Magnetism, superconductivity, and pairing symmetry in iron-based superconductors

TLDR

The study uses an itinerant model of small electron and hole pockets near (0,0) and (π,π) to analyze antiferromagnetism and superconductivity in Fe‑based superconductors. The authors find that antiferromagnetism and superconductivity are governed by logarithmically flowing interactions that become equal at low energies, so that a magnetic instability dominates for equal pocket sizes but is overtaken by superconductivity upon doping; the resulting nodeless extended‑s gap changes sign between Fermi surfaces, and its spin susceptibility and NMR relaxation rate are exponential only at very low temperatures but can be described by power laws over a broad range below Tc.

Abstract

We analyze antiferromagnetism and superconductivity in novel Fe-based superconductors within the itinerant model of small electron and hole pockets near (0,0) and $(\ensuremath{\pi},\ensuremath{\pi})$. We argue that the effective interactions in both channels logarithmically flow toward the same values at low energies; i.e., antiferromagnetism and superconductivity must be treated on equal footing. The magnetic instability comes first for equal sizes of the two pockets, but loses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces (extended $s$-wave symmetry). We argue that the $T$ dependencies of the spin susceptibility and NMR relaxation rate for such a state are exponential only at very low $T$, and can be well fitted by power laws over a wide $T$ range below ${T}_{c}$.