$S^{++}_{ee}$-wave as a hidden $S^{\pm}_{eh}$-wave pairing in iron-selenide superconductors    [PDF]

Yuan-Yuan Xiang, Yang Yang, Wan-Sheng Wang, Zheng-Zao Li, Qiang-Hua Wang
In iron-selenide supreconductors the hole bands sink below the Fermi level, causing a challenge to the $S^{\pm}_{eh}$ pairing based on the quasi-nesting between the electron and hole pockets. The proposed candidate pairing symmetry ranges from nodeless d-wave to $S^{\pm}_{ee}$ between the hybridization-split electron pockets. By functional renomalization group study we show that for the hybridization relevant to experiments the gap functions are in phase on the electron pockets. This $S^{++}_{ee}$-wave pairing is in fact a hidden $S^{\pm}_{eh}$-wave pairing as if the hole pockets were present. The underlying mechanism is the competition between the checkerboard spin fluctuations on electron pockets and Cooperon excitations on (virtual) hole pockets. As a hidden $S^{\pm}_{eh}$ pairing, the $S^{++}_{ee}$ pairing leads to renormalized spin fluctuations consistent with neutron scattering data. The in-phase gap on the fermi surface makes it robust against impurities and could benefit from electron-phonon couplings. We further propose that the quasiparticle interference pattern in scanning tunneling microscopy and the Andreev reflection in out-of-plane contact tunneling are efficient probes of $S^{++}_{ee}$ versus $S^{\pm}_{ee}$ pairing.
View original: http://arxiv.org/abs/1305.1806