Magnetic anisotropy in hole-doped superconducting Ba0.67K0.33Fe2

Abstract

We use polarized inelastic neutron scattering (INS) to study spin excitations of optimally holedoped superconductor Ba0.67K0.33Fe2As2 (Tc = 38 K). In the normal state, the imaginary part of the dynamic susceptibility, χ ′′ (Q, ω), shows magnetic anisotropy for energies below ∼7 meV with c-axis polarized spin excitations larger than that of the in-plane component. Upon entering into the superconducting state, previous unpolarized INS experiments have shown that spin gaps at ∼5 and 0.75 meV open at wave vectors Q = (0.5, 0.5, 0) and (0.5, 0.5, 1), respectively, with a broad neutron spin resonance at Er = 15 meV. Our neutron polarization analysis reveals that the large difference in spin gaps is purely due to different spin gaps in the c-axis and in-plane polarized spin excitations, resulting resonance with different energy widths for the c-axis and in-plane spin excitations. The observation of spin anisotropy in both opitmally electron and hole-doped BaFe2As2 is due to their proximity to the AF ordered BaFe2As2 where spin anisotropy exists below TN .

Temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped NaFe0.9785Co0.0215As

“…It is noteworthy that anisotropic fluctuations typically appear above T c [Figs. 7(c) and 7(e)], and develop into an anisotropic resonance mode or become partially gapped inside the superconducting state [34][35][36][37]42,52]. BaFe 2 As 1.4 P 0.6 appears to be an exception, with anisotropic spin fluctuations only present in the superconducting state [44].…”

“…In the normal state [ Fig. 4(d)], spin anisotropy is also observed, similar to electron-and hole-doped BaFe 2 As 2 [36,37,52], but different from isovalently doped BaFe 2 As 1.4 P 0.6 [44].…”

“…In BaFe 2 As 2 -derived superconductors near optimal doping, a prominent anisotropic contribution to the resonance mode is also observed [34,36,37,44] …”

“…Spin-orbit coupling also accounts for the "XYZ" spin anisotropy in parent compounds of iron pnictides [31][32][33] and spin anisotropy in iron-based superconductors [34,35]. Spin anisotropy is manifested in the resonance mode of superconducting iron pnictides up to the optimally or slightly overdoped regime in electron-and hole-doped BaFe 2 As 2 [36][37][38], leading to resonance modes that exhibit an anisotropic part in addition to an isotropic part in spin space. The resonance mode becomes fully isotropic well into the overdoped regime [39].…”

Spin anisotropy due to spin-orbit coupling in optimally hole-dopedBa0.67K0.33Fe2As2

“…To elucidate the relevance of SOC to superconductivity, it is instructive to compare the behavior of the spin anisotropy in hole-doped and electron-doped BaFe 2 As 2 , since the maximum values of T c are quite different in these two cases -T c ≈ 38 K for optimally hole-doped Ba 0.67 K 0.33 Fe 2 As 2 and T c ≈ 25 K for optimally electron-doped BaFe 1.86 Co 0.14 As 2 . Previous analysis of the electron-doped case revealed that the spin anisotropy persists in the paramagnetic tetragonal phase for doping levels up to or slightly beyond optimal doping [34][35][36][37][38][39] , but vanishes in the well-overdoped regime 38,40 . In this paper, we present polarized neutron scattering studies of spin excitations in optimally hole-doped Ba 0.67 K 0.33 Fe 2 As 2 17,37 .…”