A Numerical Renormalization Group Study of the Superconducting and Spin Density Wave Instabilities in MFeAsO$_{1-x}$F$_x$ Compounds

TL;DR

This study uses the fermion renormalization group method to analyze instabilities in FeAs-based superconductors, revealing spin density wave order at half filling and unconventional pairing tendencies away from half filling.

Contribution

It applies a numerical fermion renormalization group approach to a two-band model, uncovering detailed pairing symmetries and magnetic orderings in FeAs-based materials.

Findings

Spin density wave order at half filling with nodal gaps.

Unconventional s-wave and d-wave pairing instabilities away from half filling.

Inter-pocket pair hopping as the pairing mechanism.

Abstract

We apply the fermion renormalization group method, implemented numerically by Honerkamp et.al., to a two-band model of FeAs-based materials. At half filling we find the $(\pi,0)$ or $(0,\pi)$ spin density wave order and a sub-dominant superconducting pairing tendency. Due to a topological reason, the spin density wave gap has nodes on the fermi surfaces. Away from half filling we find an unconventional s-wave and a sub-dominant $d_{x^2-y^2}$ pairing instability. The former has $s$ symmetry around the hole fermi surface but exhibits $s+d_{x^2-y^2}$ symmetry around the electron pockets where the 90 degree rotation is broken. The pairing mechanism is inter-pocket pair hopping. Interestingly, the same interaction also drives the antiferromagnetism.