Phonon anomalies in some iron telluride materials

The detailed temperature dependence of the infrared-active mode in Fe1.03Te (TN 68 K) and Fe1.13Te (TN 56 K) has been examined, and the position, width, strength, and asymmetry parameter have been 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 TN 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 spinor electron-phonon coupling. The frequency of the infrared-active mode and the magnitude of the shift below TN are predicted reasonably well by first-principles calculations; however, the predicted splitting of the mode is not observed. In superconducting FeTe0.55Se0.45 (Tc 14 K) the infrared-active Eu mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 and 200 K, indicating the presence of either spinor electron-phonon coupling, which may be a necessary prerequisite for superconductivity in this class of materials.