Browsing by Author "Wang, Zhentao"

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    Frustrated Magnetism in Strongly Correlated Electron Systems
    (2016-04-21) Wang, Zhentao; Nevidomskyy, Andriy H
    A deep understanding of magnetism is essential for its application in magnetic semiconductors, spintronic devices and unconventional superconductors. In this work, we study magnetic structures and their corresponding excitations in several strongly correlated electron systems, where exotic orderings can be induced as a result of magnetic frustration and quantum fluctuations. We show that emergent spin textures can arise close to a magnetic field-induced quantum critical point, when the single magnon excitations have several degenerate non-coplanar minima. In this case, quantum fluctuations can lift such degeneracy and lead to the crystallization of the magnetic vortex strings. Magnetic frustration also plays an important role in Fe-based superconductors. We analyze the spin excitations in the ordered as well as paramagnetic phase of these materials, and find that higher order spin exchanges are essential for understanding the inelastic neutron scattering experiments (INS). The presence of such higher order spin interactions has far-reaching consequences, potentially resulting in more exotic phases, such as the multipolar orders. In particular, we find propensity to ferro-quadrupolar order, which we propose as a candidate for the ground state of the iron selenide FeSe. We find that the calculated spin excitations in this quadrupolar state closely resemble the results of recent INS measurements. In addition to electron spins, orbital physics also plays a prominent role in Fe-based superconductors. We study the interplay between spin and orbital degrees of freedom and show that the so-called nematic order can be naturally understood as the decoupling of the two transitions, when orbital ordering preempts long-range magnetic spin order. Our results reveal that magnetic frustration plays an important role in several strongly correlated electron systems, and elucidating its consequences is crucial for the understanding and potential application of these materials.
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    Spin dynamics of a J1- J2-K model for the paramagnetic phase of iron pnictides
    (American Physical Society, 2012) Yu, Rong; Wang, Zhentao; Goswami, Pallab; Nevidomskyy, Andriy H.; Si, Qimiao; Abrahams, Elihu
    We study the finite-temperature spin dynamics of the paramagnetic phase of iron pnictides within an antiferromagnetic J1-J2 Heisenberg model on a square lattice with a biquadratic coupling −K(Si · Sj )2 between the nearest-neighbor spins. Our focus is on the paramagnetic phase in the parameter regime of this J1-J2-K model where the ground state is a (π,0) collinear antiferromagnet. We treat the biquadratic interaction via a Hubbard-Stratonovich decomposition and study the resulting effective quadratic-coupling model using both modified spin wave and Schwinger boson mean-field theories; the results for the spin dynamics derived from the two methods are very similar. We show that the spectral weight of dynamical structure factor S(q,ω) is peaked at ellipses in the momentum space at low excitation energies.With increasing energy, the elliptic features expand towards the zone boundary and gradually split into two parts, forming a pattern around (π,π). Finally, the spectral weight is anisotropic, being larger along the major axis of the ellipse than along its minor axis. These characteristics of the dynamical structure factor are consistent with the recent measurements of the inelastic neutron scattering spectra of BaFe2As2 and SrFe2As2.
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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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    Three-Dimensional Crystallization of Vortex Strings in Frustrated Quantum Magnets
    (American Physical Society, 2015) Wang, Zhentao; Kamiya, Yoshitomo; Nevidomskyy, Andriy H.; Batista, Cristian D.
    We demonstrate that frustrated exchange interactions can produce exotic 3D crystals of vortex strings near the saturation field (H=Hsat) of body- and face-centered cubic Mott insulators. The combination of cubic symmetry and frustration leads to a magnon spectrum of the fully polarized spin state (H>Hsat) with degenerate minima at multiple noncoplanar Q vectors. This spectrum becomes gapless at the quantum critical point H=Hsat and the magnetic ordering below Hsat can be formally described as a condensate of a dilute gas of bosons. By expanding in the lattice gas parameter, we find that different vortex crystals span sizable regions of the phase diagrams for isotropic exchange and are further stabilized by symmetric exchange anisotropy.