Phonon, two-magnon, and electronic Raman scattering of Fe${}_{1+y}$Te${}_{1-x}$Se${}_x$

We have measured Raman-scattering spectra of single-crystalline FeTe${}_{0.6}$Se${}_{0.4}$ ($T_c~$ 14.5 K) and its parent compound Fe${}_{1.074}$Te at various temperatures. In the parent compound Fe${}_{1.074}$Te, $A_{1g}$ and $B_{1g}$ modes have been observed at 158 and 202 cm${}^{-1}$, respectively, at 5 K. These frequencies agree qualitatively with the calculated results. Two-magnon excitation has been observed around 2300 cm${}^{-1}$ for both compounds. A temperature dependence between the electronic Raman spectra below and above $T_c$ has been observed, and $2\Delta$ and $2\Delta /k_B{T}_C$ have been estimated as 5.0 meV and 4.0, respectively.

Figure 1

Results of magnetic measurements for (a) Fe${}_{1.074}$Te and (b) FeSe${}_{0.6}$Te${}_{0.4}$. $T_s$ and $T_c$ have been confirmed as $~58$ and 14.5 K, respectively.

Figure 2

Temperature-dependent phonon Raman spectra of Fe${}_{1.074}$Te. $A_{1g}$ and $B_{1g}$ modes have been observed at 158 and 202 cm${}^{-1}$, respectively, at 5 K.

Figure 3

Atomic-displacement patterns of Raman-active optical modes of Fe(Te,Se).

Figure 4

Comparison of phonon Raman spectra between the parent compound Fe${}_{1.074}$Te and the superconducting FeTe${}_{0.6}$Se${}_{0.4}$. $A_{1g}$ and $B_{1g}$ modes of FeTe${}_{0.6}$Se${}_{0.4}$ have been observed at 161 and 202 cm${}^{-1}$, respectively, at 5 K.

Figure 5

Temperature-dependent Raman spectra of Fe${}_{1.074}$Te with the wider-energy region up to 7000 cm${}^{-1}$. Broad-peak structures around 2300 cm${}^{-1}$ are assigned to the two-magnon excitation. Broad weak-edge structures around 300–800 cm${}^{-1}$ observed below $T_s$ are denoted by the arrow in the ($\mathit{ab}$) configuration spectra and are considered to be related to the magnetic transition.

Figure 6

Comparison of two-magnon spectra between the parent compound Fe${}_{1.074}$Te and the superconducting FeTe${}_{0.6}$Se${}_{0.4}$.

Figure 7

Electronic Raman spectra of FeTe${}_{0.6}$Se${}_{0.4}$ above and below $T_c$ and difference spectra. Dip structures are denoted by the arrows in the ($\mathit{aa}$) and ($\mathit{xx}$) configuration spectra.

Figure 8

The band structure of the unfolded five-band tight-binding model reproduced from Ref. 20. Colors are distinguished by the contributions of each orbital. The Fermi level has shifted to reproduce three hole FS observed by ARPES.[46] (b) Fermi surfaces deduced from the five-band tight-binding model. (c) The top, middle, and bottom panels show the Raman form factors $|\frac{m}{\hslash {}^2}\partial {}^2\epsilon /\partial k_{\alpha }\partial k_{\beta }|{}^2$ of all the measured polarization configurations for the $\mathit{XZ}$, $\mathit{YZ}$, and $X^2$-$Y^2$ bands, respectively. The contributions to the electronic Raman scattering are significant in the white or red regions overlapping with each FS sheet represented by the yellow ($\mathit{XZ}$ band), green ($\mathit{YZ}$ band), and blue ($X^2$-$Y^2$ band) lines, respectively.