Browsing by Author "Lee, D.H."

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    Experimental elucidation of the origin of the ‘double spin resonances’ in Ba(Fe1−xCox)2As2
    (American Physical Society, 2016) Wang, Meng; Yi, M.; Sun, H. L.; Valdivia, P.; Kim, M.G.; Xu, Z. J.; Berlijn, T.; Christianson, A.D.; Chi, Songxue; Hashimoto, M.; Lu, D.H.; Li, X.D.; Bourret-Courchesne, E.; Dai, Pengcheng; Lee, D.H.; Maier, T.A.; Birgeneau, R.J.
    We report a combined study of the spin resonances and superconducting gaps for underdoped (Tc=19 K), optimally doped (Tc=25 K), and overdoped (Tc=19 K) Ba(Fe1−xCox)2As2 single crystals with inelastic neutron scattering and angle resolved photoemission spectroscopy. We find a quasi-two-dimensional spin resonance whose energy scales with the superconducting gap in all three compounds. In addition, anisotropic low energy spin excitation enhancements in the superconducting state have been deduced and characterized for the under and optimally doped compounds. Our data suggest that the quasi-two-dimensional spin resonance is a spin exciton that corresponds to the spin singlet-triplet excitations of the itinerant electrons. However, the intensity enhancements of the anisotropic spin excitations are dominated by the out-of-plane spin excitations of the ordered moments due to the suppression of damping in the superconducting state. Hence we offer an interpretation of the double energy scales differing from previous interpretations based on anisotropic superconducting energy gaps and systematically explain the doping-dependent trend across the phase diagram.
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    Mott localization in a pure stripe antiferromagnet ${\mathrm{Rb}}_{1\ensuremath{-}\ensuremath{\delta}}{\mathrm{Fe}}_{1.5\ensuremath{-}\ensuremath{\sigma}}{\mathrm{S}}_{2}$
    (American Physical Society, 2015) Wang, Meng; Yi, Ming; Cao, Huibo; de la Cruz, C.; Mo, S.K.; Huang, Q.Z.; Bourret-Courchesne, E.; Dai, Pengcheng; Lee, D.H.; Shen, Z.X.; Birgeneau, R.J.
    A combination of neutron diffraction and angle-resolved photoemission spectroscopy measurements on a pure antiferromagnetic stripe Rb1−δFe1.5−σS2 is reported. A neutron diffraction experiment on a powder sample shows that a 98% volume fraction of the sample is in the antiferromagnetic stripe phase with rhombic iron vacancy order and a refined composition of Rb0.66Fe1.36S2, and that only 2% of the sample is in the block antiferromagnetic phase with √5×√5 iron vacancy order. Furthermore, a neutron diffraction experiment on a single crystal shows that there is only a single phase with the stripe antiferromagnetic order with the refined composition of Rb0.78Fe1.35S2, while the phase with block antiferromagnetic order is absent. Angle-resolved photoemission spectroscopy measurements on the same crystal with the pure stripe phase reveal that the electronic structure is gapped at the Fermi level with a gap larger than 0.325 eV. The data collectively demonstrate that the extra 10% iron vacancies in addition to the rhombic iron vacancy order effectively impede the formation of the block antiferromagnetic phase; the data also suggest that the stripe antiferromagnetic phase with rhombic iron vacancy order is a Mott insulator.
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    Mott localization in a pure stripe antiferromagnet Rb1−δFe1.5−σS2
    (American Physical Society, 2015) Wang, Meng; Yi, Ming; Cao, Huibo; de la Cruz, C.; Mo, S.K.; Huang, Q.Z.; Bourret-Courchesne, E.; Dai, Pengcheng; Lee, D.H.; Shen, Z.X.; Birgeneau, R.J.
    A combination of neutron diffraction and angle-resolved photoemission spectroscopy measurements on a pure antiferromagnetic stripe Rb1−δFe1.5−σS2 is reported. A neutron diffraction experiment on a powder sample shows that a 98% volume fraction of the sample is in the antiferromagnetic stripe phase with rhombic iron vacancy order and a refined composition of Rb0.66Fe1.36S2, and that only 2% of the sample is in the block antiferromagnetic phase with 5√×5√ iron vacancy order. Furthermore, a neutron diffraction experiment on a single crystal shows that there is only a single phase with the stripe antiferromagnetic order with the refined composition of Rb0.78Fe1.35S2, while the phase with block antiferromagnetic order is absent. Angle-resolved photoemission spectroscopy measurements on the same crystal with the pure stripe phase reveal that the electronic structure is gapped at the Fermi level with a gap larger than 0.325 eV. The data collectively demonstrate that the extra 10% iron vacancies in addition to the rhombic iron vacancy order effectively impede the formation of the block antiferromagnetic phase; the data also suggest that the stripe antiferromagnetic phase with rhombic iron vacancy order is a Mott insulator.
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    Spin waves and spatially anisotropic exchange interactions in the $S=2$ stripe antiferromagnet ${\mathrm{Rb}}_{0.8}{\mathrm{Fe}}_{1.5}{\mathrm{S}}_{2}$
    (American Physical Society, 2015) Wang, Meng; Valdivia, P.; Yi, Ming; Chen, J.X.; Zhang, W.L.; Ewings, R.A.; Perring, T.G.; Zhao, Yang; Harriger, L.W.; Lynn, J.W.; Bourret-Courchesne, E.; Dai, Pengcheng; Lee, D.H.; Yao, D. X.; Birgeneau, R.J.
    An inelastic neutron scattering study of the spin waves corresponding to the stripe antiferromagnetic order in insulating Rb0.8Fe1.5S2 throughout the Brillouin zone is reported. The spin wave spectra are well described by a Heisenberg Hamiltonian with anisotropic in-plane exchange interactions. Integrating the ordered moment and the spin fluctuations results in a total moment squared of 27.6±4.2μ2B/Fe, consistent with S≈2. Unlike XFe2As2 (X=Ca, Sr, and Ba), where the itinerant electrons have a significant contribution, our data suggest that this stripe antiferromagnetically ordered phase in Rb0.8Fe1.5S2 is a Mott-like insulator with fully localized 3d electrons and a high-spin ground state configuration. Nevertheless, the anisotropic exchange couplings appear to be universal in the stripe phase of Fe pnictides and chalcogenides.
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    Spin waves and spatially anisotropic exchange interactions in the S=2 stripe antiferromagnet Rb0.8Fe1.5S2
    (American Physical Society, 2015) Wang, Meng; Valdivia, P.; Yi, Ming; Chen, J.X.; Zhang, W.L.; Ewings, R.A.; Perring, T.G.; Zhao, Yang; Harriger, L.W.; Lynn, J.W.; Bourret-Courchesne, E.; Dai, Pengcheng; Lee, D.H.; Yao, D.X.; Birgeneau, R.J.
    An inelastic neutron scattering study of the spin waves corresponding to the stripe antiferromagnetic order in insulating Rb0.8Fe1.5S2 throughout the Brillouin zone is reported. The spin wave spectra are well described by a Heisenberg Hamiltonian with anisotropic in-plane exchange interactions. Integrating the ordered moment and the spin fluctuations results in a total moment squared of 27.6±4.2μ2B/Fe, consistent with S≈2. Unlike XFe2As2 (X=Ca, Sr, and Ba), where the itinerant electrons have a significant contribution, our data suggest that this stripe antiferromagnetically ordered phase in Rb0.8Fe1.5S2 is a Mott-like insulator with fully localized 3d electrons and a high-spin ground state configuration. Nevertheless, the anisotropic exchange couplings appear to be universal in the stripe phase of Fe pnictides and chalcogenides.