Anomalous optical phonons in FeTe chalcogenides: Spin state, magnetic order, and lattice anharmonicity

Polarized Raman-scattering spectra of nonsuperconducting, single-crystalline FeTe are investigated as a function of temperature. We have found a relation between the magnitude of ordered magnetic moments and the linewidth of $A_{1g}$ phonons at low temperatures. This relation is attributed to the intermediate spin state ($S$ $=$ 1) and the orbital degeneracy of Fe ions. Spin-phonon coupling constants have been estimated based on microscopic modeling using density functional theory and analysis of the local spin density. Our observations show the importance of orbital degrees of freedom for Fe-based superconductors with large ordered magnetic moments, while the small magnetic moment of Fe ions in some iron pnictides reflects the low spin state of Fe ions in those systems.

Figure 1

Raman spectra of single crystal Fe${}_{1.051}$Te taken in quasibackscattering from the $\mathit{ab}$ plane at two temperatures. For clarity, the green curves are shifted in the vertical direction as indicated.

Figure 2

Parameters of two phonon modes in Fe${}_{1.051}$Te. (a) and (d) Temperature dependence of the frequency (solid circles) together with the fit using Eq. (1) (dashed lines). (b) and (e) Linewidth, FWHM. (c) and (f) Integrated intensity. Solid lines in (b), (c), (e), and (f) are a guide to the eyes.

Figure 3

Magnetic structure of FeTe in the monoclinic phase $P$2${}_1$/$m$ with the magnetic propagation vector $k_I$ $=$ (π/$a$, 0). Magnetic moments are shown by dark arrows drawn through the atoms. $A_{1g}$(Te) and $B_{1g}$(Fe) ions’ displacements are shown by arrows drawn from the ions.