Theory of magnetic excitations in iron-based layered superconductors

TLDR

Normal‑state spin excitations in Fe‑pnictides are dominated by interorbital antiferromagnetic and intraband spin‑density wave fluctuations, but unconventional superconductivity alters this response. The study analyzes the spin response of Fe‑pnictide superconductors in normal and superconducting states using an effective four‑band model. Using this model, the authors compute the spin susceptibility in both phases to investigate magnetic excitations. A well‑defined spin‑exciton resonance appears only for interband scattering at the antiferromagnetic momentum for the extended s‑wave order parameter, vanishes rapidly for q < QAFM, and is extremely weak for d‑wave symmetry, allowing experimental discrimination of the superconducting gap 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 the form of the well-defined spin exciton occurs only for the interband scattering at the antiferromagnetic momentum ${\mathbf{Q}}_{\text{AFM}}$ for the ${s}_{\ifmmode\pm\else\textpm\fi{}}$ (extended $s$ wave) superconducting order parameter and it disappears rapidly for $\mathbf{q}<{\mathbf{Q}}_{\text{AFM}}$. The resonance feature is extremely weak for the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave order parameter due to the specific Fermi-surface topology of these compounds. The essential difference between ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave symmetries for the magnetic excitations can be used for experimental determination of the superconducting wave-function symmetry.