We use neutron polarization analysis to study temperature dependence of the spin excitation anisotropy in BaFe2As2, which has a tetragonal-to-orthorhombic structural distortion at Ts and antiferromagnetic (AF) phase transition at TN with ordered moments along the orthorhombic aaxis below Ts ≈ TN ≈ 136 K. In the paramagnetic tetragonal state at 160 K, spin excitations are isotropic in spin space with Ma = M b = Mc, where Ma, M b , and Mc are spin excitations polarized along the a, b, and c-axis directions of the orthorhombic lattice, respectively. On cooling towards TN , significant spin excitation anisotropy with Ma > M b ≈ Mc develops below 3 meV with a diverging Ma at TN . The in-plane spin excitation anisotropy in the tetragonal phase of BaFe2As2 is similar to those seen in the tetragonal phase of its electron and hole-doped superconductors, suggesting that spin excitation anisotropy is a direct probe of doping dependence of spin-orbit coupling and its connection to superconductivity in iron pnictides.

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High-temperature superconductivity occurs near antiferromagnetic instabilities and the nematic state. Debate remains on the origin of nematic order in FeSe and its relation with superconductivity. Here, we use transport, neutron scattering and Fermi surface measurements to demonstrate that hydrothermo grown superconducting FeS, an isostructure of FeSe, is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Only stripe-type spin excitations are observed up to 100 meV. No direct coupling between spin excitations and superconductivity in FeS is found, suggesting that FeS is less correlated and the nematic order in FeSe is due to competing checkerboard and stripe spin fluctuations.npj Quantum Materials (2017) 2:14 ; doi:10.1038/s41535-017-0019-6 INTRODUCTION High-transition temperature superconductivity in copper oxides and iron-based materials occurs near checkerboard and stripe antiferromagnetic (AF) instabilities, respectively. [1][2][3] Although there is also ample evidence for the existence of a nematic order, where a translationally invariant metallic phase spontaneously breaks rotational symmetry, 4-8 and for a nematic quantum critical point near optimal superconductivity in iron-based superconductors, 9, 10 much remains unclear concerning its microscopic origin and its relationship to superconductivity. 2,3 In particular, recent debates focus on whether nematic order in superconducting FeSe below the tetragonal-to-orthorhombic transition temperature T s = 91 K without static AF order 11-13 is due to competing magnetic instabilities or orbital ordering.14-22 Here, we use transport, neutron scattering and Fermi surface measurements to demonstrate that superconducting FeS, an isostructure of FeSe, 23, 24 is a tetragonal paramagnet without nematic order and with a quasiparticle mass significantly reduced from that of FeSe. Our neutron scattering experiments in the energy regime below 100 meV reveal only stripe-type spin fluctuations in FeS that are not directly coupled to superconductivity. These properties suggest that FeS is a weakly correlated analog of FeSe and, moreover, that the nematic order in FeSe is due to the frustrated magnetic interactions underlying the competing checkerboard and stripe spin fluctuations. 16-18A key to understanding the physics of the iron-based superconductors is to determine the role played by magnetism and by electronic nematicity to superconductivity. [1][2][3][5][6][7] In a typical AF ordered iron-pnictide, a tetragonal-to-orthorhombic lattice distortion T s occurs at temperatures above or at the AF ordering temperature T N , 2 and the nematic phase is observed in the paramagnetic orthorhombic phase between T s and T N . [5][6][7] Although iron chalcogenide FeSe single crystals [ Fig. 1a, b] also undergo a nematic transition at T s and become superconducting at T c = 9.3 K, 11 the low-temperature static AF ordered phase is absent. 12,13 This has fueled debates concerning the role of AF order and spin fluctuations...

We use inelastic neutron scattering to study energy and wave vector dependence of spin fluctuations in SrCo2As2, derived from SrFe2−xCoxAs2 iron pnictide superconductors. Our data reveals the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) spin fluctuations at wave vectors Q AF =(1,0) and Q FM =(0,0)/(2,0), respectively. By comparing neutron scattering results with those of dynamic mean field theory calculation and angle-resolved photoemission spectroscopy experiments, we conclude that both AF and FM spin fluctuations in SrCo2As2 are closely associated with a flat band of the eg orbitals near the Fermi level, different from the t2g orbitals in superconducting SrFe2−xCoxAs2. Therefore, Co-substitution in SrFe2−xCoxAs2 induces a t2g to eg orbital switching, and is responsible for FM spin fluctuations detrimental to the singlet pairing superconductivity.Flat electronic bands can give rise to a plethora of interaction-driven quantum phases, including ferromagnetism [1], Mott insulating phase due to electron correlations [2], and superconductivity [3]. Therefore, an understanding how the flat electronic bands can influence the electronic, magnetic, and superconducting properties of solids is an important topic in condensed matter physics. In iron pnictide superconductors such as AFe 2−x Co x As 2 (A = Ba, Sr) [Figs. 1(a)-1(d)], the dominate interactions are stripe antiferromagnetic (AF) order, and superconductivity, which has singlet electron pairing, arises by doping electron with Co-substitution to suppress static AF order [4][5][6]. While AF spin fluctuations and superconductivity in iron pnictides are believed to arise from nested hole Fermi surfaces at Γ and electron Fermi surfaces at M [ Fig. 1(e)] [7], the density functional theory (DFT) calculations suggest the competing ferromagnetic (FM) and AF spin fluctuations with the balance controlled by doping [8,9]. For Co-overdoped ACo 2 As 2 [10, 11], where the DFT calculations find a tendency for both the FM and AF order, neutron scattering revealed only the AF spin fluctuations [12] while angle resolved photoemission spectroscopy (ARPES) experiments found no evidence of the Fermi surface nesting [13,14]. On the other hand, nuclear magnetic resonance (NMR) measurements on AFe 2−x Co x As 2 provided evidence for FM spin fluctuations at all Co-doping levels in addition to the AF spin fluctuations [15,16]. In particular, strong FM spin fluctuations in AFe 2−x Co x As 2 are believed to compete with AF spin fluctuations and prevent superconductivity for Co-overdoped samples [15,16], contrary to the Fermi surface nesting picture where superconductivity is suppressed via vanishing hole Fermi surfaces with increasing Co-doping [7,17]. Finally, action of physical, chemical pressure, or aliovalent substitution in BCo 2 As 2 (B = Eu, Ca) can drive these AF materials into ferromagnets [18]. In particular, CaCo 1.84 As 2 with a collapsed tetragonal structure [19] forms A-type AF ground state with coexisting FM spin fluctuations within the CoAs layer and A-type AF s...