Phonon anomalies in some iron-telluride materials

TL;DR

This study investigates the temperature-dependent behavior of infrared-active phonon modes in iron-telluride compounds, revealing mode coupling to spin or charge excitations and implications for superconductivity.

Contribution

It provides detailed experimental analysis of phonon anomalies and their coupling to electronic excitations in iron-telluride materials, with comparisons to first-principles predictions.

Findings

Mode frequency increases as temperature decreases

Mode asymmetry indicates coupling to spin or charge excitations

Superconducting FeTeSe shows persistent phonon asymmetry

Abstract

The detailed temperature dependence of the infrared-active mode in Fe$_{1.03}$Te ($T_N\simeq 68$ K) and Fe$_{1.13}$Te ($T_N\simeq 56$ K) has been examined, and the position, width, strength, and asymmetry parameter determined using an asymmetric Fano profile superimposed on an electronic background. In both materials the frequency of the mode increases as the temperature is reduced; however, there is also a slight asymmetry in the line shape, indicating that the mode is coupled to either spin or charge excitations. Below $T_N$ there is an anomalous decrease in frequency and the mode shows little temperature dependence, at the same time becoming more symmetric, suggesting a reduction in spin- or electron-phonon coupling. The frequency of the infrared-active mode and the magnitude of the shift below $T_N$ are predicted reasonably well by first-principles calculations; however, the predicted splitting of the mode is not observed. In superconducting FeTe$_{0.55}$Se$_{0.45}$ ($T_c\simeq 14$ K) the infrared-active $E_u$ mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 - 200 K, indicating the presence of either spin- or electron-phonon coupling, which may be a necessary prerequisite for superconductivity in this class of materials.