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

TL;DR

This paper investigates the interplay of magnetism and superconductivity in Fe-based superconductors, revealing that both phenomena are closely linked through similar low-energy interactions, with a sign-changing s-wave gap and specific temperature dependencies.

Contribution

It demonstrates that antiferromagnetism and superconductivity in Fe-based superconductors are governed by the same low-energy interactions, with a detailed analysis of the pairing symmetry and temperature behavior.

Findings

Superconducting gap has no nodes but changes sign between Fermi surfaces.

Magnetic and superconducting instabilities are linked through similar interactions.

Temperature dependencies of spin susceptibility and NMR relaxation rate can be fitted by power-laws.

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 $(\pi,\pi)$. We argue that the effective interactions in both channels logarithmically flow towards the same values at low energies, {\it i.e.}, antiferromagnetism and superconductivity must be treated on equal footings. The magnetic instability comes first for equal sizes of the two pockets, but looses 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 state are exponential only at very low $T$, and can be well fitted by power-laws over a wide $T$ range below $T_c$.