Browsing by Author "Kulda, J."

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    Anisotropic neutron spin resonance in underdoped superconducting NaFe1−xCoxAs
    (American Physical Society, 2014) Zhang, Chenglin; Song, Yu; Regnault, L.-P.; Su, Yixi; Enderle, M.; Kulda, J.; Tan, Guotai; Sims, Zachary C.; Egami, Takeshi; Si, Qimiao; Dai, Pengcheng
    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 axes, 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.
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    Direct observation of spin excitation anisotropy in the paramagnetic orthorhombic state of BaFe2−xNixAs2
    (American Physical Society, 2018) Man, Haoran; Zhang, Rui; Park, J.T.; Lu, Xingye; Kulda, J.; Ivanov, A.; Dai, Pengcheng
    We use transport and inelastic neutron-scattering measurements to investigate single crystals of iron pnictide BaFe2−xNixAs2(x=0,0.03), which exhibit a tetragonal-to-orthorhombic structural transition at Ts and stripe antiferromagnetic order at TN(Ts≥TN). Using a tunable uniaxial pressure device, we detwin the crystals and study their transport and spin excitation properties at antiferromagnetic wave-vector S1(1,0) and its 90∘ rotated wave-vector S2(0,1) under different pressure conditions. We find that uniaxial pressure necessary to detwin and maintain the single domain orthorhombic antiferromagnetic phase of BaFe2−xNixAs2 induces resistivity and spin excitation anisotropy at temperatures above zero pressure Ts. In the uniaxial pressure-free detwinned sample, spin excitation anisotropy between S1(1,0) and S2(0,1) first appears in the paramagnetic orthorhombic phase below Ts. These results are consistent with predictions of spin nematic theory, suggesting the absence of structural or nematic phase transition above Ts in iron pnictides.
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    Electronic nematic correlations in the stress-free tetragonal state of BaFe2−xNixAs2
    (American Physical Society, 2015) Man, Haoran; Lu, Xingye; Chen, Justin S.; Zhang, Rui; Zhang, Wenliang; Luo, Huiqian; Kulda, J.; Ivanov, A.; Keller, T.; Morosan, Emilia; Si, Qimiao; Dai, Pengcheng
    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 states. 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**(