Electron doping evolution of the magnetic excitations in BaFe2−xNixAs2

Abstract

We use inelastic neutron scattering (INS) spectroscopy to study the magnetic excitations spectra throughout the Brioullion zone in electron-doped iron pnictide superconductors BaFe2−xNixAs2 with x = 0.096, 0.15, 0.18. While the x = 0.096 sample is near optimal superconductivity with Tc = 20 K and has coexisting static incommensurate magnetic order, the x = 0.15, 0.18 samples are electronoverdoped with reduced Tc of 14 K and 8 K, respectively, and have no static antiferromagnetic (AF) order. In previous INS work on undoped (x = 0) and electron optimally doped (x = 0.1) samples, the effect of electron-doping was found to modify spin waves in the parent compound BaFe2As2 below ∼100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. While the new data collected on the x = 0.096 sample confirms the overall features of the earlier work, our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its Q-width below Tc similar to that of the optimally hole-doped iron pnictides Ba0.67K0.33Fe2As2. In addition, we establish the dispersion of the resonance and find it to change from commensurate to transversely incommensurate with increasing energy. Upon further electron-doping to overdoped iron pnictides with x = 0.15 and 0.18, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above ∼100 meV are still not much affected. Our absolute spin excitation intensity measurements throughout the Brillouin zone for x = 0.096, 0.15, 0.18 confirm the notion that the low-energy spin excitation coupling with itinerant electron is important for superconductivity in these materials, even though the high-energy spin excitations are weakly doping dependent.

Electron doping evolution of the magnetic excitations inNaFe1−xCoxAs

“…Large discrepencies in magnetic scattering intensity are observed only to small energy transfers with intensities becoming comparable around E ≈ 50 meV. This observation is reflected in previous measurements in BaFe 2−x Ni x As 2 , in which strong doping dependence is observed below E ≈ 80 meV 24,25,28,29 .…”

“…However, high-energy (E > 100 meV) spin excitations remain weakly electron-doping dependent and are reminiscent of spin waves in the undoped BaFe 2 As 2 24-29 . In concert, these results suggest that low-energy spin excitations in electron-doped BaFe 2 As 2 family of materials arise from itinerant electrons and Fermi surface nesting [40][41][42] , while high-energy spin excitations are related to local moments and are insensitive to changes in Fermi surfaces 24,28,29,[43][44][45][46] .…”

“…The transverse dispersion from the AF ordering wave vector is clearly visible at all dopings, as is a feature stemming from the zone boundary wave vector positions (±1, ±1). When energy is increased to E = 119 ± 7 meV, spin excitations form a ring like structure centered around the wave vector (±1, ±1), and again are weakly Co-doping dependent of NaFe 1−x Co x As not observed in BaFe 2−x Ni x As 2 family of materials 22,29 . However, recent ToF INS measurements of the FeSe family of materials reveal spin excitations at both of these wave vectors that are interpreted as competition between stripe magnetic order and Néel order 63 .The presence of similar features in the NaFe 1−x Co x As family of materials suggests that magnetic frustrations may also play an important role in determining the rather low Néel temperature and weakly ordered moment of the undoped NaFeAs 51 .…”

“…If spin excitations are mediating the electron pairing for high-T c superconductivity, the superconducting condensation energy U should be accounted for by the change in magnetic exchange energy ∆E ex (T ) = 2J[ S i+x · S i N − S i+x · S i S ], where J is the nearest neighbor magnetic exchange coupling and S i+x · S i is the magnetic scattering in absolute units at the normal (N ) and superconducting (S) phases at zero temperature 6 , within an isotropic t-J model 56 . Since the effective magnetic exchange coupling constants of NaFeAs 52 are considerably smaller than those of BaFe 2 As 2 22 , it will be instructive to systematically map out the overall spin excitations spectra in NaFe 1−x Co x As and compare the result with those of BaFe 2−x Ni x As 2 family of materials 24,25,28,29 . In previous INS experiments on NaFe 1−x Co x As probing low-energy spin excitations using triple-axis spectrometry, we find the presence of a single, sharp neutron spin resonance in a sample with nearly optimal Co-doping, similar to the resonance in electron-doped BaFe 2 As 2 57 , while an underdoped sample with coexisting superconductivity and AF order exhibits a double resonance 58,59 .…”

Presence of magnetic excitations in SmFeAsO

“…On the dynamical side, spin wave-like excitations were also observed in the AF phases of several compounds by inelastic neutron scattering (INS) and Resonant Inelastic X-Ray Scattering (RIXS) [10,[14][15][16][17][18][19]. The importance of spin fluctuations has been further confirmed by their persistence within the superconducting phase [10,[15][16][17][18][19], even though a conclusive picture of their role is still under development.…”