Two energy scales in the magnetic resonance spectrum of electron and hole doped pnictide superconductors

TL;DR

This paper investigates multiple spin resonances in multiband pnictide superconductors, revealing how their energies and momenta depend on doping, Fermi surface topology, and gap symmetry, using RPA calculations based on experimental data.

Contribution

It demonstrates the existence of multiple spin resonances in pnictides due to multiband and multiple gaps, and analyzes their doping dependence and relation to Fermi surface topology.

Findings

Two distinct spin resonances are identified with different energies and momenta.

Resonance energies are strongly doping-dependent and proportional to gap amplitudes.

The spin excitation spectra evolve with changes in gap symmetry and Fermi surface topology.

Abstract

We argue that a multiband superconductor with sign-changing gaps may have multiple spin resonances. We calculate the RPA-based spin resonance spectra of a pnictide superconductor by using the five band tight-binding model or angle-resolved photoemission spectroscopy (ARPES) Fermi surface (FS) and experimental values of superconducting (SC) gaps. The resonance spectra split in both energy and momenta due to the effects of multiband and multiple gaps in $s^{\pm}-$pairing; the higher energy peak appears around the commensurate momenta due to scattering between $\alpha-$FS to $\gamma/\delta-$FS pockets. The second resonance is incommensurate coming from $\beta-$FS to $\gamma/\delta-$FS scatterings and its $q-$vector is doping-dependent and hence on the FS topology. Energies of both resonances $\omega^{1,2}_{res}$ are strongly doping dependent and are proportional to the gap amplitudes at the contributing FSs. We also discuss the evolution of the spin excitation spectra with various other possible gap symmetries, which may be relevant when either both the electron pockets or both the hole pockets completely disappear.