Symplectic-N t-J model and s[subscript ±] superconductors

Abstract

The possible discovery of s[subscript ±] superconducting gaps in the moderately correlated iron-based superconductors has raised the question of how to properly treat s[subscript ±] gaps in strongly correlated superconductors. Unlike the case of the d-wave cuprates, the Coulomb repulsion does not vanish by symmetry, and a careful treatment is essential. Thus far, only weak-correlation approaches have included this Coulomb pseudopotential, which has motivated us to introduce a symplectic-N treatment of the t − J model that incorporates the strong Coulomb repulsion through the complete elimination of on-site pairing. Through a proper extension of time-reversal symmetry to the large-N limit, symplectic N allows, for the first time, a superconducting large-N solution of the t − J model. For d-wave superconductors, the previous uncontrolled mean-field solutions are reproduced, while for s[subscript ±] superconductors, the SU(2) constraint enforcing single occupancy acts as a pair chemical potential adjusting the location of the gap nodes. This adjustment can capture the wide variety of gaps proposed for the iron-based superconductors: line and point nodes, as well as two different, but related full gaps on different Fermi surfaces.