We use neutron resonance spin echo and Larmor diffraction to study the effect of uniaxial pressure on the tetragonal-to-orthorhombic structural (Ts) and antiferromagnetic (AF) phase transitions in iron pnictides BaFe2−xNixAs2 (x = 0, 0.03, 0.12), SrFe1.97Ni0.03As2, and BaFe2(As0.7P0.3)2. In antiferromagnetically ordered BaFe2−xNixAs2 and SrFe1.97Ni0.03As2 with TN and Ts (TN ≤ Ts), a uniaxial pressure necessary to detwin the sample also increases TN , smears out the structural transition, and induces an orthorhombic lattice distortion at all temperatures. By comparing temperature and doping dependence of the pressure induced lattice parameter changes with the elastoresistance and nematic susceptibility obtained from transport and ultrasonic measurements, we conclude that the in-plane resistivity anisotropy found in the paramagnetic state of electron underdoped iron pnictides depends sensitively on the nature of the magnetic phase transition and a strong coupling between the uniaxial pressure induced lattice distortion and electronic nematic susceptibility.

We use transport and neutron scattering to study electronic, structural, and magnetic properties of the electron-doped BaFe2−xNixAs2 iron pnictides in uniaxial strained and external stress free detwinned state. Using a specially designed in-situ mechanical detwinning device, we demonstrate that the in-plane resistivity anisotropy observed in the uniaxial strained tetragonal state of BaFe2−xNixAs2 below a temperature T * , previously identified as a signature of the electronic nematic phase, is also present in the stress free tetragonal phase below T * * (< T * ). By carrying out neutron scattering measurements on BaFe2As2 and BaFe1.97Ni0.03As2, we argue that the resistivity anisotropy in the stress free tetragonal state of iron pnictides arises from the magnetoelastic coupling associated with antiferromagnetic order. These results thus indicate that the local lattice distortion and nematic spin correlations are responsible for the resistivity anisotropy in the tetragonal state of stress free iron pnictides, and suggest that resistivity anisotropy, spin excitation anisotropy, and orbital ordering found in the paramagnetic state of uniaxial strained iron pnictides are due to the externally applied uniaxial strain and its coupling to nematic susceptibility.

The magnetic insulator yttrium iron garnet (YIG) with a ferrimagnetic transition temperature of ∼560 K has been widely used in microwave and spintronic devices. Anomalous features in the spin Seeback effect (SSE) voltages have been observed in Pt/YIG and attributed to the magnon-phonon coupling. Here we use inelastic neutron scattering to map out low-energy spin waves and acoustic phonons of YIG at 100 K as a function of increasing magnetic field. By comparing the zero and 9.1 T data, we find that instead of splitting and opening up gaps at the spin wave and acoustic phonon dispersion intersecting points, magnon-phonon coupling in YIG enhances the hybridized scattering intensity. These results are different from expectations of conventional spin-lattice coupling, calling for new paradigms to understand the scattering process of magnon-phonon interactions and the resulting magnon-polarons.Spin waves (magnons) and phonons are propagating disturbance of the ordered magnetic moment and lattice vibrations, respectively. They constitute two fundamental quasiparticles in a solid and can couple together to form a hybrid quasiparticle [1,2]. Since our current understandings of these quasiparticles are based on linearized models that ignore all the high-order terms than quadratic terms and neglect interactions among the quasiparticle themselves [3], magnons and phonons are believed to be stable and unlikely to interact and breakdown for most purposes [4]. Therefore, discovering and understanding how the otherwise stable magnons and phonons can couple and interact with each other to influence the electronic properties of solids are one of the central themes in modern condensed matter physics.In general, spin-lattice (magnon-phonon) coupling can modify magnon in two different ways. First, the static lattice distortion induced by the magnetic order may affect the anisotropy of magnon exchange couplings, as seen in the spin waves of iron pnictides with large inplane magnetic exchange anisotropy [5]. Second, the dynamic lattice vibrations interact with time-dependent spin waves may give rise to significant magnon-phonon coupling [6,7]. One possible consequence of such coupling is to create energy gaps in the magnon dispersion at the nominal intersections of the magnon and phonon modes [8,9], as seen in antiferromagnet (Y,Lu)MnO 3 [10]. Alternatively, magnon-phonon coupling may give rise to spin-wave broadening at the magnon-phonon crossing points [11]. In both cases, we expect the integrated intensity of hybridized excitations at the intersecting points to be the sum of separate magnon and phonon scattering intensity without spin-lattice coupling [8]. Finally, if magnon and phonon lifetime-broadening is smaller than their interaction strength, the resulting mixed quasiparticles can form magnon polarons [6,7].Here we use inelastic neutron scattering to study lowenergy ferromagnetic magnons and acoustic phonons in the ferrimagnetic insulator yttrium iron garnet (YIG) with chemical formula -14]. At zero field and 100 K, we confir...

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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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.

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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...