Browsing by Author "Yin, Zhiping"

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    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, Pengcheng
    Superconductivity 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.
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    Coexistence of Ferromagnetic and Stripe Antiferromagnetic Spin Fluctuations in SrCo2As2
    (American Physical Society, 2019) Li, Yu; Yin, Zhiping; Liu, Zhonghao; Wang, Weiyi; Xu, Zhuang; Song, Yu; Tian, Long; Huang, Yaobo; Shen, Dawei; Abernathy, D.L.; Niedziela, J.L.; Ewings, R.A.; Perring, T.G.; Pajerowski, Daniel M.; Matsuda, Masaaki; Bourges, Philippe; Mechthild, Enderle; Su, Yixi; Dai, Pengcheng
    We use inelastic neutron scattering to study energy and wave vector dependence of spin fluctuations in SrCo2As2, derived from SrFe2−xCoxAs2 iron pnictide superconductors. Our data reveal the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) spin fluctuations at wave vectors QAF=(1,0) and QFM=(0,0)/(2,0), respectively. By comparing neutron scattering results with those of dynamic mean field theory calculation and angle-resolved photoemission spectroscopy experiments, we conclude that both AF and FM spin fluctuations in SrCo2As2 are closely associated with a flatband of the eg orbitals near the Fermi level, different from the t2g orbitals in superconducting SrFe2−xCoxAs2. Therefore, Co substitution in SrFe2−xCoxAs2 induces a t2g to eg orbital switching, and is responsible for FM spin fluctuations detrimental to the singlet pairing superconductivity.
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    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, Pengcheng
    We 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.
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    Orbital Selective Spin Excitations and their Impact on Superconductivity of LiFe1−xCoxAs
    (American Physical Society, 2016) Li, Yu; Yin, Zhiping; Wang, Xiancheng; Tam, David W.; Abernathy, D.L.; Podlesnyak, A.; Zhang, Chenglin; Wang, Meng; Xing, Lingyi; Jin, Changqing; Haule, Kristjan; Kotliar, Gabriel; Maier, Thomas A.; Dai, Pengcheng
    We use neutron scattering to study spin excitations in single crystals of LiFe0.88Co0.12As, which is located near the boundary of the superconducting phase of LiFe1−xCoxAs and exhibits non-Fermi-liquid behavior indicative of a quantum critical point. By comparing spin excitations of LiFe0.88Co0.12As with a combined density functional theory and dynamical mean field theory calculation, we conclude that wave-vector correlated low energy spin excitations are mostly from the dxy orbitals, while high-energy spin excitations arise from the dyz and dxz orbitals. Unlike most iron pnictides, the strong orbital selective spin excitations in the LiFeAs family cannot be described by an anisotropic Heisenberg Hamiltonian. While the evolution of low-energy spin excitations of LiFe1−xCoxAs is consistent with the electron-hole Fermi surface nesting conditions for the dxy orbital, the reduced superconductivity in LiFe0.88Co0.12As suggests that Fermi surface nesting conditions for the dyz and dxz orbitals are also important for superconductivity in iron pnictides.
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    Spin excitations in metallic kagome lattice FeSn and CoSn
    (Springer Nature, 2021) Xie, Yaofeng; Chen, Lebing; Chen, Tong; Wang, Qi; Yin, Qiangwei; Stewart, J. Ross; Stone, Matthew B.; Daemen, Luke L.; Feng, Erxi; Cao, Huibo; Lei, Hechang; Yin, Zhiping; MacDonald, Allan H.; Dai, Pengcheng
    In two-dimensional (2D) metallic kagome lattice materials, destructive interference of electronic hopping pathways around the kagome bracket can produce nearly localized electrons, and thus electronic bands that are flat in momentum space. When ferromagnetic order breaks the degeneracy of the electronic bands and splits them into the spin-up majority and spin-down minority electronic bands, quasiparticle excitations between the spin-up and spin-down flat bands should form a narrow localized spin-excitation Stoner continuum coexisting with well-defined spin waves in the long wavelengths. Here we report inelastic neutron scattering studies of spin excitations in 2D metallic kagome lattice antiferromagnetic FeSn and paramagnetic CoSn, where angle resolved photoemission spectroscopy experiments found spin-polarized and nonpolarized flat bands, respectively, below the Fermi level. Our measurements on FeSn and CoSn reveal well-defined spin waves extending above 140 meV and correlated paramagnetic scattering around Γ point below 90 meV, respectively. In addition, we observed non-dispersive excitations at ~170 meV and ~360 meV arising mostly from hydrocarbon scattering of the CYTOP-M used to glue the samples to aluminum holder. Therefore, our results established the evolution of spin excitations in FeSn and CoSn, and identified anomalous flat modes overlooked by the neutron scattering community for many years.
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    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, Pengcheng
    We 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.
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    Two-Dimensional Massless Dirac Fermions in Antiferromagnetic AFe2As2 (A=Ba,Sr)
    (American Physical Society, 2017) Chen, Zhi-Guo; Wang, Luyang; Song, Yu; Lu, Xingye; Luo, Huiqian; Zhang, Chenglin; Dai, Pengcheng; Yin, Zhiping; Haule, Kristjan; Kotliar, Gabriel
    We report infrared studies of AFe2As2 (A=Ba, Sr), two representative parent compounds of iron-arsenide superconductors, at magnetic fields (B) up to 17.5 T. Optical transitions between Landau levels (LLs) were observed in the antiferromagnetic states of these two parent compounds. Our observation of a √B dependence of the LL transition energies, the zero-energy intercepts at B=0  T under the linear extrapolations of the transition energies and the energy ratio (∼2.4) between the observed LL transitions, combined with the linear band dispersions in two-dimensional (2D) momentum space obtained by theoretical calculations, demonstrates the existence of massless Dirac fermions in the antiferromagnet BaFe2As2. More importantly, the observed dominance of the zeroth-LL-related absorption features and the calculated bands with extremely weak dispersions along the momentum direction kz indicate that massless Dirac fermions in BaFe2As2 are 2D. Furthermore, we find that the total substitution of the barium atoms in BaFe2As2 by strontium atoms not only maintains 2D massless Dirac fermions in this system, but also enhances their Fermi velocity, which supports that the Dirac points in iron-arsenide parent compounds are topologically protected.