Double magnetic resonance and spin anisotropy in Fe-based superconductors due to static and fluctuating antiferromagnetic orders

Abstract

Motivated by recent neutron scattering experiments in Fe-based superconductors, we study how the magnetic resonance in the superconducting state is affected by the simultaneous presence of either static or fluctuating magnetic orders using the random phase approximation. We find that for the underdoped materials with coexisting superconducting and antiferromagnetic orders, spin rotational symmetry is explicitly broken at the ordering momentum $Q_1=(\pi ,0)$. Only the longitudinal susceptibility exhibits the resonance mode, whereas a spin-wave Goldstone mode develops in the transverse component. Meanwhile, at the frustrated momentum $Q_2=(0,\pi )$, the susceptibility becomes isotropic in spin space and the magnetic resonance exists for both components. Furthermore, the resonance energies at $Q_1$ and $Q_2$ have distinct scales, which provides a natural explanation for the recently observed double resonance peaks. In addition, we show that near optimal doping the existence of strong magnetic fluctuations, which are modeled here via a Gaussian mode, can still induce the spin anisotropy in the magnetic susceptibility.

9 pages, 4 figures, accepted version in PRB

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• Anisotropic neutron spin resonance in underdoped superconducting NaFe1-xCoxAs
• Spin Resonance and Magnetic Order in an Unconventional Superconductor
• Spin fluctuation anisotropy as a probe of orbital-selective hole-electron quasiparticle excitations in detwinned Ba(Fe1-xCox)2As2
• Electron doping evolution of the neutron spin resonance in NaFe${}_{1-x}$Co${}_x$As
• Suppression of low-energy longitudinal spin-excitations in Co-underdoped BaFe2As2
• Experimental elucidation of the origin of the `double spin resonances' in Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$
• Strong Spin Resonance Mode associated with suppression of soft magnetic ordering in Hole-doped Ba1-xNaxFe2As2
• Orbital selective neutron spin resonance in underdoped superconducting NaFe${}_{\text{0.985}}$Co${}_{\text{0.015}}$As
• Unusual suppression of a spin resonance mode with magnetic field in underdoped NaFe${}_{1-x}$Co${}_x$As: Evidence for orbital-selective pairing
• Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped NaFe${}_{0.985}$Co${}_{0.015}$As
• Impurity effects on spin dynamics in magnetic and superconducting iron pnictides and chalcogenides