Spin waves in J1a−J1b−J2 orthorhombic square-lattice Heisenberg models: Application to iron pnictide materials
R. Applegate, J. Oitmaa, R. R. P. Singh
DOI 10.1103/PhysRevB.81.024505 · Physical Review B
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Abstract
Motivated by the observation of spatially anisotropic exchange constants in the iron pnictide materials, we study the spin-wave spectra of the J1a−J1b−J2 Heisenberg models on a square lattice with nearest-neighbor exchange J1a along x and J1b along y axis and a second-neighbor exchange J2. We focus on the regime, where the spins order at (π,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%, depending on parameters. Extreme anisotropy, consisting of a ferromagnetic J1b=−|JF|, is best distinguished from a weak anisotropy (J1a≈J1b=J1, J2>J1/2) by the nature of the spin waves near the wave vectors (0,π) or (π,π). The reported spectra for the pnictide material CaFe2As2 clearly imply such an extreme anisotropy.
Source-reported materials — not catalogue approval
| Formula | Reported Tc (K) | Pressure (GPa) | Type |
|---|---|---|---|
| CaFe2As2 Archive — visibility unverified Source-occurrence policy only; no material identity or catalogue acceptance is inferred from the formula. | — | Pressure not reported | unknown |
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