Observation of iron d-orbitals modifications in superconducting FeSe by Raman spectra study

We have studied the Raman spectra of superconducting FeSe and non-superconducting Cu-doped FeSe crystals at different temperatures. The spectral range covers the phonon modes at low frequency regime to electronic Raman scattering at high frequency regime, up to 2000 cm⁻¹. A hardening in frequency of B_1g mode, which is related to the spin-phonon coupling is observed. At high frequency region, our results on superconducting FeSe crystal show that the d_zx and d_yz orbitals of Fe split at ∼130 K and then become closer to achieve balance as temperature decreases. This result indicates that a modification of d-orbital electron state of iron happens at temperature much higher than the structural transition, implicating a precursor order in iron-based superconductor. In comparison with non-superconducting Cu_0.03Fe_0.97Se crystal, our observations suggest that the orbital modification could be an important clue for understanding the mechanism of superconductivity in iron-based superconductors.