Spins, magnons, and phonons in the quasi-1D material MoI₃

Quasi-1D materials are interesting for both basic and applied physics. MoI₃ has attracted special attention due to its strongly 1D character. In the Pmmn phase, each Mo atom is surrounded by 6 chalcogen atoms in distorted octahedral coordination forming dimerized chains. The chains are weakly coupled to neighboring chains in a triangular lattice. Density functional theory calculations find an antiferromagnetic (AFM) ground state with an easy plane perpendicular to the chains, alternating spins along the dimerized chains, and a spin spiral texture from chain to chain. The calculated magnetic moments of ~3 μ_B per Mo ion are consistent with Mo³⁺ with 3 electrons in the t_2g ground state resulting from the octahedral crystal field splitting. Exchange coupling constants are extracted for a Heisenberg-type magnetic Hamiltonian, which is then solved for the magnon dispersion using linear spin wave theory. The phonon spectrum is determined, and the frequencies of the phonon and magnon spectra match well with measured Raman peaks [1]. The anisotropic thermal conductivity is also determined from the phonon Boltzmann transport equation.

[1] F. Kargar et al., arXiv preprint arXiv:2210.05081 (2022).