Ergueta, Patricia BilbaoNevidomskyy, Andriy H.2016-06-102016-06-102015Ergueta, Patricia Bilbao and Nevidomskyy, Andriy H.. "Ising-nematic order in the bilinear-biquadratic model for the iron pnictides." <i>Physical Review B,</i> 92, no. 16 (2015) American Physical Society: 165102. http://dx.doi.org/10.1103/PhysRevB.92.165102.https://hdl.handle.net/1911/90497Motivated by the recent inelastic neutron scattering (INS) measurements in the iron pnictides which show a strong anisotropy of spin excitations even above the magnetic transition temperature TN, we study the spin dynamics within the frustrated Heisenberg model with biquadratic spin-spin exchange interactions. Using the Dyson-Maleev (DM) representation, which proves appropriate for all temperature regimes, we find that the spin-spin dynamical structure factors are in excellent agreement with experiment, exhibiting breaking of the C4 symmetry even into the paramagnetic region TN<T<Tσ, which we refer to as the Ising-nematic phase. In addition to the Heisenberg spin interaction, we include the biquadratic coupling −K(Si⋅Sj)2 and study its effect on the dynamical temperature range Tσ−TN of the Ising-nematic phase. We find that this range reduces dramatically when even small values of the interlayer exchange Jc and biquadratic coupling K are included. To supplement our analysis, we benchmark the results obtained using full decoupling in the DM method against those from different nonlinear spin-wave theories, including the recently developed generalized spin-wave theory (GSWT), and find good qualitative agreement among the different theoretical approaches as well as experiment for both the spin-wave dispersions and the dynamical structure factors.engArticle is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.Journal articlehttp://dx.doi.org/10.1103/PhysRevB.92.165102