Orbital selective neutron spin resonance in underdoped superconducting NaFe0.985Co0.015As

Abstract

We use neutron scattering to study the electron-doped superconducting NaFe0.985Co0.015As (Tc = 14 K), which has co-existing static antiferromagnetic (AF) order (TN = 31 K) and exhibits two neutron spin resonances (Er1 ≈ 3.5 meV and Er2 ≈ 6 meV) at the in-plane AF ordering wave vector Q AF = Q1 = (1, 0) in reciprocal space. In the twinned state below the tetragonal-to-orthorhombic structural transition Ts, both resonance modes appear at Q1 but cannot be distinguished from Q2 = (0, 1). By detwinning the single crystal with uniaxial pressure along the orthorhombic b-axis, we find that both resonances appear only at Q1 with vanishing intensity at Q2. Since electronic bands of the orbital dxz and dyz characters split below Ts with the dxz band sinking ∼ 10 meV below the Fermi surface, our results indicate that the neutron spin resonances in NaFe0.985Co0.015As arise mostly from quasi-particle excitations between the hole and electron Fermi surfaces with the dyz orbital character.

Three-dimensional electronic structure of the nematic and antiferromagnetic phases of NaFeAs from detwinned angle-resolved photoemission spectroscopy

“…The one-ellipse structure would naturally lead to a marked in-plane resistivity anisotropy [8,12]. Moreover the low-energy spin susceptibility will be much larger along the a orth direction; the recent observation of neutron spin-resonances at (π,0,π) but no intensity at (0,π,π) in detwinned inelastic neutron scattering measurements of Na(Fe 0.985 Co 0.015 )As [55] could be compatible with this picture. If the system remains non-magnetic but becomes superconducting, the absence of states to pair with on the electron pocket along b orth would naturally lead to a substantially anisotropic twofold symmetric gap structure within the context of a spin-fluctuation pairing mechanism, which could explain recent results in FeSe [56].…”

Strong spin resonance mode associated with suppression of soft magnetic ordering in hole-doped Ba1-xNaxFe2As2

“…[28][29][30][31][32] Concerning the split SRM's, the two best studied series are both electron doped: Ba(Fe 1−x Co x ) 2 As 2 and NaFe 1−x Co x As. [12][13][14][15]33 For underdoped Ba(Fe 1−x Co x ) 2 As 2 (x = 4.5%), nematic and AFM order with sizeable moments precede SC resulting in a large gap in the longitudinal spin fluctuations (polarized along the ordered moment in orthorhombic a direction). In consequence only anisotropic resonance modes emerge in the transverse channels, 13 which clearly disagree with the isotropic spin-exciton model.…”

Orbital selective spin waves in detwinned NaFeAs

“…From the inelastic slice containing 4 E 9 meV with E i = 40 meV [Fig. 1(h)], we find that the integrated intensities at the strong (I s at Q strong ) and weak (I w at Q weak ) positions can give η = (I s − I w )/(I s + I w ) ≈ 50%, or a 3:1 domain ratio, only slightly smaller than η ≈ 62.4% obtained on one single crystal of NaFe 0.985 Co 0.015 As [22].…”

“…1(g)] [38]. Since it is not possible to rotate the "weak" magnetic 054430-3 Bragg peak at (0, ±1, 1.5) positions onto the detector bank at fixed E i , one cannot use the above-mentioned method to directly measure the sample assembly detwinning ratio, defined as η = [I (1, 0, 1.5) − I (0, 1, 1.5)]/[I (1, 0, 1.5) + I (0, 1, 1.5)], where I (1, 0, 1.5) and I (0, 1, 1.5) correspond to the strong and weak magnetic Bragg peaks, respectively [22]. Nevertheless, one can still determine the sample detwinning ratio η using the low-energy inelastic part of the spin fluctuation spectrum, which at least provides a firm lower bound, and was in fact observed to have one-to-one correspondence to η in previous experiments on the NaFeAs and BaFe 2 As 2 systems [22,23].…”

“…Differentiation of these mechanisms has important consequences for the origins of superconductivity [20]. Inelastic neutron scattering experiments on detwinned FeSe [21], underdoped superconducting NaFe 0.985 Co 0.015 As [22], and Ba(Fe 1−x Co x ) 2 As 2 [23] suggest that the neutron spin resonance, a collective magnetic excita-tion coupled to the superconducting order parameter [24][25][26][27][28][29], likely arises from orbital selective electron-hole Fermi surface nesting of quasiparticles within the single d yz orbital submanifold.…”