Browsing by Author "Roessli, Bertrand"

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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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    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, Pengcheng
    We 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.
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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.