Electron doping evolution of the anisotropic spin excitations in BaFe2−xNixAs2

Abstract

We use inelastic neutron scattering to systematically investigate the Ni-doping evolution of the low-energy spin excitations in BaFe2−xNixAs2 spanning from underdoped antiferromagnet to overdoped superconductor (0.03 ≤ x ≤ 0.18). In the undoped state, BaFe2As2 changes from paramagnetic tetragonal phase to orthorhombic antiferromagnetic (AF) phase below about 138 K, where the low-energy (≤∼ 80) meV spin waves form transversely elongated ellipses in the [H, K] plane of the reciprocal space. Upon Ni-doping to suppress the static AF order and induce superconductivity, the c-axis magnetic exchange coupling is rapidly suppressed and the momentum distribution of spin excitations in the [H, K] plane is enlarged in both the transverse and longitudinal directions with respect to the in-plane AF ordering wave vector of the parent compound. As a function of increasing Ni-doping x, the spin excitation widths increase linearly but with a larger rate along the transverse direction. These results are in general agreement with calculations of dynamic susceptibility based on the random phase approximation (RPA) in an itinerant electron picture. For samples near optimal superconductivity at x ≈ 0.1, a neutron spin resonance appears in the superconducting state. Upon further increasing the electron-doping to decrease the superconducting transition temperature Tc, the intensity of the low-energy magnetic scattering decreases and vanishes concurrently with vanishing superconductivity in the overdoped side of the superconducting dome. Comparing with the low-energy spin excitations centered at commensurate AF positions for underdoped and optimally doped materials (x ≤ 0.1), spin excitations in the over-doped side (x = 0.15) form transversely incommensurate spin excitations, consistent with the RPA calculation. Therefore, the itinerant electron approach provides a reasonable description to the low-energy AF spin excitations in BaFe2−xNixAs2.

Effect of Li-deficiency impurities on the electron-overdoped LiFeAs superconductor

“…This nesting condition is consistent with previous work on LiFeAs [21] and our own measurements. These results are also in agreement with the Fermi surface nesting interpretation of the low-energy spin excitations in the electron- [30,31] and hole-doped [39,40] BaFe 2 As 2 , and thus suggest that the stoichiometric LiFeAs is an intrinsically electron-overdoped superconductor. This is also consistent with the fact that further electron-doping in the SC LiFeAs via Ni and Co substition can systematically reduce T c [11].…”

“…The quasiparticle excitations between the mismatched hole and electron Fermi surfaces due to the self electron-doping should produce incommensurate spin fluctuations along the direction transverse to the AF ordering wave vector Q = (1, 0) (Figs. 1d and 1e) consistent with the calculated spin susceptibility χ ′′ (Q, ω) based on a random phase approximation of a three-dimensional 5-orbital tight-binding model for BaFe 2 As 2 [30,31,38]. Experimentally, the transverse incommensurate spin fluctuations with δ K ≈ 0.1 were found at E = 7 meV for the electron overdoped BaFe 2−x Ni x As 2 at x = 0.15 [31].…”

“…1d and 1e) consistent with the calculated spin susceptibility χ ′′ (Q, ω) based on a random phase approximation of a three-dimensional 5-orbital tight-binding model for BaFe 2 As 2 [30,31,38]. Experimentally, the transverse incommensurate spin fluctuations with δ K ≈ 0.1 were found at E = 7 meV for the electron overdoped BaFe 2−x Ni x As 2 at x = 0.15 [31]. Using the ARPES measurements (Fig.…”

“…We find that a few percent Li-deficiency can completely suppress superconductivity and change transport properties but without significant effect on the sizes of the Fermi surfaces [20] and incommensurate spin excitations [21]. By comparing our results with previous work on the SC LiFeAs [20,21], NaFe 1−x Co x As [26][27][28], BaFe 2−x T x As 2 [29][30][31], and LaFe 1−y Zn y AsO 1−x F x [32], we conclude that the stoichiometric LiFeAs is an intrinsically electronoverdoped superconductor similar to NaFe 1−x Co x with x ≈ 0.065, and that Li deficiencies affect its SC properties similar to the Zn impurity effects in the electronoverdoped iron pnictide superconductors [32]. These results naturally explain the absence of the static AF order in Li 1−x FeAs, and why superconductivity in LiFeAs is so sensitive to the Li-deficiency.…”

“…1a) without static AF order [26]. The electron doping of x ≈ 0.065 is roughly estimated from a comparison of incommensurate spin excitations in BaFe 2−x Ni x As 2 [31] with the expected location of incommensurate spin excitations in NaFe 1−x Co x As [27]. The quasiparticle excitations between the mismatched hole and electron Fermi surfaces due to the self electron-doping should produce incommensurate spin fluctuations along the direction transverse to the AF ordering wave vector Q = (1, 0) (Figs.…”