Phonon anomalies in some iron telluride materials

The detailed temperature dependence of the infrared-active mode in ${\mathrm{Fe}}_{1.03}\mathrm{Te}$ ($T_N\simeq 68$ K) and ${\mathrm{Fe}}_{1.13}\mathrm{Te}$ ($T_N\simeq 56$ 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 $T_N$ 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 ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ ($T_c\simeq 14$ K) the infrared-active $E_u$ mode displays asymmetric line shape at all temperatures, which is most pronounced between 100 and 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.

Figure 1

(a) The real part of the optical conductivity of ${\mathrm{Fe}}_{1.03}\mathrm{Te}$ above and below $T_N\simeq 68$ K in the region of the infrared-active mode at $\simeq 196{\mathrm{cm}}^{-1}$. There is no evidence that this mode splits below $T_N$. (b) Fano oscillator fit (short-dashed line) to ${\sigma }_1\left(\omega \right)$ at 30 K with the Drude-Lorentz background (long-dashed line) and (c) the simultaneous fit to ${\sigma }_2\left(\omega \right)$; note the almost perfectly symmetric profile.

Figure 2

The temperature dependence of the (a) infrared-active mode compared with the anharmonic decay model (dotted line), (b) linewidth and predicted dependence (dotted line), (c) oscillator strength, and (d) asymmetry parameter in ${\mathrm{Fe}}_{1.03}\mathrm{Te}$ ($T_N\simeq 68$ K); the dotted lines in the last two panels are drawn as a guide to the eye.

Figure 3

The temperature dependence of the (a) infrared-active mode compared with the anharmonic decay model (dotted line), (b) linewidth and predicted dependence (dotted line), (c) oscillator strength, and (d) asymmetry parameter in ${\mathrm{Fe}}_{1.13}\mathrm{Te}$ ($T_N\simeq 56$ K); the dotted lines in the last two panels are drawn as a guide to the eye.

Figure 4

(a) The real part of the optical conductivity of ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ above and below $T_c\simeq 14$ K in the region of the infrared-active $E_u$ mode at $\simeq 213{\mathrm{cm}}^{-1}$. (b) Fano oscillator fit (short-dashed line) to ${\sigma }_1\left(\omega \right)$ at 30 K with the Drude-Lorentz background (long-dashed line) and (c) the simultaneous fit to ${\sigma }_2\left(\omega \right)$.

Figure 5

The temperature dependence of the (a) infrared-active $E_u$ mode compared with the anharmonic decay model (dotted line), (b) linewidth and predicted dependence (dotted line), (c) oscillator strength, and (d) asymmetry parameter in ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ ($T_c=14$ K); the dotted lines in the last two panels are drawn as a guide to the eye.