Spin excitations as a probe of itinerant electrons and local moments in hole-doped Ba0. 67K0. 33Fe2As2 superconductor (original) (raw)
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Physical Review B, 2013
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 .
Physical Review B, 2010
We use elastic and inelastic neutron scattering to systematically investigate the evolution of the low-energy spin excitations of the iron arsenide superconductor BaFe 2−x Ni x As 2 as a function of nickel doping x. In the undoped state, BaFe 2 As 2 exhibits a tetragonal-to-orthorhombic structural phase transition and simultaneously develops a collinear antiferromagnetic ͑AF͒ order below T N = 143 K. Upon electron doping of x = 0.075 to induce bulk superconductivity with T c = 12.2 K, the AF ordering temperature reduces to T N Ϸ 58 K. We show that the appearance of bulk superconductivity in BaFe 1.925 Ni 0.075 As 2 coincides with a dispersive neutron spin resonance in the spin excitation spectra and a reduction in the static ordered moment. For optimally doped BaFe 1.9 Ni 0.1 As 2 ͑T c =20 K͒ and overdoped BaFe 1.85 Ni 0.15 As 2 ͑T c =14 K͒ superconductors, the static AF longrange order is completely suppressed and the spin excitation spectra are dominated by a resonance and spin gap at lower energies. We determine the electron-doping dependence of the neutron spin resonance and spin gap energies and demonstrate that the three-dimensional nature of the resonance survives into the overdoped regime. If spin excitations are important for superconductivity, these results would suggest that the threedimensional characters of the electronic superconducting gaps are prevalent throughout the phase diagram and may be critical for superconductivity in these materials.
Physical Review B, 2010
We use inelastic neutron scattering to show that for the optimally electron-doped BaFe 1.9 Ni 0.1 As 2 ͑T c =20 K͒ iron arsenide superconductor, application of a magnetic field that partially suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. These results demonstrate that the energy of the resonance is intimately connected to the electron pairing energy, and thus indicate that the mode is a direct probe for measuring electron pairing and superconductivity in iron arsenides.
Physical Review Letters, 2013
We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe 0:985 Co 0:015 As induces a dispersive sharp resonance near E r1 ¼ 3:25 meV and a broad dispersionless mode at E r2 ¼ 6 meV. However, similar measurements on overdoped superconducting NaFe 0:935 Co 0:045 As find only a single sharp resonance at E r ¼ 7 meV. We connect these results with the observations of angleresolved photoemission spectroscopy that the superconducting gaps in the electron Fermi pockets are anisotropic in the underdoped material but become isotropic in the overdoped case. Our analysis indicates that both the double neutron spin resonances and gap anisotropy originate from the orbital dependence of the superconducting pairing in the iron pnictides. Our discovery also shows the importance of the inelastic neutron scattering in detecting the multiorbital superconducting gap structures of iron pnictides.
Physical Review B, 2012
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 mostly isotropic in the optimal and electron overdoped iron pnictides, consistent with a singlet to triplet excitation and isotropic paramagnetic scattering.
Double neutron spin resonances and gap anisotropy in underdoped superconducting NaFe0. 985Co0. 015As
We use inelastic neutron scattering to show that superconductivity in electron-underdoped NaFe0.985Co0.015As induces a dispersive sharp resonance near Er1 = 3.25 meV and a broad dispersionless mode at Er2 = 6 meV. However, similar measurements on overdoped superconducting NaFe0.955Co0.045As find only a single sharp resonance at Er = 7 meV. We connect these results with the observations of angle-resolved photoemission spectroscopy that the superconducting gaps in the electron Fermi pockets are anisotropic in the underdoped material but become isotropic in the overdoped case. Our analysis indicates that both the double neutron spin resonances and gap anisotropy originate from the orbital dependence of the superconducting pairing in the iron pnictides. Our discovery also shows the importance of the inelastic neutron scattering in detecting the multiorbital superconducting gap structures of iron pnictides.
Effect of Fermi Surface Nesting on Resonant Spin Excitations in Ba_{1-x}K_{x}Fe_{2}As_{2}
Physical Review Letters, 2011
We report inelastic neutron scattering measurements of the resonant spin excitations in Ba1−xKxFe2As2 over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s±-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight caused by the weakening of electron-electron correlations.
Physical Review B, 2010
Inelastic neutron scattering from superconducting (SC) Ba(Fe0.926Co0.074)2As2 reveals anisotropic and quasi-two-dimensional (2D) magnetic excitations close to QAFM = ( 1 2 1 2 ) -the 2D antiferromagnetic (AFM) wave-vector of the parent BaFe2As2 compound. The correlation length anisotropy of these low energy fluctuations is consistent with spin nematic correlations in the J1-J2 model with J1/J2 ∼ 1. The spin resonance at ∼8.3 meV in the SC state displays the same anisotropy. The anisotropic fluctuations experimentally evolve into two distinct maxima only along the direction transverse to QAFM above ∼80 meV indicating unusual quasi-propagating excitations.