and leads to electron-doped superconductivity. By using a two-orbital Hubbard model in the strong-coupling limit, we find that the s-wave pairing is more favorable at small Hund's coupling, and d x 2 −y 2 -wave pairing is more favorable at large Hund's coupling.
The magnetic-field-induced antiferromagnetic phase of the underdoped cuprates is studied within the t − t − J model. A magnetic field suppresses the pairing amplitude, which, in turn, may induce antiferromagnetism. We apply our theory to interpret the recently reported quantum oscillations in high magnetic field in ortho-II YBa2Cu3O6.5 [1] and propose that the total hole density abstracted from the oscillation period is reduced by 50% due to doubling of the unit cell.
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