Spin-waves in the $J_{1a}-J_{1b}-J_{2}$ orthorombic square-lattice Heisenberg models: Application to the iron pnictide materials

Details Spin-waves in the $J_{1a}-J_{1b}-J_{2}$ orthorombic square-lattice Heisenberg models: Application to the iron pnictide materials Spin-waves in the $J_{1a}-J_{1b}-J_{2}$ orthorombic square-lattice Heisenberg models: Application to the iron pnictide materials

2009-10-09

arxiv.org/abs/0910.1793v1

Phys. Rev. B 81, 024505 (2010)

Physics

Condensed Matter

Strongly Correlated Electrons

6 pages, 10 figures

Scientific paper

Motivated by the observation of spatially anisotropic exchange constants in the iron pnictide materials, we study the spin-wave spectra of the $J_{1a}-J_{1b}-J_{2}$ Heisenberg models on a square-lattice with nearest neighbor exchange $J_{1a}$ along x and $J_{1b}$ along y axis and a second neighbor exchange $J_2$. We focus on the regime, where the spins order at ($\pi,0$), and compute the spectra by systematic expansions around the Ising limit. We study both spin-half and spin-one Heisenberg models as well as a range of parameters to cover various cases proposed for the iron pnictide materials. The low-energy spectra have anisotropic spin-wave velocities and are renormalized with respect to linear spin-wave theory by up to 20 percent, depending on parameters. Extreme anisotropy, consisting of a ferromagnetic $J_{1b}=- |J_F|$, is best distinguished from a weak anisotropy ($J_{1a}\approx J_{1b}=J_1$, $J_2>J_1/2$) by the nature of the spin-waves near the wavevectors ($0,\pi$) or ($\pi,\pi$). The reported spectra for the pnictide material CaFe$_2$As$_2$ clearly imply such an extreme anisotropy.

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