Dichotomic nature of spin and electronic fluctuations in FeSe

We report on the symmetry-dependent Raman scattering studies of a FeSe single crystal over a broad frequency range of 60-7000 cm(-1). Our experimental data feature abundant excitation spectra of phonon, spin, charge, and electronic degrees of freedom. The 207-cm(-1) B-1g mode, involving the out-of-plane vibrations of the Fe ions, undergoes a large frequency shift by 14 cm(-1) due to orbital-phonon coupling. The Raman spectra are well separated into strongly symmetry-dependent excitations at energies below 3000 cm(-1) and weakly symmetry-dependent electron-hole excitations above 3000 cm(-1). This is taken as evidence for the coexistence of itinerant and localized spins. A prominent low-energy Raman response peaked at 531 cm(-1) observed only in Be g symmetry is interpreted in terms of coupled spin and charge quadrupole fluctuations. Our Raman results suggest that the dichotomic character of electrons together with the small energy scale involved in orbital ordering of the degenerated d(xz) and d(yz) orbitals promotes the intriguing physics of FeSe.