Theory of magnetic excitations in iron-based layered superconductors

TL;DR

This paper models magnetic excitations in iron-based superconductors, revealing how different superconducting symmetries affect spin resonance features, aiding experimental identification of the pairing symmetry.

Contribution

It provides a theoretical analysis of spin response in Fe-pnictides using a four-band model, highlighting the distinct magnetic signatures of different superconducting symmetries.

Findings

Resonance peak appears only for interband scattering in s±-wave superconductors.

The resonance is weak or absent for d-wave symmetry due to Fermi surface topology.

Magnetic excitation differences can help determine the superconducting pairing symmetry.

Abstract

Based on the effective four-band model we analyze the spin response in the normal and superconducting states of the Fe-pnictide superconductors. While the normal state spin excitations are dominated by the continuum of the interorbital antiferromagnetic fluctuations and the intraband spin density wave fluctuations, the unconventional superconductivity yields different feedback. The resonance peak in form of the well-defined spin exciton occurs {\it only} for the interband scattering at the antiferromagnetic momentum ${\bf Q}_{AFM}$ for the $s_\pm$ (extended s-wave) superconducting order parameter and it disappears rapidly for ${\bf q} < {\bf Q}_{AFM}$. The resonance feature is extremely weak for the $d_{x^2 -y^2}$-wave order parameter due to specific Fermi surface topology of these compounds. The essential difference between $s_\pm$-wave and $d_{x^2 -y^2}$-wave symmetries for the magnetic excitations can be used for experimental determination of the superconducting wave function symmetry.