Browsing by Author "Park, J.T."

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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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    Effect of Nematic Order on the Low-Energy Spin Fluctuations in Detwinned BaFe1.935Ni0.065As2
    (American Physical Society, 2016) Zhang, Wenliang; Park, J.T.; Lu, Xingye; Wei, Yuan; Ma, Xiaoyan; Hao, Lijie; Dai, Pengcheng; Meng, Zi Yang; Yang, Yi-feng; Luo, Huiqian; Li, Shiliang
    The origin of nematic order remains one of the major debates in iron-based superconductors. In theories based on spin nematicity, one major prediction is that the spin-spin correlation length at (0,π) should decrease with decreasing temperature below the structural transition temperature Ts. Here, we report inelastic neutron scattering studies on the low-energy spin fluctuations in BaFe1.935Ni0.065As2 under uniaxial pressure. Both intensity and spin-spin correlation start to show anisotropic behavior at high temperature, while the reduction of the spin-spin correlation length at (0,π) happens just below Ts, suggesting the strong effect of nematic order on low-energy spin fluctuations. Our results favor the idea that treats the spin degree of freedom as the driving force of the electronic nematic order.
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    Impact of uniaxial pressure on structural and magnetic phase transitions in electron-doped iron pnictides
    (American Physical Society, 2016) Lu, Xingye; Tseng, Kuo-Feng; Keller, T.; Zhang, Wenliang; Hu, Ding; Song, Yu; Man, Haoran; Park, J.T.; Luo, Huiqian; Li, Shiliang; Nevidomskyy, Andriy H.; Dai, Pengcheng
    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.
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    Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$
    (American Physical Society, 2015) Zhang, Chenglin; Park, J.T.; Lu, Xingye; Yu, Rong; Li, Yu; Zhang, Wenliang; Zhao, Yang; Lynn, J.W.; Si, Qimiao; Dai, Pengcheng
    We use inelastic neutron scattering (INS) to study the spin excitations in partially detwinned NaFe0.985Co0.015As which has coexisting static antiferromagnetic (AF) order and superconductivity (Tc=15 K, TN=30 K). In previous INS work on a twinned sample, spin excitations formed a dispersive sharp resonance near Er1=3.25 meV and a broad dispersionless mode at Er1=6 meV at the AF ordering wave vector QAF=Q1=(1,0) and its twinned domain Q2=(0,1). For partially detwinned NaFe0.985Co0.015As with the static AF order mostly occurring at QAF=(1,0), we still find a double resonance at both wave vectors with similar intensity. Since Q1=(1,0) characterizes the explicit breaking of the spin rotational symmetry associated with the AF order, these results indicate that the double resonance cannot be due to the static and fluctuating AF orders but originate from the superconducting gap anisotropy.
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    Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped NaFe0.985Co0.015As
    (American Physical Society, 2015) Zhang, Chenglin; Park, J.T.; Lu, Xingye; Yu, Rong; Li, Yu; Zhang, Wenliang; Zhao, Yang; Lynn, J.W.; Si, Qimiao; Dai, Pengcheng
    We use inelastic neutron scattering (INS) to study the spin excitations in partially detwinned NaFe0.985Co0.015As which has coexisting static antiferromagnetic (AF) order and superconductivity (Tc=15 K, TN=30 K). In previous INS work on a twinned sample, spin excitations formed a dispersive sharp resonance near Er1=3.25ﾠmeV and a broad dispersionless mode at Er1=6 meV at the AF ordering wave vector QAF=Q1=(1,0) and its twinned domain Q2=(0,1). For partially detwinned NaFe0.985Co0.015As with the static AF order mostly occurring at QAF=(1,0), we still find a double resonance at both wave vectors with similar intensity. Since Q1=(1,0) characterizes the explicit breaking of the spin rotational symmetry associated with the AF order, these results indicate that the double resonance cannot be due to the static and fluctuating AF orders but originate from the superconducting gap anisotropy.