Browsing by Author "Hu, Wen-Jun"

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    Spin Ferroquadrupolar Order in the Nematic Phase of FeSe
    (American Physical Society, 2016) Wang, Zhentao; Hu, Wen-Jun; Nevidomskyy, Andriy H.
    We provide evidence that spin ferroquadrupolar (FQ) order is the likely ground state in the nonmagnetic nematic phase of stoichiometric FeSe. By studying the variational mean-field phase diagram of a bilinear-biquadratic Heisenberg model up to the 2nd nearest neighbor, we find the FQ phase in close proximity to the columnar antiferromagnet commonly realized in iron-based superconductors; the stability of the FQ phase is further verified by the density matrix renormalization group. The dynamical spin structure factor in the FQ state is calculated with flavor-wave theory, which yields a qualitatively consistent result with inelastic neutron scattering experiments on FeSe at both low and high energies. We verify that FQ can coexist with C4 breaking environments in the mean-field calculation, and further discuss the possibility that quantum fluctuations in FQ act as a source of nematicity.
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    Spin-isotropic continuum of spin excitations in antiferromagnetically orderedﾠFe1.07Te
    (American Physical Society, 2018) Song, Yu; Lu, Xingye; Regnault, L.-P.; Su, Yixi; Lai, Hsin-Hua; Hu, Wen-Jun; Si, Qimiao; Dai, Pengcheng; Rice Center for Quantum Materials
    Unconventional superconductivity typically emerges in the presence of quasidegenerate ground states, and the associated intense fluctuations are likely responsible for generating the superconducting state. Here we use polarized neutron scattering to study the spin space anisotropy of spin excitations in Fe1.07Te exhibiting bicollinear antiferromagnetic (AF) order, the parent compound of FeTe1−xSexsuperconductors. We confirm that the low-energy spin excitations are transverse spin waves, consistent with a local-moment origin of the bicollinear AF order. While the ordered moments lie in the ab plane in Fe1.07Te, it takes less energy for them to fluctuate out of plane, similar to BaFe2As2 and NaFeAs. At energies above E≳20 meV, we find magnetic scattering to be dominated by an isotropic continuum that persists up to at least 50 meV. Although the isotropic spin excitations cannot be ascribed to spin waves from a long-range-ordered local-moment antiferromagnet, the continuum can result from the bicollinear magnetic order ground state of Fe1.07Te being quasidegenerate with plaquette magnetic order.
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    Unified spin model for magnetic excitations in iron chalcogenides
    (American Physical Society, 2017) Ergueta, Patricia Bilbao; Hu, Wen-Jun; Nevidomskyy, Andriy H.
    Recent inelastic neutron scattering (INS) measurements on FeSe and Fe(Te1−xSex) have sparked intense debate over the nature of the ground state in these materials. Here we propose an effective bilinear-biquadratic spin model, which is shown to consistently describe the evolution of low-energy spin excitations in FeSe, both under applied pressure and upon Se/Te substitution. The phase diagram, studied using a combination of variational mean-field, flavor-wave calculations and density-matrix renormalization group (DMRG), exhibits a sequence of transitions between the columnar antiferromagnet common to the iron pnictides, the nonmagnetic ferroquadrupolar phase attributed to FeSe, and the double-stripe antiferromagnetic order known to exist in Fe1+yTe. The calculated spin structure factor in these phases mimics closely that observed with INS in the Fe(Te1−xSex) series. In addition to the experimentally established phases, the possibility of incommensurate magnetic order is also predicted.