Browsing by Author "Tan, Guotai"
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Item 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, PengchengWe 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.Item c-axis pressure-induced antiferromagnetic order in optimally P-doped BaFe2(As0.70P0.30)2 superconductor(Springer Nature, 2018) Hu, Ding; Wang, Weiyi; Zhang, Wenliang; Wei, Yuan; Gong, Dongliang; Tam, David W.; Zhou, Panpan; Li, Yu; Tan, Guotai; Song, Yu; Georgii, Robert; Pedersen, Björn; Cao, Huibo; Tian, Wei; Roessli, Bertrand; Yin, Zhiping; Dai, PengchengSuperconductivity in BaFe2(As1−xPx)2 iron pnictides emerges when its in-plane two-dimensional (2D) orthorhombic lattice distortion associated with nematic phase at Ts and three-dimensional (3D) collinear antiferromagnetic order at TN (Ts = TN) are gradually suppressed with increasing x, reaching optimal superconductivity around x = 0.30 with Tc ≈ 30 K. Here we show that a moderate uniaxial pressure along the c-axis in BaFe2(As0.70P0.30)2 spontaneously induces a 3D collinear antiferromagnetic order with TN = Ts > 30 K, while only slightly suppresses Tc. Although a ~ 400 MPa pressure compresses the c-axis lattice while expanding the in-plane lattice and increasing the nearest-neighbor Fe–Fe distance, it barely changes the average iron-pnictogen height in BaFe2(As0.70P0.30)2. Therefore, the pressure-induced antiferromagnetic order must arise from a strong in-plane magnetoelastic coupling, suggesting that the 2D nematic phase is a competing state with superconductivity.Item Unknown Effect of Pnictogen Height on Spin Waves in Iron Pnictides(American Physical Society, 2014) Zhang, Chenglin; Harriger, Leland W.; Yin, Zhiping; Lv, Weicheng; Wang, Miaoyin; Tan, Guotai; Song, Yu; Abernathy, D.L.; Tian, Wei; Egami, Takeshi; Haule, Kristjan; Kotliar, Gabriel; Dai, PengchengWe use inelastic neutron scattering to study spin waves in the antiferromagnetic ordered phase of iron pnictide NaFeAs throughout the Brillouin zone. Comparing with the well-studied AFe2As2 (A=Ca, Sr, Ba) family, spin waves in NaFeAs have considerably lower zone boundary energies and more isotropic effective in-plane magnetic exchange couplings. These results are consistent with calculations from a combined density functional theory and dynamical mean field theory and provide strong evidence that pnictogen height controls the strength of electron-electron correlations and consequently the effective bandwidth of magnetic excitations.Item Unknown Electron doping evolution of structural and antiferromagnetic phase transitions in NaFe1−xCoxAs iron pnictides(American Physical Society, 2016) Tan, Guotai; Song, Yu; Zhang, Chenglin; Lin, Lifang; Xu, Zhuang; Hou, Tingting; Tian, Wei; Cao, Huibo; Li, Shiliang; Feng, Shiping; Dai, PengchengWe use transport and neutron diffraction to study the electronic phase diagram of NaFe1−xCoxAs. In the undoped state, NaFeAs exhibits a tetragonal-to-orthorhombic structural transition below Tsfollowed by a collinear antiferromagnetic (AF) order below TN. Upon codoping to form NaFe1−xCoxAs,Ts and TN are gradually suppressed, leading to optimal superconductivity near Co-doping x=0.025. While transport experiments on these materials reveal an anomalous behavior suggesting the presence of a quantum critical point (QCP) near optimal superconductivity, our neutron diffraction results indicate that commensurate AF order becomes transversely incommensurate with TN>Tc before vanishing abruptly at optimal superconductivity. These results are remarkably similar to electron-doping and isovalent-doping evolution of the AF order in BaFe2−xNixAs2 and BaFe2(As1−xPx)2, thus suggesting a universal behavior in the suppression of the magnetic order in iron pnictides as superconductivity is induced.Item Unknown Electron doping evolution of the magnetic excitations in NaFe1−xCoxAs(American Physical Society, 2016) Carr, Scott V.; Zhang, Chenglin; Song, Yu; Tan, Guotai; Li, Yu; Abernathy, D.L.; Stone, M.B.; Granroth, G.E.; Perring, T.G.; Dai, PengchengWe use time-of-flight (TOF) inelastic-neutron-scattering (INS) spectroscopy to investigate the doping dependence of magnetic excitations across the phase diagram of NaFe 1 − x Co x As with x = 0 , 0.0175, 0.0215, 0.05, and 0.11 . The effect of electron doping by partially substituting Fe by Co is to form resonances that couple with superconductivity, broaden, and suppress low-energy ( E ≤ 80 meV) spin excitations compared with spin waves in undoped NaFeAs. However, high-energy ( E > 80 meV) spin excitations are weakly Co-doping-dependent. Integration of the local spin dynamic susceptibility χ ' ' ( ω ) of NaFe 1 − x Co x As reveals a total fluctuating moment of 3.6 μ 2 B /Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in Co-overdoped nonsuperconducting NaFe 0.89 Co 0.11 As suggests that Fermi surface nesting is responsible for low-energy spin excitations. These results parallel the Ni-doping evolution of spin excitations in BaFe 2 − x Ni x As 2 in spite of the differences in crystal structures and Fermi surface evolution in these two families of iron pnictides, thus confirming the notion that low-energy spin excitations coupling with itinerant electrons are important for superconductivity, while weakly doping-dependent high-energy spin excitations result from localized moments.Item Unknown Electron doping evolution of the neutron spin resonance in NaFe1−xCoxAs(American Physical Society, 2016) Zhang, Chenglin; Lv, Weicheng; Tan, Guotai; Song, Yu; Carr, Scott V.; Chi, Songxue; Matsuda, M.; Christianson, A.D.; Fernandez-Baca, J.A.; Harriger, L.W.; Dai, PengchengNeutron spin resonance, a collective magnetic excitation coupled to superconductivity, is one of the most prominent features shared by a broad family of unconventional superconductors including copper oxides, iron pnictides, and heavy fermions. In this paper, we study the doping evolution of the resonances in NaFe1−xCoxAs covering the entire superconducting dome. For the underdoped compositions, two resonance modes coexist. As doping increases, the low-energy resonance gradually loses its spectral weight to the high-energy one but remains at the same energy. By contrast, in the overdoped regime we only find one single resonance, which acquires a broader width in both energy and momentum but retains approximately the same peak position even when Tc drops by nearly a half compared to optimal doping. These results suggest that the energy of the resonance in electron overdoped NaFe1−xCoxAs is neither simply proportional to Tc nor the superconducting gap but is controlled by the multiorbital character of the system and doped impurity scattering effect.Item Unknown In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFe2As2(Springer Nature, 2020) Liu, Panpan; Klemm, Mason L.; Tian, Long; Lu, Xingye; Song, Yu; Tam, David W.; Schmalzl, Karin; Park, J. T.; Li, Yu; Tan, Guotai; Su, Yixi; Bourdarot, Frédéric; Zhao, Yang; Lynn, Jeffery W.; Birgeneau, Robert J.; Dai, PengchengA small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe2As2, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature Ts. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature TN ( < Ts) and create in-plane resistivity anisotropy above Ts. Here we use neutron polarization analysis to show that such a strain on BaFe2As2 also induces a static or quasi-static out-of-plane (c-axis) AF order and its associated critical spin fluctuations near TN/Ts. Therefore, uniaxial pressure necessary to detwin single crystals of BaFe2As2 actually rotates the easy axis of the collinear AF order near TN/Ts, and such effects due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system.Item Unknown Influence of doping on the spin dynamics and magnetoelectric effect in hexagonalﾠY0.7Lu0.3MnO3(American Physical Society, 2014) Tian, W.; Tan, Guotai; Liu, Liu; Zhang, Jinxing; Winn, Barry; Hong, Tao; Fernandez-Baca, J.A.; Zhang, Chenglin; Dai, PengchengWe use inelastic neutron scattering and dielectric constant measurements to study the doping influence on the spin dynamics and magnetoelectric (ME) effect in hexagonalY0.7Lu0.3MnO3. In undoped YMnO3 and LuMnO3, the Mn trimerization distortion has been suggested to play a key role in determining the magnetic structure and the magnetoelectric effect. In Y0.7Lu0.3MnO3, at the antiferromagnetic zone center, we observed a much smaller Δ12≈0.52 meV gap (which is ∼2.5meV for both YMnO3 and LuMnO3) that coincides with a weaker in-plane dielectric anomaly at TN; both can be attributed to a weaker Mn trimerization distortion in Y0.7Lu0.3MnO3 compared to YMnO3 and LuMnO3. The results provide strong evidence that the magnitude of ME coupling is linked to the strength of the trimerization distortion, suggesting the Mn trimerization is responsible for the ME effect in Y1−yLuyMnO3.Item Unknown Phase diagram and neutron spin resonance of superconducting NaFe1−xCuxAs(American Physical Society, 2017) Tan, Guotai; Song, Yu; Zhang, Rui; Lin, Lifang; Xu, Zhuang; Tian, Long; Chi, Songxue; Graves-Brook, M.K.; Li, Shiliang; Dai, PengchengWe use transport and neutron scattering to study the electronic phase diagram and spin excitations of NaFe1−xCuxAs single crystals. Similar to Co- and Ni-doped NaFeAs, a bulk superconducting phase appears near x≈2% with the suppression of stripe-type magnetic order in NaFeAs. Upon further increasing Cu concentration the system becomes insulating, culminating in an antiferromagnetically ordered insulating phase near x≈50%. Using transport measurements, we demonstrate that the resistivity in NaFe1−xCuxAs exhibits non-Fermi-liquid behavior near x≈1.8%. Our inelastic neutron scattering experiments reveal a single neutron spin resonance mode exhibiting weak dispersion along c axis in NaFe0.98Cu0.02As. The resonance is high in energy relative to the superconducting transition temperature Tc but weak in intensity, likely resulting from impurity effects. These results are similar to other iron pnictides superconductors despite that the superconducting phase in NaFe1−xCuxAs is continuously connected to an antiferromagnetically ordered insulating phase near x≈50% with significant electronic correlations. Therefore, electron correlations is an important ingredient of superconductivity in NaFe1−xCuxAs and other iron pnictides.Item Unknown Spin anisotropy due to spin-orbit coupling in optimally hole-doped Ba0:67K0:33Fe2As2(American Physical Society, 2016) Song, Yu; Man, Haoran; Zhang, Rui; Lu, Xingye; Zhang, Chenglin; Wang, Meng; Tan, Guotai; Regnault, L.-P.; Su, Yixi; Kang, Jian; Fernandes, Rafael M.; Dai, PengchengWe 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 pronounced 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 with electron-doped BaFe2−xTMxAs2 (TM=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 hole- and electron-doped iron pnictides near optimal superconductivity onset at temperatures similar to the temperatures at which the elastoresistance deviates 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.Item Unknown Superconductivity and antiferromagnetism in Ba0.75K0.25Fe2As2 single crystals as seen by 57Fe Mossbauer spectroscopy(American Physical Society, 2013) Munevar, J.; Micklitz, H.; Aguero, J.; Tan, Guotai; Zhang, Chenglin; Dai, Pengcheng; Baggio-Saitovitch, E.Item Unknown Superconductivity and electronic fluctuations in Ba1−xKxFe2As2 studied by Raman scattering(American Physical Society, 2017) Wu, S.-F.; Richard, P.; Ding, H.; Wen, H.-H.; Tan, Guotai; Wang, Meng; Zhang, Chenglin; Dai, Pengcheng; Blumberg, G.Using polarization-resolved electronic Raman scattering we study underdoped, optimally doped, and overdoped Ba1−xKxFe2As2 samples in the normal and superconducting states. We show that low-energy nematic fluctuations are universal for all studied doping ranges. In the superconducting state, we observe two distinct superconducting pair-breaking peaks corresponding to one large and one small superconducting gap. In addition, we detect a collective mode below the superconducting transition in the B2g channel and determine the evolution of its binding energy with doping. Possible scenarios are proposed to explain the origin of the in-gap collective mode. In the superconducting state of the underdoped regime, we detect a reentrance transition below which the spectral background changes and the collective mode vanishes.Item Unknown Temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped NaFe0.9785Co0.0215As(American Physical Society, 2017) Song, Yu; Wang, Weiyi; Zhang, Chenglin; Gu, Yanhong; Lu, Xingye; Tan, Guotai; Su, Yixi; Bourdarot, Frédéric; Christianson, A.D.; Li, Shiliang; Dai, PengchengWe use unpolarized and polarized neutron scattering to study the temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped NaFe0.9785Co0.0215As, with coexisting superconductivity (Tc≈19 K) and weak antiferromagnetic order (TN≈30 K, ordered moment ≈0.02μB/Fe). A single spin resonance mode with intensity tracking the superconducting order parameter is observed, although energy of the mode only softens slightly upon approaching Tc. Polarized neutron scattering reveals that the single resonance is mostly isotropic in spin space, similar to overdoped NaFe0.935Co0.045As but different from optimal electron-, hole-, and isovalently doped BaFe2As2 compounds, all featuring an additional prominent anisotropic component. Spin anisotropy in NaFe0.9785Co0.0215As is instead present at energies below the resonance, which becomes partially gapped below Tc, similar to the situation in optimally doped YBa2Cu3O6.9. Our results indicate that anisotropic spin fluctuations in NaFe1−xCoxAs appear in the form of a resonance in the underdoped regime, become partially gapped below Tc near optimal doping, and disappear in overdoped compounds.Item Unknown Unusual suppression of a spin resonance mode by magnetic field in underdoped NaFe1−xCoxAs: Evidence for orbital-selective pairing(American Physical Society, 2018) Song, Yu; Tan, Guotai; Zhang, Chenglin; Toft-Petersen, Rasmus; Yu, Rong; Dai, PengchengWe use inelastic neutron scattering to study the fate of the two spin resonance modes in underdoped superconducting NaFe1−xCoxAs(x=0.0175) under applied magnetic fields. While an in-plane magnetic field of B=12 T only modestly suppresses superconductivity and enhances static antiferromagnetic order, the two spin resonance modes display disparate responses. The spin resonance mode at higher energy is mildly suppressed, consistent with the field effect in other unconventional superconductors. The spin resonance mode at lower energy, on the other hand, is almost completely suppressed. Such dramatically different responses to applied magnetic field indicate distinct origins of the two spin resonance modes, resulting from the strongly orbital-selective nature of spin excitations and Cooper pairing in iron-based superconductors.