Browsing by Author "Chen, Genfu"

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    Role of the 245 phase in alkaline iron selenide superconductors revealed by high-pressure studies
    (American Physical Society, 2014) Gao, Peiwen; Yu, Rong; Sun, Liling; Wang, Hangdong; Wang, Zhen; Wu, Qi; Fang, Minghu; Chen, Genfu; Guo, Jing; Zhang, Chao; Gu, Dachun; Tian, Huanfang; Li, Jianqi; Liu, Jing; Li, Yanchun; Li, Xiaodong; Jiang, Sheng; Yang, Ke; Li, Aiguo; Si, Qimiao; Zhao, Zhongxian
    There is considerable interest in uncovering the physics of iron-based superconductivity from the alkaline iron selenides, a materials class containing an insulating phase (245 phase) and a superconducting (SC) phase. Due to the microstructural complexity of these superconductors, the role of the 245 phase in the development of the superconductivity has been a puzzle. Here we demonstrate a comprehensive high-pressure study on the insulating samples with pure 245 phase and biphasic SC samples. We find that the insulating behavior can be completely suppressed by pressure in the insulating samples and also identify an intermediate metallic (M′) state. The Mott insulating (MI) state of the 245 phase and the M′ state coexist over a significant range of pressure up to ∼10 GPa, the same pressure at which the superconductivity of the SC samples vanishes. Our results reveal the M′ state as a pathway that connects the insulating and SC phases of the alkaline iron selenides and indicate that the coexistence and interplay between the MI and M′ states is a necessary condition for superconductivity. Finally, we interpret the M′ state in terms of an orbital selectivity of the correlated 3d electrons.
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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.
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    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, Pengcheng
    We 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.