Polarized neutron scattering studies of magnetic excitations in electron-overdoped superconducting BaFe1.85Ni0.15As

Abstract

We use polarized inelastic neutron scattering to study low-energy spin excitations and their spatial anisotropy in electron-overdoped superconducting BaFe1.85Ni0.15As2 (Tc = 14 K). In the normal state, the imaginary part of the dynamic susceptibility, χ ′′ (Q, ω), at the antiferromagnetic (AF) wave vector Q = (0.5, 0.5, 1) increases linearly with energy for E ≤ 13 meV. Upon entering the superconducting state, a spin gap opens below E ≈ 3 meV and a broad neutron spin resonance appears at E ≈ 7 meV. Our careful neutron polarization analysis reveals that χ ′′ (Q, ω) is isotropic for the in-plane and out-of-plane components in both the normal and superconducting states. A comparison of these results with those of undoped BaFe2As2 and optimally electron-doped BaFe1.9Ni0.1As2 (Tc = 20 K) suggests that the spin anisotropy observed in BaFe1.9Ni0.1As2 is likely due to its proximity to the undoped BaFe2As2. Therefore, the neutron spin resonance is isotropic in the overdoped regime, consistent with a singlet to triplet excitation.

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

“…7. In one extreme, there is FeSe with the resonance entirely anisotropic (although it is unclear at what energy fluctuations become isotropic), and in the other extreme the resonance is fully isotropic as found in overdoped iron pnictides [39,42]. In most systems reported so far, M y M z is observed; namely c-or a-axis (in-plane longitudinal direction at the stripe vector) polarized excitations are at least as intense as b-axis (in-plane transverse direction at the stripe vector) polarized excitations.…”

“…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]. With unpolarized neutron scattering the two parts are difficult to resolve, and it has been suggested that the two parts may have different origins [40].…”

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

“…In this notation, the vector Q in three-dimensional reciprocal space inÅ −1 is defined as Q = Ha scattering 23 . Figure 1 shows energy scans above and below T c at wave vectors Q = (0.5, 0.5, 0) and (0.5, 0.5, 2).…”

Field-induced magnetic transitions inCa10(Pt3As8)((Fe1−xPtxWatson1, McCollam2, Blake3 et al. 2014Phys. Rev. B191170View full textAdd to dashboardCite

“…On this point, it is important to keep in mind that the magnetic anisotropy in these materials is not universal: neutron scattering studies suggest that in LaFeAsO, SrFe 2 As 2 and BaFe 2 As 2 the spins are aligned in-plane [1], but in NdFeAsO [21] along c. In the latter case the spin flop may be masked by spin-reorientation transitions in the Nd subsystem. Additionally, there is evidence in polarized neutron scattering measurements that the anisotropy of low-energy spin fluctuations [22] in electron-doped Ba(Fe 1−x Ni x ) 2 As 2 does not reflect the easy axis of the ordered phase.…”