Raman scattering study of the lattice dynamics of superconducting LiFeAs

We report an investigation of the lattice dynamical properties of LiFeAs using inelastic light scattering. Five of the six expected phonon modes are observed. The temperature evolution of their frequencies and linewidths is in good agreement with an anharmonic-decay model. We find no evidence for substantial electron-phonon coupling, and no superconductivity-induced phonon anomalies.

Figure 1

(a) Atomic displacement patterns of the Raman-active optical modes of LiFeAs. (b) Room-temperature Raman spectra in $z\left(xy\right)z$, $z\left(x^{\prime }{y}^{\prime }\right)z$, $z\left(x^{\prime }{x}^{\prime }\right)z$, $z\left(xx\right)z$, $y\left(xz\right)y$, and $y\left(zz\right)y$ configurations [note that the spectra in $z\left(x^{\prime }{y}^{\prime }\right)z$ and $z\left(x^{\prime }{x}^{\prime }\right)z$ configurations have been recorded in a lower-resolution mode to maximize the signal]. Spectra have been shifted vertically for clarity. Starting from the left, the peaks are assigned to $A_{1g}$(As), $B_{1g}$(Fe), $E_g$(1), $E_g$(2), and $A_{1g}$(Li) vibrations. (c) $A_{1g}$(As) and $B_{1g}$(Fe) phonons for selected temperatures (room temperature, $T\sim T_c$, and the base temperature of our cryostat). Black squares are the data; the red line is the result of a fit following the procedure described in the text. The spectra have been shifted vertically for clarity. (d) Same plot for $E_g$(1), $E_g$(2), and $A_{1g}$(Li) phonons.

Figure 2

Comparison of the temperature dependence of the frequencies of the high-energy modes at 585 and 620 cm${}^{-1}$ with the sums of the frequencies of modes observed in $y\left(zz\right)y$ and $y\left(xz\right)y$ polarizations.

Figure 3

Temperature dependence of (a)–(e) frequencies and (f)–(j) linewidths of the five observed Raman-active modes. The pink dashed line marks $T_c$, and the green line is the result of a conventional phonon anharmonic model (see text).