Anisotropic neutron spin resonance in underdoped superconductingNaFe1−xCoxAs

Abstract

We use polarized inelastic neutron scattering (INS) to study spin excitations in superconducting NaFe0.985Co0.015As (C15) with static antiferromagnetic (AF) order along the a-axis of the orthorhombic structure and NaFe0.935Co0.045As (C45) without AF order. In previous unpolarized INS work, spin excitations in C15 were found to have a dispersive sharp resonance near Er1 = 3.25 meV and a broad dispersionless mode at Er2 = 6 meV. Our neutron polarization analysis reveals that the dispersive resonance in C15 is highly anisotropic and polarized along the a-and c-axis, while the dispersionless mode is isotropic similar to that of C45. Since the a-axis polarized spin excitations of the anisotropic resonance appear below Tc, our data suggests that the itinerant electrons contributing to the magnetism are also coupled to the superconductivity.

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

“…Combined with previous results [42], our present work allows for a systematic understanding of how anisotropic spin dynamics evolve in NaFe 1−x Co x As, as sketched in Fig. 7.…”

“…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.…”

“…It is noteworthy that anisotropic fluctuations typically appear above T c [Figs. 7(c) and 7(e)], and develop into an anisotropic resonance mode or become partially gapped inside the superconducting state [34][35][36][37]42,52]. BaFe 2 As 1.4 P 0.6 appears to be an exception, with anisotropic spin fluctuations only present in the superconducting state [44].…”

“…Using polarized neutron scattering, it was further found that the mode at lower energy is anisotropic in spin space while the one at higher energy is isotropic [42]. With increasing doping the low-energy mode gradually loses spectral weight, while the mode at higher energy is present across the superconducting dome [20]; however it is unclear how doping affects spin fluctuations in terms of spin anisotropy.…”

Electron doping evolution of the magnetic excitations inNaFe1−xCoxAs

“…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 . To illuminate the rest of the story, the Co-doping evolution of high-energy spin excitations in superconducting and non-superconducting NaFe 1−x Co x As needs to be established.…”