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

Abstract

We use polarized inelastic neutron scattering to study the temperature and energy dependence of spin space anisotropies in the optimally hole-doped iron pnictide Ba0.67K0.33Fe2As2 (Tc = 38 K). In the superconducting state, while the high-energy part of the magnetic spectrum is nearly isotropic, the low-energy part displays a pronouced anisotropy, manifested by a c-axis polarized resonance. We also observe that the spin anisotropy in superconducting Ba0.67K0.33Fe2As2 extends to higher energies compared to electron-doped BaFe2−xT MxAs2 (T M =Co, Ni) and isovalent-doped BaFe2As1.4P0.6, suggesting a connection between Tc and the energy scale of the spin anisotropy. In the normal state, the low-energy spin anisotropy for optimally hole-and electron-doped iron pnictides onset at temperatures similar to the temperatures at which the elastoresistance deviate from Curie-Weiss behavior, pointing to a possible connection between the two phenomena. Our results highlight the relevance of the spin-orbit coupling to the superconductivity of the iron pnictides.

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

“…The persistence of CW behavior down to a similar temperature is also observed in nearly optimally doped NaFe 0.985 Ni 0.015 As [55]. These results indicate that compared to electron-doped BaFe 2 As 2 [53], CW behavior in nematic susceptibility persists to lower temperatures in electron-doped NaFeAs, and confirms the link between development of anisotropic magnetic fluctuations and the nematic susceptibility deviating from CW behavior [34].…”

“…However, for electron-and hole-doped BaFe 2 As 2 superconductors, deviations from CW behavior were found at temperatures above T c , although CW behavior was found down to T c in isovalently doped BaFe 2 As 1.4 P 0.6 [53]. It was noted that the temperatures at which nematic susceptibility deviates from CW behavior correspond to temperatures at which spin anisotropy onset occurs in these systems, suggesting that anisotropic magnetic fluctuations may be responsible for the deviation from CW behavior in nematic susceptibility [34].…”

“…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.…”

Phase diagram and neutron spin resonance of superconducting NaFe1−xCuxAs

“…3(b), (d) and (f)] may be due to the mode at L = 0 and 1 being at a higher energy, and experience stronger interactions with particle-hole excitations that broaden the mode. Alternatively, spin-orbit coupling found to be present in many iron pnictide superconductors 51 and causes energysplitting of resonance modes polarized along different crystallographic directions 52,53 , also results in broadening of the resonance mode seen in unpolarized neutron scattering experiments. Polarized neutron scattering experiments are needed to distinguish between these scenarios.…”