Abstract
We have studied the Raman spectra of superconducting FeSe crystals at different temperatures. The spectral range covers the phonon modes at low frequency regime to electron Raman scattering at high frequency regime, up to 2000 cm-1. The phonon mode near 130 cm-1 shows a sharp decrease in the frequency, and the line-width of the mode near 181 cm-1 increases above 90 K, which is a consequence of the structural transition as reported in x-ray and neutron diffraction experiments. At high frequency region, the mode at 1519 cm-1 splits into two modes as temperature below 130 K. These two modes are attributed to the electronic Raman scattering from zx/yz to (x2-y2) d-orbitals of Fe. The energy difference of these two modes decreases as temperature decreases. These behaviors indicate that the dzx and dyz orbitals split at ~130 K and then become closer to achieve balance as temperature decreases. Surprisingly, the other Raman mode near 1004 cm-1 appears only below the same temperature. These results indicate that a modification of d-orbital electron state of iron happens at temperature much higher than the structural transition, which was also observed in the transient optical reflectivity study, implicating a precursor order in iron-based superconductor. In comparison with non-superconducting Cu0.03Fe0.97Se crystal, it shows no structural distortion, and the crystal field splitting cannot be observed in Raman spectrum. This indicates that the orbital modification could be an important clue for understanding the mechanism of superconductivity in iron-based superconductors.