Browsing by Author "Luo, Huiqian"
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Item Doping effects of Cr on the physical properties of BaFe1.9−xNi0.1CrxAs2(American Physical Society, 2018) Gong, Dongliang; Xie, Tao; Zhang, Rui; Birk, Jonas; Niedermayer, Christof; Han, Fei; Lapidus, S.H.; Dai, Pengcheng; Li, Shiliang; Luo, HuiqianWe present a systematic study on the heavily Cr doped iron pnictides BaFe1.9−xNi0.1CrxAs2 by using elastic neutron scattering, high-resolution synchrotron x-ray diffraction (XRD), resistivity, and Hall transport measurements. When the Cr concentration increases from x=0 to 0.8, neutron diffraction experiments suggest that the collinear antiferromagnetism persists in the whole doping range, where the Néel temperature TN coincides with the tetragonal-to-orthorhombic structural transition temperature Ts, and both of them keeps around 35 K. The magnetic ordered moment, on the other hand, increases within increasing x until x=0.5, and then decreases with further increasing x. Detailed refinement of the powder XRD patterns reveals that the Cr substitutions actually stretch the FeAs4 tetrahedron along the c axis and lift the arsenic height away from the Fe-Fe plane. Transport results indicate that the charge carriers become more localized upon Cr doping, then changes from electron type to hole type around x=0.5. Our results suggest that the ordered moment and the ordered temperature of static magnetism in iron pnictides can be decoupled and tuned separately by chemical doping.Item Doping evolution of antiferromagnetism and transport properties in nonsuperconducting BaFe2−2xNixCrxAs2(American Physical Society, 2015) Zhang, Rui; Gong, Dongliang; Lu, Xingye; Li, Shiliang; Laver, Mark; Niedermayer, Christof; Danilkin, Sergey; Deng, Guochu; Dai, Pengcheng; Luo, HuiqianWe report elastic neutron scattering and transport measurements on the Ni and Cr equivalently doped iron pnictide BaFe2−2xNixCrxAs2. Compared with the electron-doped BaFe2−xNixAs2, the long-range antiferromagnetic (AF) order in BaFe2−2xNixCrxAs2 is gradually suppressed with vanishing ordered moment and Néel temperature near x=0.20 without the appearance of superconductivity. A detailed analysis on the transport properties of BaFe2−xNixAs and BaFe2−2xNixCrxAs2 suggests that the non-Fermi-liquid behavior associated with the linear resistivity as a function of temperature may not correspond to the disappearance of the static AF order. From the temperature dependence of the resistivity in overdoped compounds without static AF order, we find that the transport properties are actually affected by Cr impurity scattering, which may induce a metal-to-insulator crossover in highly doped BaFe1.7−yNi0.3CryAs2.Item Doping evolution of antiferromagnetism and transport properties in nonsuperconducting BaFe2−2xNixCrxAs2(American Physical Society, 2015) Zhang, Rui; Gong, Dongliang; Lu, Xingye; Li, Shiliang; Laver, Mark; Niedermayer, Christof; Danilkin, Sergey; Deng, Guochu; Dai, Pengcheng; Luo, HuiqianWe report elastic neutron scattering and transport measurements on the Ni and Cr equivalently doped iron pnictide BaFe2−2xNixCrxAs2. Compared with the electron-doped BaFe2−xNixAs2, the long-range antiferromagnetic (AF) order in BaFe2−2xNixCrxAs2 is gradually suppressed with vanishing ordered moment and Néel temperature near x=0.20 without the appearance of superconductivity. A detailed analysis on the transport properties of BaFe2−xNixAs and BaFe2−2xNixCrxAs2 suggests that the non-Fermi-liquid behavior associated with the linear resistivity as a function of temperature may not correspond to the disappearance of the static AF order. From the temperature dependence of the resistivity in overdoped compounds without static AF order, we find that the transport properties are actually affected by Cr impurity scattering, which may induce a metal-to-insulator crossover in highly doped BaFe1.7−yNi0.3CryAs2.Item 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, ShiliangThe 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.Item 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, PengchengWe 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**(Item Electronic specific heat in BaFe2−xNixAs2(American Physical Society, 2016) Gong, Dongliang; Xie, Tao; Lu, Xingye; Ren, Cong; Shan, Lei; Zhang, Rui; Dai, Pengcheng; Yang, Yi-feng; Luo, Huiqian; Li, ShiliangWe have systematically studied the low-temperature specific heat of the BaFe2−xNixAs2 single crystals covering the whole superconducting dome. Using the nonsuperconducting heavily overdoped x=0.3 sample as a reference for the phonon contribution to the specific heat, we find that the normal-state electronic specific heats in the superconducting samples may have a nonlinear temperature dependence, which challenges previous results in the electron-doped Ba-122 iron-based superconductors. A model based on the presence of ferromagnetic spin fluctuations may explain the data between x=0.1 and x=0.15, suggesting the important role of Fermi-surface topology in understanding the normal-state electronic states.Item Energy dependence of the spin excitation anisotropy in uniaxial-strained BaFe1.9Ni0.1As2(American Physical Society, 2015) Song, Yu; Lu, Xingye; Abernathy, D.L.; Tam, David W.; Niedziela, J.L.; Tian, Wei; Luo, Huiqian; Si, Qimiao; Dai, PengchengWe use inelastic neutron scattering to study the temperature and energy dependence of the spin excitation anisotropy in uniaxial-strained electron-doped iron pnictide BaFe1.9Ni0.1As2 near optimal superconductivity (Tc=20K). Our work has been motivated by the observation of in-plane resistivity anisotropy in the paramagnetic tetragonal phase of electron-underdoped iron pnictides under uniaxial pressure, which has been attributed to a spin-driven Ising-nematic state or orbital ordering. Here we show that the spin excitation anisotropy, a signature of the spin-driven Ising-nematic phase, exists for energies below ∼60 meV in uniaxial-strained BaFe1.9Ni0.1As2. Since this energy scale is considerably larger than the energy splitting of the dxz and dyz bands of uniaxial-strained Ba(Fe1−xCox)2As2 near optimal superconductivity, spin Ising-nematic correlations are likely the driving force for the resistivity anisotropy and associated electronic nematic correlations.Item Excess-iron driven spin glass phase in Fe1 + yTe1 - xSex(IOP Publishing Ltd, 2021) Tian, Long; Liu, Panpan; Hong, Tao; Seydel, Tilo; Lu, Xingye; Luo, Huiqian; Li, Shiliang; Dai, PengchengThe iron-chalcogenide superconductor FeTe1–xSex displays a variety of exotic features distinct from iron pnictides. Although much effort has been devoted to understanding the interplay between magnetism and superconductivity near x = 0.5, the existence of a spin glass phase with short-range magnetic order in the doping range (x ∼ 0.1–0.3) has rarely been studied. Here, we use DC/AC magnetization and (quasi) elastic neutron scattering to confirm the spin-glass nature of the short-range magnetic order in a Fe1.07Te0.8Se0.2 sample. The AC-frequency dependent spin-freezing temperature Tf generates a frequency sensitivity ΔTf(ω)/[Tf(ω)Δlog10 ω] ≈ 0.028 and the description of the critical slowing down with τ = τ0(Tf/TSG – 1)−z v gives TSG ≈ 22 K and zv ≈ 10, comparable to that of a classical spin-glass system. We have also extended the frequency-dependent Tf to the smaller time scale using energy-resolution-dependent neutron diffraction measurements, in which the TN of the short-range magnetic order increases systematically with increasing energy resolution. By removing the excess iron through annealing in oxygen, the spin-freezing behavior disappears, and bulk superconductivity is realized. Thus, the excess Fe is the driving force for the formation of the spin-glass phase detrimental to bulk superconductivity.Item 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, PengchengWe 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.Item Longitudinal Spin Excitations and Magnetic Anisotropy in Antiferromagnetically Ordered BaFe2As2(American Physical Society, 2013) Wang, Chong; Zhang, Rui; Wang, Fa; Luo, Huiqian; Regnault, L.P.; Dai, Pengcheng; Li, YuanWe report on a spin-polarized inelastic neutron-scattering study of spin waves in the antiferromagnetically ordered state of BaFe2As2. Three distinct excitation components are identified, with spins fluctuating along the c axis, perpendicular to the ordering direction in the ab plane and parallel to the ordering direction. While the first two “transverse” components can be described by a linear spin-wave theory with magnetic anisotropy and interlayer coupling, the third “longitudinal” component is generically incompatible with the local-moment picture. It points toward a contribution of itinerant electrons to the magnetism that is already in the parent compound of this family of Fe-based superconductors.Item Nematic Crossover in BaFe2As2 under Uniaxial Stress(American Physical Society, 2015) Ren, Xiao; Duan, Lian; Hu, Yuwen; Li, Jiarui; Zhang, Rui; Luo, Huiqian; Dai, Pengcheng; Li, YuanRaman scattering can detect spontaneous point-group symmetry breaking without resorting to single-domain samples. Here, we use this technique to study BaFe2As2, the parent compound of the “122” Fe-based superconductors. We show that an applied compression along the Fe-Fe direction, which is commonly used to produce untwinned orthorhombic samples, changes the structural phase transition at temperature Ts into a crossover that spans a considerable temperature range above Ts. Even in crystals that are not subject to any applied force, a distribution of substantial residual stress remains, which may explain phenomena that are seemingly indicative of symmetry breaking above Ts. Our results are consistent with an onset of spontaneous nematicity only below Ts.Item Nematic Fluctuations in the Non-Superconducting Iron Pnictide BaFe1.9−xNi0.1CrxAs2(Frontiers Media S.A., 2022) Gong, Dongliang; Yi, Ming; Wang, Meng; Xie, Tao; Zhang, Wenliang; Danilkin, Sergey; Deng, Guochu; Liu, Xinzhi; Park, Jitae T.; Ikeuchi, Kazuhiko; Kamazawa, Kazuya; Mo, Sung-Kwan; Hashimoto, Makoto; Lu, Donghui; Zhang, Rui; Dai, Pengcheng; Birgeneau, Robert J.; Li, Shiliang; Luo, Huiqian; Rice Center for Quantum MaterialsThe main driven force of the electronic nematic phase in iron-based superconductors is still under debate. Here, we report a comprehensive study on the nematic fluctuations in a non-superconducting iron pnictide system BaFe1.9−xNi0.1CrxAs2 by electronic transport, angle-resolved photoemission spectroscopy (ARPES), and inelastic neutron scattering (INS) measurements. Previous neutron diffraction and transport measurements suggested that the collinear antiferromagnetism persists to x = 0.8, with similar Néel temperature TN and structural transition temperature Ts around 32 K, but the charge carriers change from electron type to hole type around x = 0.5. In this study, we have found that the in-plane resistivity anisotropy also highly depends on the Cr dopings and the type of charge carriers. While ARPES measurements suggest possibly weak orbital anisotropy onset near Ts for both x = 0.05 and x = 0.5 compounds, INS experiments reveal clearly different onset temperatures of low-energy spin excitation anisotropy, which is likely related to the energy scale of spin nematicity. These results suggest that the interplay between the local spins on Fe atoms and the itinerant electrons on Fermi surfaces is crucial to the nematic fluctuations of iron pnictides, where the orbital degree of freedom may behave differently from the spin degree of freedom, and the transport properties are intimately related to the spin dynamics.Item Nematic magnetoelastic effect contrasted between Ba(Fe1−xCox)2As2 and FeSe(American Physical Society, 2016) Hu, Yuwen; Ren, Xiao; Zhang, Rui; Luo, Huiqian; Kasahara, Shigeru; Watashige, Tatsuya; Shibauchi, Takasada; Dai, Pengcheng; Zhang, Yan; Matsuda, Yuji; Li, YuanTo elucidate the origin of nematic order in Fe-based superconductors, we report a Raman scattering study of lattice dynamics, which quantify the extent of C4-symmetry breaking, in BaFe2As2 and FeSe. FeSe possesses a nematic ordering temperature Ts and orbital-related band-energy split below Ts that are similar to those in BaFe2As2, but unlike BaFe2As2 it has no long-range magnetic order. We find that the Eg phonon-energy split in FeSe becomes substantial only well below Ts, and its saturated value is much smaller than that in BaFe2As2. Together with reported results for the Ba(Fe1−xCox)2As2 family, the data suggest that magnetism exerts a major influence on the lattice.Item Nematic Quantum Critical Fluctuations in BaFe2−xNixAs2(American Physical Society, 2016) Liu, Zhaoyu; Gu, Yanhong; Zhang, Wei; Gong, Dongliang; Zhang, Wenliang; Xie, Tao; Lu, Xingye; Ma, Xiaoyan; Zhang, Xiaotian; Zhang, Rui; Zhu, Jun; Ren, Cong; Shan, Lei; Qiu, Xianggang; Dai, Pengcheng; Yang, Yi-feng; Luo, Huiqian; Li, ShiliangWe have systematically studied the nematic fluctuations in the electron-doped iron-based superconductor BaFe2−xNixAs2 by measuring the in-plane resistance change under uniaxial pressure. While the nematic quantum critical point can be identified through the measurements along the (110) direction, as studied previously, quantum and thermal critical fluctuations cannot be distinguished due to similar Curie-Weiss-like behaviors. Here we find that a sizable pressure-dependent resistivity along the (100) direction is present in all doping levels, which is against the simple picture of an Ising-type nematic model. The signal along the (100) direction becomes maximum at optimal doping, suggesting that it is associated with nematic quantum critical fluctuations. Our results indicate that thermal fluctuations from striped antiferromagnetic order dominate the underdoped regime along the (110) direction. We argue that either there is a strong coupling between the quantum critical fluctuations and the fermions, or more exotically, a higher symmetry may be present around optimal doping.Item Nodeless superconductivity in the presence of spin-density wave in pnictide superconductors: The case of BaFe2−xNixAs2(American Physical Society, 2015) Abdel-Hafiez, Mahmoud; Zhang, Yuanyuan; He, Zheng; Zhao, Jun; Bergmann, Christoph; Krellner, Cornelius; Duan, Chun-Gang; Lu, Xingye; Luo, Huiqian; Dai, Pengcheng; Chen, Xiao-JiaThe characteristics of Fe-based superconductors are manifested in their electronic, magnetic properties, and pairing symmetry of the Cooper pair, but the latter remain to be explored. Usually in these materials, superconductivity coexists and competes with magnetic order, giving unconventional pairing mechanisms. We report on the results of the bulk magnetization measurements in the superconducting state and the low-temperature specific heat down to 0.4 K for BaFe2−xNixAs2 single crystals. The electronic specific heat displays a pronounced anomaly at the superconducting transition temperature and a small residual part at low temperatures in the superconducting state. The normal-state Sommerfeld coefficient increases with Ni doping for x=0.092, 0.096, and 0.10, which illustrates the competition between magnetism and superconductivity. Our analysis of the temperature dependence of the superconducting-state specific heat and the London penetration depth provides strong evidence for a two-band s-wave order parameter. Further, the data of the London penetration depth calculated from the lower critical field follow an exponential temperature dependence, characteristic of a fully gapped superconductor. These observations clearly show that the superconducting gap in the nearly optimally doped compounds is nodeless.Item Short-range cluster spin glass near optimal superconductivity in BaFe2−xNixAs2(American Physical Society, 2014) Lu, Xingye; Tam, David W.; Zhang, Chenglin; Luo, Huiqian; Wang, Meng; Zhang, Rui; Harriger, Leland W.; Keller, T.; Keimer, B.; Regnault, L.-P.; Maier, Thomas A.; Dai, PengchengHigh-temperature superconductivity in iron pnictides occurs when electrons are doped into their antiferromagnetic (AF) parent compounds. In addition to inducing superconductivity, electron doping also changes the static commensurate AF order in the undoped parent compounds into short-range incommensurate AF order near optimal superconductivity. Here we use neutron scattering to demonstrate that the incommensurate AF order in BaFe2−xNixAs2 is not a spin-density wave arising from the itinerant electrons in nested Fermi surfaces, but is consistent with a cluster spin glass in the matrix of the superconducting phase. Therefore, optimal superconductivity in iron pnictides coexists and competes with a mesoscopically separated cluster spin glass phase, much different from the homogeneous coexisting AF and superconducting phases in the underdoped regime.Item Spin excitation anisotropy in the optimally isovalent-doped superconductor BaFe2(As0.7P0.3)2(American Physical Society, 2017) Hu, Ding; Zhang, Wenliang; Wei, Yuan; Roessli, Bertrand; Skoulatos, Markos; Regnault, Louis Pierre; Chen, Genfu; Song, Yu; Luo, Huiqian; Li, Shiliang; Dai, PengchengWe use neutron polarization analysis to study spin excitation anisotropy in the optimally isovalent-doped superconductor BaFe2(As0.7P0.3)2 (Tc=30 K). Different from optimally hole- and electron-doped BaFe2As2, where there is a clear spin excitation anisotropy in the paramagnetic tetragonal state well above Tc, we find no spin excitation anisotropy for energies above 2 meV in the normal state of BaFe2(As0.7P0.3)2. Upon entering the superconducting state, significant spin excitation anisotropy develops at the antiferromagnetic (AF) zone center QAF=(1,0,L=odd), while the magnetic spectrum is isotropic at the zone boundary Q=(1,0,L=even). By comparing the temperature, wave vector, and polarization dependence of the spin excitation anisotropy in BaFe2(As0.7P0.3)2 and hole-doped Ba0.67K0.33Fe2As2 (Tc=38 K), we conclude that such anisotropy arises from spin-orbit coupling and is associated with the nearby AF order and superconductivity.Item Spin excitations in optimally P-dopedᅠBaFe2(As0.7P0.3)2 superconductor(American Physical Society, 2016) Hu, Ding; Yin, Zhiping; Zhang, Wenliang; Ewings, R.A.; Ikeuchi, Kazuhiko; Nakamura, Mitsutaka; Roessli, Bertrand; Wei, Yuan; Zhao, Lingxiao; Chen, Genfu; Li, Shiliang; Luo, Huiqian; Haule, Kristjan; Kotliar, Gabriel; Dai, PengchengWe use inelastic neutron scattering to study the temperature and energy dependence of spin excitations in an optimally P-doped BaFe2(As0.7P0.3)2 superconductor (Tc=30 K) throughout the Brillouin zone. In the undoped state, spin waves and paramagnetic spin excitations of BaFe2As2stem from an antiferromagnetic (AF) ordering wave vector QAF=(±1,0), and peak near the zone boundary at (±1,±1) around 180 meV. Replacing 30% As by smaller P to induce superconductivity, low-energy spin excitations of BaFe2(As0.7P0.3)2 form a resonance in the superconducting state and high-energy spin excitations now peak around 220 meV near (±1,±1). These results are consistent with calculations from a combined density functional theory and dynamical mean field theory, and suggest that the decreased average pnictogen height in BaFe2(As0.7P0.3)2 reduces the strength of electron correlations and increases the effective bandwidth of magnetic excitations.Item Spin Waves in Detwinned BaFe2As2(American Physical Society, 2018) Lu, Xingye; Scherer, Daniel D.; Tam, David W.; Zhang, Wenliang; Zhang, Rui; Luo, Huiqian; Harriger, Leland W.; Walker, H.C.; Adroja, D.T.; Andersen, Brian M.; Dai, PengchengUnderstanding magnetic interactions in the parent compounds of high-temperature superconductors forms the basis for determining their role for the mechanism of superconductivity. For parent compounds of iron pnictide superconductors such as AFe_{2}As_{2} (A=Ba, Ca, Sr), although spin excitations have been mapped out throughout the entire Brillouin zone, the respective measurements were carried out on twinned samples and did not allow for a conclusive determination of the spin dynamics. Here we use inelastic neutron scattering to completely map out spin excitations of ∼100% detwinned BaFe_{2}As_{2}. By comparing observed spectra with theoretical calculations, we conclude that the spin excitations can be well described by an itinerant model when taking into account moderate electronic correlation effects.Item Structural and Magnetic Phase Transitions near Optimal Superconductivity in BaFe2(As1−xPx)2(American Physical Society, 2015) Hu, Ding; Lu, Xingye; Zhang, Wenliang; Luo, Huiqian; Li, Shiliang; Wang, Peipei; Chen, Genfu; Han, Fei; Banjara, Shree R.; Sapkota, A.; Kreyssig, A.; Goldman, A.I.; Yamani, Z.; Niedermayer, Christof; Skoulatos, Markos; Georgii, Robert; Keller, T.; Wang, Pengshuai; Yu, Weiqiang; Dai, PengchengWe use nuclear magnetic resonance (NMR), high-resolution x-ray, and neutron scattering studies to study structural and magnetic phase transitions in phosphorus-doped BaFe2(As1−xPx)2. Previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at x=0.3. However, we show that the tetragonal-to-orthorhombic structural (Ts) and paramagnetic to antiferromagnetic (AF, TN) transitions in BaFe2(As1−xPx)2 are always coupled and approach TN≈Ts≥Tc (≈29 K) for x=0.29 before vanishing abruptly for x≥0.3. These results suggest that AF order in BaFe2(As1−xPx)2 disappears in a weakly first-order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP.