Anomalous transverse optic phonons in SnTe and PbTe -- revisited

TL;DR

This study revisits the soft transverse optic phonon modes in SnTe and PbTe, combining neutron scattering experiments with ab-initio calculations, revealing similarities and challenging previous interpretations of phonon behavior related to ferroelectric proximity.

Contribution

It provides new experimental and theoretical insights into the spectral features of soft phonons in SnTe and PbTe, especially highlighting the asymmetric spectral weight distribution and the persistent double-peak signature in PbTe.

Findings

The soft mode in SnTe has an asymmetric spectral weight distribution similar to PbTe.

The phonon peak widths are comparable in SnTe and PbTe.

PbTe shows a double-peak signature down to 5 K, questioning phonon-phonon scattering explanations.

Abstract

We present a study of the soft transverse optic phonon mode in SnTe in comparison to the corresponding mode in PbTe using inelastic neutron scattering and ab-initio lattice dynamical calculations. In contrast to previous reports our calculations predict that the soft mode in SnTe features a strongly asymmetric spectral weight distribution qualitatively similar to that found in PbTe. Experimentally, we find that the overall width in energy of the phonon peaks is comparable in our neutron scattering spectra for SnTe and PbTe. We observe the well-known double-peak-like signature of the TO mode in PbTe even down to $T$ = 5 K questioning its proposed origin purely based on phonon-phonon scattering. The proximity to the incipient ferroelectric transition in PbTe likely plays an important role not included in current models.