Anomalous optical phonons in FeTe pnictides: spin state, magnetic order, and lattice anharmonicity

TL;DR

This study investigates the relationship between magnetic moments, spin states, and lattice vibrations in FeTe using polarized Raman spectroscopy, revealing the significance of orbital degrees of freedom and spin-phonon coupling.

Contribution

It provides new insights into the spin state and magnetic order in FeTe, highlighting the role of orbital degeneracy and spin-phonon interactions in Fe-based materials.

Findings

Magnetic moments correlate with phonon linewidths at low temperatures.

Intermediate spin state (S=1) and orbital degeneracy are identified in FeTe.

Spin-phonon coupling constants are estimated from microscopic modeling.

Abstract

Polarized Raman-scattering spectra of non-superconducting, single-crystalline FeTe are investigated as function of temperature. We have found a relation between the magnitude of ordered magnetic moments and the linewidth of A1g phonons at low temperatures. This relation is attributed to the intermediate spin state (S=1) and the orbital degeneracy of the 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 the Fe-based superconductors with large ordered magnetic moments, while small magnetic moment of Fe ions in some iron pnictides reflects the low spin state of Fe ions in those systems.