Electron doping evolution of the neutron spin resonance inNaFe1−xCoxAs

Abstract

Neutron spin resonance, a collective magnetic excitation coupled to superconductivity, is one of the most prominent features shared by a broad family of unconventional superconductors including copper oxides, iron pnictides, and heavy fermions. In this work, we study the doping evolution of the resonances in NaFe1−xCoxAs covering the entire superconducting dome. For the underdoped compositions, two resonance modes coexist. As doping increases, the low-energy resonance gradually loses its spectral weight to the high-energy one but remains at the same energy. By contrast, in the overdoped regime we only find one single resonance, which acquires a broader width in both energy and momentum, but retains approximately the same peak position even when Tc drops by nearly a half compared to optimal doping. These results suggest that the energy of the resonance in electron overdoped NaFe1−xCoxAs is neither simply proportional to Tc nor the superconducting gap, but is controlled by the multi-orbital character of the system and doped impurity scattering effect.

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

“…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.…”

“…The energy of the resonance mode (E r ) well inside the superconducting state has been proposed to universally scale with either the superconducting transition temperature T c (E r ≈ 4-6 k B T c ) [16,17] or the superconducting gap 2 (E r ≈ 0.64 × 2 ) [18], although some iron-based superconductors deviate from such scalings [19][20][21][22]. The energy of the resonance mode was also found to track the superconducting order parameter as a function of temperature in optimally electron-doped BaFe 1.85 Co 0.15 As 2 [23].…”

Phase diagram and neutron spin resonance of superconducting NaFe1−xCuxAs

“…In electron-doped NaFe 1−x Co x As, two resonance modes are seen in underdoped compositions 24 , with the lower-energy-mode gradually losing spectral weight upon further doping before disappearing near optimal doping 33,34 . The energy of the single resonance mode in well-overdoped NaFe 1−x Co x As does not scale with T c , likely due to multi-orbital physics and impurity effects 33 .…”

“…The energy of the single resonance mode in well-overdoped NaFe 1−x Co x As does not scale with T c , likely due to multi-orbital physics and impurity effects 33 . Since superconducting domes are absent in Cu-doped AFe 2 As 2 16,35 , it is unclear if superconductivity induced by Cu-doping has similar electronic and magnetic properties as their electron/holedoped counterparts.…”

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“…Interestingly to note that the spin resonance appears below Tc in the superconductors and its energy is related to T c with the well know relation Δ reso~4 .3*T c (for pnictides) and ~5.4*T c (for cuprates) [85,86], while the spin-gap in Kondo insulators appears at a temperature below T χ max and its value is related to T K with relation Δ spin~3 * T χ max = T K [28]. Further a similar resonant mode in the non-superconducting antiferromagnetic heavy-fermion metal CeB 6 has been observed at the Q-position corresponding to antiferro-quadrupolar (AFQ) ordering (½, ½, ½) [87].…”