In-plane spin excitation anisotropy in the paramagnetic phase of NaFeAs

Abstract

We use unpolarized and polarized inelastic neutron scattering to study low-energy spin excitations in NaFeAs, which exhibits a tetragonal-to-orthorhombic lattice distortion at $T_s\approx 58$ K followed by a collinear antiferromagnetic (AF) order below $T_N\approx 45$ K. In the AF ordered state ($T<T_N$), spin waves are entirely c-axis polarized below $\sim$10 meV, exhibiting a gap of $\sim 4$ meV at the AF zone center and disperse to $\sim$7 meV near the c-axis AF zone boundary. On warming to the paramagnetic state with orthorhombic lattice distortion ($T_N<T<T_s$), spin excitations become anisotropic within the FeAs plane. Upon further warming to the paramagnetic tetragonal state ($T>T_s$), spin excitations become more isotropic. Since similar magnetic anisotropy is also observed in the paramagnetic tetragonal phase of superconducting BaFe${}_{1.904}$Ni${}_{0.096}$As${}_2$, our results suggest that the spin excitation anisotropy in superconducting iron pnictides originates from similar anisotropy already present in their parent compounds.

7 pages, 4 figures

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• Prominent role of spin-orbit coupling in FeSe
• Orbital Order and Spin Nematicity in the Tetragonal Phase of Electron-doped Iron-Pnictides NaFe${}_{1-x}$Co${}_x$As
• Neutron spin resonance in the 112-type iron-based superconductor
• Neutron-scattering measurements of the spin excitations in LaFeAsO and Ba(Fe${}_{0.953}$Co${}_{0.047}$)${}_2$As${}_2$: Evidence for a sharp enhancement of spin fluctuations by nematic order
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• Emergent magnetic degeneracy in iron pnictides due to the interplay between spin-orbit coupling and quantum fluctuations
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• Double magnetic resonance and spin anisotropy in Fe-based superconductors due to static and fluctuating antiferromagnetic orders
• Spin anisotropy due to spin-orbit coupling in optimally hole-doped Ba${}_{0.67}$K${}_{0.33}$Fe${}_2$As${}_2$
• Magnetic phase diagram of the iron pnictides in the presence of spin-orbit coupling: Frustration between $C_2$ and $C_4$ magnetic phases
• In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFe${}_2$As${}_2$
• Strong Spin Resonance Mode associated with suppression of soft magnetic ordering in Hole-doped Ba1-xNaxFe2As2
• Phase diagram and neutron spin resonance of superconducting NaFe${}_{1-x}$Cu${}_x$As
• Spin excitation anisotropy in the paramagnetic tetragonal phase of BaFe2 As2
• Orbital selective neutron spin resonance in underdoped superconducting NaFe${}_{\text{0.985}}$Co${}_{\text{0.015}}$As
• Spin-isotropic continuum of spin excitations in antiferromagnetically ordered Fe${}_{1.07}$Te
• Strain-induced spin-nematic state and nematic susceptibility arising from $2\times 2$ Fe clusters in KFe${}_{0.8}$Ag${}_{1.2}$Te${}_2$
• Temperature and polarization dependence of low-energy magnetic fluctuations in nearly-optimal-doped NaFe${}_{0.9785}$Co${}_{0.0215}$As
• Spin dynamics in NaFeAs and NaFe${}_{0.53}$Cu${}_{0.47}$As probed by resonant inelastic X-ray scattering
• Longitudinal modes of spin fluctuations in iron-based superconductors
• The influence of the structural transition on magnetic fluctuations in NaFeAs
• Twinning of domains and spin anisotropy in K${}_5$Fe${}_4$Ag${}_6$Te${}_{10}$