Pairing symmetries of several iron-based superconductor families and some similarities with cuprates and heavy-fermions

Abstract

Abstract. We show that, by using the unit-cell transformation between 1 Fe per unit cell to 2 Fe per unit cell, one can qualitatively understand the pairing symmetry of several families of iron-based superconductors. In iron-pnictides and iron-chalcogenides, the nodeless s ± -pairing and the resulting magnetic resonance mode transform nicely between the two unit cells, while retaining all physical properties unchanged. However, when the electron-pocket disappears from the Fermi surface with complete doping in KFe 2 As 2 , we find that the unitcell invariant requirement prohibits the occurrence of s ± -pairing symmetry (caused by inter-hole-pocket nesting). However, the intra-pocket nesting is compatible here, which leads to a nodal d-wave pairing. The corresponding Fermi surface topology and the pairing symmetry are similar to Ce-based heavy-fermion superconductors. Furthermore, when the Fermi surface hosts only electron-pockets in K y Fe 2-x Se 2 , the interelectron-pocket nesting induces a nodeless and isotropic d-wave pairing. This situation is analogous to the electron-doped cuprates, where the strong antiferromagnetic order creates similar disconnected electron-pocket Fermi surface, and hence nodeless d-wave pairing appears. The unit-cell transformation in K y Fe 2-x Se 2 exhibits that the d-wave pairing breaks the translational symmetry of the 2 Fe unit cell, and thus cannot be realized unless a vacancy ordering forms to compensate for it. These results are consistent with the coexistence picture of a competing order and nodeless d-wave superconductivity in both cuprates and K y Fe 1.6 Se 2 .

Evolution of London penetration depth with scattering in single crystals of K1−x Nax Kim1, Tanatar2, Liu3 et al. 2014Phys. Rev. B

“…Several theoretical explanations of this fact were suggested. [18][19][20] The observed doping evolution was explained in s ± pairing scenario as a result of the competition between inter-band pairing and intra-band Coulomb repulsion. 19,21 Alternatively, it was explained to be due to a phase transition between s ± -wave and d-wave superconducting states.…”

“…Several theoretical explanations of this fact were suggested [18][19][20]. The observed doping evolution was explained in s ± pairing scenario as a result of the competition between interband pairing and intraband Coulomb repulsion [19,21].…”