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Spin dynamics of a J_1-J_2-K model for the paramagnetic phase of iron pnictides

机译:J_1-J_2-K模型对铁离子顺磁性相的自旋动力学

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We study the finite-temperature spin dynamics of the paramagnetic phase of iron pnictides within an antiferromagnetic J_1-J_2 Heisenberg model on a square lattice with a biquadratic coupling - K(S_i · S_j)2 between the nearest-neighbor spins. Our focus is on the paramagnetic phase in the parameter regime of this J_1-J_2-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 BaFe_2As_2 and SrFe_2AS_2.
机译:我们在最近邻自旋之间具有双二次耦合-K(S_i·S_j)2的方格上研究了反铁磁J_1-J_2 Heisenberg模型中铁素化物的顺磁性相的顺温有限温度自旋动力学。我们的重点是在此J_1-J_2-K模型的参数范围内的顺磁相位,其中基态是(π,0)共线反铁磁体。我们通过Hubbard-Stratonovich分解处理双二次相互作用,并使用修正的自旋波和Schwinger玻色子均场理论研究由此产生的有效二次耦合模型;从这两种方法得出的自旋动力学结果非常相似。我们表明,动态结构因子S(q,ω)的频谱权重在低激发能下的动量空间中的椭圆处达到峰值。随着能量的增加,椭圆形特征向区域边界扩展并逐渐分成两部分,从而在(π,π)周围形成图案。最后,频谱权重是各向异性的,沿着椭圆的长轴大于沿着其短轴。动力学结构因子的这些特征与BaFe_2As_2和SrFe_2AS_2的非弹性中子散射光谱的最新测量结果一致。

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